Re: [llvm-dev] llvm-dev Digest, Vol 167, Issue 3

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Re: [llvm-dev] llvm-dev Digest, Vol 167, Issue 3

Joel E. Denny via llvm-dev
Hi all,
We have enabled Thin LTO and LTO for a specific target architecture. What can be the possible scopes of improvement depending on the target  after we enable the basic LTO and thin LTO.?  Wanted to know the possible approach we can think to see how it performs compared to a not LTO enabled case and also what all possible directions can we think to improve upon it depending on the arch. I am looking for a general approach of improvement. Any suggestion will be valuable.
Thanks,
Siddharth

On Tue, May 1, 2018 at 11:15 PM, via llvm-dev <[hidden email]> wrote:
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Today's Topics:

   1. Re: Need guidance to work on NEW PASS managers bugs
      (Kaylor, Andrew via llvm-dev)
   2. Re: Disabling Exception in LLVM (Chris Bieneman via llvm-dev)
   3. Re: Disabling Exception in LLVM
      (Siddharth Shankar Swain via llvm-dev)
   4. Re: ThinLTO + CFI (via llvm-dev)
   5. Re: Disabling Exception in LLVM (Chris Bieneman via llvm-dev)
   6. Re: RFC: LLVM Assembly format for ThinLTO Summary
      (Teresa Johnson via llvm-dev)


----------------------------------------------------------------------

Message: 1
Date: Tue, 1 May 2018 17:22:26 +0000
From: "Kaylor, Andrew via llvm-dev" <[hidden email]>
To: vivek pandya <[hidden email]>
Cc: "[hidden email]" <[hidden email]>
Subject: Re: [llvm-dev] Need guidance to work on NEW PASS managers
        bugs
Message-ID:
        <[hidden email]>

Content-Type: text/plain; charset="utf-8"

Hi Vivek,

Have you read the mailing list threads on this topic? I don’t believe we’re quite ready to make the switch yet. There was a discussion last October about what was left to be done. I’m sure it has been discussed since then too. Here’s a link to the start of the October discussion.

http://lists.llvm.org/pipermail/llvm-dev/2017-October/118280.html

If you’d like to get involved, one possible area you could contribute is adding optbisect/optnone support as mentioned in this bug:

https://bugs.llvm.org/show_bug.cgi?id=28316

If that looks like something you’re interested in I can offer some guidance with it.

Thanks,
Andy

From: llvm-dev [mailto:[hidden email]] On Behalf Of vivek pandya via llvm-dev
Sent: Saturday, April 28, 2018 9:23 AM
To: llvm-dev <[hidden email]>
Subject: [llvm-dev] Need guidance to work on NEW PASS managers bugs

Hello LLVM-Devs,

I am trying to get some starting points for working on following new pass manager related bugs:

https://bugs.llvm.org/show_bug.cgi?id=28322 [PM] Remove use of old PM in the middle-end.

https://bugs.llvm.org/show_bug.cgi?id=28323 [PM] Use new PM in production for Clang, LLD, libLTO, etc. middle-end
https://bugs.llvm.org/show_bug.cgi?id=28321 [PM] Remove use of old PM in the backend

I read related code but did not get a good starting point.
Can someone guide me through this? Can we add more details to these bugs? Or can we further divide these bugs to smaller workable items?

Any help will be appreciated.

Thanks,
Vivek
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Message: 2
Date: Tue, 01 May 2018 10:32:14 -0700
From: Chris Bieneman via llvm-dev <[hidden email]>
To: Siddharth Shankar Swain <[hidden email]>
Cc: llvm-dev <[hidden email]>
Subject: Re: [llvm-dev] Disabling Exception in LLVM
Message-ID: <[hidden email]>
Content-Type: text/plain; charset="utf-8"

Siddharth,

I'm not sure what coding standards you refer to when you say "some C++ coding standard". This question is answered in the LLVM Coding Standards document here:

https://www.llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions <https://www.llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions>

As such LLVM's coding standards prohibit the use of exceptions and RTTI.

-Chris

> On May 1, 2018, at 10:02 AM, Siddharth Shankar Swain via llvm-dev <[hidden email]> wrote:
>
> Hi all,
> Can anyone explain why exceptions are disabled in LLVM, even if some C++ coding standard tells to use exceptions ?
> Thanks,
> Siddharth
> _______________________________________________
> LLVM Developers mailing list
> [hidden email]
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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Message: 3
Date: Tue, 1 May 2018 23:08:17 +0530
From: Siddharth Shankar Swain via llvm-dev <[hidden email]>
To: Chris Bieneman <[hidden email]>, llvm-dev
        <[hidden email]>
Subject: Re: [llvm-dev] Disabling Exception in LLVM
Message-ID:
        <CAMkbrzKtMUrYVSA_Ke3YJA=[hidden email]>
Content-Type: text/plain; charset="utf-8"

Hi Chris,
Thanks for answering, Can u clarify on this comment mentioned in
https://github.com/Z3Prover/z3/issues/861 .

  cplusplus no exception support · Issue #861 · Z3Prover/z3 · GitHub

   - LLVM's *source code* does not use exceptions for performance reasons
   and so is compiled by default with -fno-exceptions. When using LLVM's
   libraries via it's C++ interface it is important to match how LLVM was
   built (i.e. do not mix code built with and without exception support).
   - The Clang compiler which is built on top of LLVM is also compiled
   without exceptions by default. However the built Clang can compile C++ code
   with or without exceptions (using the -fno-exceptions). flag. Said
   another way Clang's implementation doesn't use exceptions but Clang itself
   can compile C++ code with or without exception support.


Can anyone clarify on this comment ?

Thanks,
Siddharth

On Tue, May 1, 2018 at 11:02 PM, Chris Bieneman <[hidden email]>
wrote:

> Siddharth,
>
> I'm not sure what coding standards you refer to when you say "some C++
> coding standard". This question is answered in the LLVM Coding Standards
> document here:
>
> https://www.llvm.org/docs/CodingStandards.html#do-not-
> use-rtti-or-exceptions
>
> As such LLVM's coding standards prohibit the use of exceptions and RTTI.
>
> -Chris
>
> On May 1, 2018, at 10:02 AM, Siddharth Shankar Swain via llvm-dev <
> [hidden email]> wrote:
>
> Hi all,
> Can anyone explain why exceptions are disabled in LLVM, even if some C++
> coding standard tells to use exceptions ?
> Thanks,
> Siddharth
> _______________________________________________
> LLVM Developers mailing list
> [hidden email]
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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Message: 4
Date: Tue, 1 May 2018 17:41:00 +0000
From: via llvm-dev <[hidden email]>
To: <[hidden email]>
Cc: [hidden email], [hidden email]
Subject: Re: [llvm-dev] ThinLTO + CFI
Message-ID: <[hidden email]>
Content-Type: text/plain; charset="utf-8"



> On Apr 30, 2018, at 5:43 PM, Vlad Tsyrklevich <[hidden email]> wrote:
>
> Hi Dmitry, the direct call patch seems like a good start; however, by falling through to the F->replaceUsesExceptBlockAddr(FDecl) at the bottom of LowerTypeTestsModule::importFunction() I believe it will replace all uses of a function (for a definition outside the TU it's defined in) with the direct function reference instead of just direct calls.

You’re right, it was meant to bail after direct call replacement.

> Also, I believe the logic for replaceDirectCalls() should be more restrictive, currently it will also rewrite calls where the function use is as a function pointer argument instead of the call target.

I’m checking if the user is a call instruction and that it’s getCalledFunction() returns non-null. I thought this would only give me direct calls. If not, what would be a good check?

I will create a Phab review to make it easier to comment.


>
> On Mon, Apr 30, 2018 at 1:05 PM <[hidden email]> wrote:
> Replacing direct calls to jump table entries with calls to real targets recovers most of the performance loss in my benchmark. Dealing with link order files is a bit cumbersome though: I can’t keep both, say “foo” and “foo.cfi” next to each other in link order, because if both exist, the linker reorders the jump table next to the real function. This is not what we want when the goal is to get rid of calls through jump tables. I had to manually pick the functions which were renamed to “foo.cfi” and replace them in the link order file. The process can be automated, but it’s a bit flaky if we have to do it to make CFI work.
>
> I attached 2 patches here: one is the direct call replacement and the other is moving type test lowering pass to run later in the pipeline. Interestingly, running the pass later seems to help with performance as well. Though the comment in PassManagerBuilder implies that this pass needs to run early. Why is it different from full LTO?
>
>
>
>
>
> > On Apr 27, 2018, at 11:04 AM, via llvm-dev <[hidden email]> wrote:
> >
> > For the test case below, I get the following IR for main() on entry to ThinLTO backend invocation of LowerTypeTestsModule::lower(). This is after I moved this pass down in the pipeline so it’s invoked after inlining.
> >
> > The declarations for foo() and bar() are read in at the time of module import, Importer.importFunctions() in lto::thinBackend(). They do not have type metadata attached to them.
> > In lowerTypeTestCall() we check if the pointer in the type test is of a known type, so we look at bitcast and then select operands. foo and bar in select are global objects with no type metadata, so the type check cannot be guaranteed to be true and it can’t be eliminated. In full LTO case this works as expected: both foo and bar are the same known type and type check is gone.
> >
> > Maybe the problem is not with renaming but with the missing type metadata in this particular case.
> > Though having so many direct calls routed through the jump table still seems problematic. Is there a feasible solution?
> >
> > --------------------------------
> > define hidden i32 @main(i32, i8** nocapture readnone) local_unnamed_addr #0 !type !3 !type !4 {
> >  %3 = add nsw i32 %0, -2
> >  store i32 %3, i32* @i, align 4, !tbaa !5
> >  %4 = icmp sgt i32 %0, 1
> >  %5 = select i1 %4, i32 ()* @foo, i32 ()* @bar
> >  %6 = bitcast i32 ()* %5 to i8*, !nosanitize !9
> >  %7 = tail call i1 @llvm.type.test(i8* nonnull %6, metadata !"_ZTSFivE"), !nosanitize !9
> >  br i1 %7, label %10, label %8, !prof !10, !nosanitize !9
> >
> > ; <label>:8:                                      ; preds = %2
> >  %9 = ptrtoint i32 ()* %5 to i64
> >  tail call void @__ubsan_handle_cfi_check_fail_abort(i8* getelementptr inbounds ({ i8, { [4 x i8]*, i32, i32 }, { i16, i16, [13 x i8] }* }, { i8, { [4 x i8]*, i32, i32 }, { i16, i16, [13 x i8] }* }* @anon.fad58de7366495db4650cfefac2fcd61.1, i64 0, i32 0), i64 %9, i64 undef) #4, !nosanitize !9
> >  unreachable, !nosanitize !9
> >
> > ; <label>:10:                                     ; preds = %2
> >  %11 = tail call i32 %5() #5
> >  ret i32 %11
> > }
> >
> > . . .
> > declare hidden i32 @foo() #3
> > declare hidden i32 @bar() #3
> >
> >
> > —————————test case---------------
> > b.c
> > =================
> > typedef int (*fptr_t) (void);
> > fptr_t get_fptr();
> > extern int i;
> >
> > int main(int argc, char *argv[])
> > {
> >  i = argc - 2;
> >  fptr_t fp = get_fptr();
> >  return fp();
> > }
> >
> > v.c
> > ================
> > int i;
> > typedef int (*fptr_t) (void);
> > int foo(void) {  return 11; }
> > int bar(void) {  return 22; }
> > fptr_t get_fptr(void) {  return (i >= 0) ? foo : bar; }
> >
> >
> >> On Apr 26, 2018, at 5:29 PM, Peter Collingbourne <[hidden email]> wrote:
> >>
> >>> We could probably tolerate a certain amount of unused jump table entries. However, I just realized that all non-inline imported calls end up going through a jump table entry. Is that correct?
> >>
> >> In fact it is all calls that go through a function pointer type that is used anywhere in the program for an indirect call, but depending on your program that could be very close to "yes".
> >>
> >>> Moving the type check lowering pass further down the pipeline (after inlining) still does not solve the problem because CFI renaming happens early and symbols attached to the jump table do not have a matching type.
> >>
> >> As far as I know, renaming happens during the LowerTypeTests pass, after the type checks are lowered.
> >> Lowering: http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/LowerTypeTests.cpp#1620
> >> Renaming: http://llvm-cs.pcc.me.uk/lib/Transforms/IPO/LowerTypeTests.cpp#1642
> >> Do you have an example of what you are seeing?
> >>
> >> Peter
> >>
> >> On Thu, Apr 26, 2018 at 4:54 PM,  <[hidden email]> wrote:
> >> Hi Peter,
> >>
> >> We could probably tolerate a certain amount of unused jump table entries. However, I just realized that all non-inline imported calls end up going through a jump table entry. Is that correct? Initially I thought you meant calls promoted from indirect. While this can be fixed by replacing direct calls to jump tables with direct calls to real targets, I found other cases where ThinLTO+CFI has issues.
> >>
> >> In ThinLTO backend, type test lowering happens very early in the pipeline, before inlining. When the type check after the call to get_fptr() is lowered (in my original example, below), the compiler cannot see that both targets belong to the same type and that the type check will always return ‘true’ and can be eliminated. Moving the type check lowering pass further down the pipeline (after inlining) still does not solve the problem because CFI renaming happens early and symbols attached to the jump table do not have a matching type.
> >>
> >> I’m trying to think if there’s a way to delay renaming until ThinLTO backend type check lowering pass. It would help with solving both problems.
> >>
> >> Thanks.
> >> Dmitry.
> >>
> >>
> >>
> >>>>>>
> >>>>>> a.c
> >>>>>> =============================
> >>>>>> typedef int (*fptr_t) (void);
> >>>>>> fptr_t get_fptr();
> >>>>>> int main(int argc, char *argv[])
> >>>>>> {
> >>>>>> fptr_t fp = get_fptr();
> >>>>>> return fp();
> >>>>>> }
> >>>>>>
> >>>>>>
> >>>>>> b.c
> >>>>>> =============================
> >>>>>> typedef int (*fptr_t) (void);
> >>>>>> int foo(void) { return 11; }
> >>>>>> int bar(void) { return 22; }
> >>>>>>
> >>>>>> static fptr_t fptr = bar;
> >>>>>> static int i = 53;
> >>>>>>
> >>>>>> fptr_t get_fptr(void)
> >>>>>> {
> >>>>>> if (i >= 0)
> >>>>>>   fptr = foo;
> >>>>>> else
> >>>>>>   fptr = bar;
> >>>>>>
> >>>>>> return fptr;
> >>>>>> }
> >>>>>>
> >>
> >>
> >>
> >>
> >>> On Apr 19, 2018, at 6:18 PM, Peter Collingbourne <[hidden email]> wrote:
> >>>
> >>> Regarding the orderfile, yes, I was thinking more about ordering the real functions.
> >>>
> >>> In that case it sounds like your best option may be to implement the optimization pass to make direct calls go directly to the real function. From a performance perspective I don't think it would make much difference if there are unused jump table entries.
> >>>
> >>> Peter
> >>>
> >>> On Thu, Apr 19, 2018 at 6:09 PM, via llvm-dev <[hidden email]> wrote:
> >>> Teresa, Peter,
> >>>
> >>> Thanks for your help!
> >>> I need to re-run my experiments as the compiler I used did not have the latest changes like r327254.
> >>> The fact that the decision about routing calls through jump table entries is made early may be problematic. In my experiments with FreeBSD kernel, ThinLTO produced thousands jump table entries compared to only dozens with full LTO. As for re-ordering jump table entries, I don’t think it’s going to work as they are placed in the same section. Including *.cfi names into a link order file will take care of re-ordering real functions routed through jump table entries, but in our case we need to force some functions to be on the same page. So not having jump table entries for the functions that don't really need them would be ideal.
> >>>
> >>> Thanks.
> >>> Dmitry.
> >>>
> >>>
> >>>> On Apr 18, 2018, at 6:11 PM, Teresa Johnson <[hidden email]> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On Wed, Apr 18, 2018 at 4:49 PM,  <[hidden email]> wrote:
> >>>> Hi Teresa,
> >>>>
> >>>> Thanks for the info!
> >>>> This example is my attempt to reduce FreeBSD kernel to something more manageable :)
> >>>>
> >>>> I will take a look at why globals are not being imported in this case. What’s the best tool to look into ThinLTO objects and their summaries? Most dumping tools don’t seem to like ThinLTO bitcode files…
> >>>>
> >>>> Sadly there isn't a really great way to dump the summaries. =( There was a patch awhile back by a GSOC student to dump in YAML format, but there was resistance from some who preferred dumping to llvm assembly via llvm-dis and support reading in the summary from llvm assembly. It's been on my list of things to do, hasn't yet risen high enough in priority to work on that. For now, you have to use llvm-bcanalyzer -dump and look at the raw format.
> >>>>
> >>>> Teresa
> >>>>
> >>>>
> >>>> Hopefully Peter can chime in regarding CFI related issues.
> >>>>
> >>>> Thanks.
> >>>> Dmitry.
> >>>>
> >>>>
> >>>>> On Apr 17, 2018, at 9:37 AM, Teresa Johnson <[hidden email]> wrote:
> >>>>>
> >>>>> Hi Dmitry,
> >>>>>
> >>>>> Sorry for the late reply. For CFI specific code generation, pcc is a better person to answer. But on the issue of global variables being optimized, that hasn't happened yet. That would be great if you wanted to pick that up!
> >>>>>
> >>>>> In your original email example, it seems like the file static i=53 could be constant propagated since there are no other defs, and the code in get_fptr simplified during the compile step, but I assume this is part of a more complex example where it is not possible to do this? Also note that with r327254 we started importing global variables. Do you know why we don't import in your case? I wonder if it has to do with it being CFI inserted code?
> >>>>>
> >>>>> Teresa
> >>>>>
> >>>>> On Tue, Apr 17, 2018 at 9:17 AM <[hidden email]> wrote:
> >>>>> I watched  Teresa’s talk on ThinLTO from last year’s CppCon, and it sounded like adding global variable information to the summaries was in the works, or at least in planning. Can someone (Teresa?) please share the current status? If it’s part of future plans, are there any specific proposals that can be picked up and worked on?
> >>>>>
> >>>>> Thanks!
> >>>>>
> >>>>>
> >>>>>> On Apr 9, 2018, at 6:51 PM, via llvm-dev <[hidden email]> wrote:
> >>>>>>
> >>>>>> Hi,
> >>>>>>
> >>>>>> I’m working on setting up ThinLTO+CFI for a C application which uses a lot of function pointers. While functionally it appears stable, it’s performance is significantly degraded, to the tune of double digit percentage points compared to regular LTO+CFI.
> >>>>>>
> >>>>>> Looking into possible causes I see that under ThinLTO+CFI iCall type checks almost always generate jump table entries for indirect calls, which creates another level of indirection for every such call. On top of that it breaks the link order layout because real function names point to jump table entries. It appears that I’m hitting a limitation in ThinLTO on how much information it can propagate across modules, particularly information about constants. In the example below, the fact that “i” is effectively a constant, is lost under ThinLTO, and the inlined copy of b.c:get_fptr() in a.c does not eliminate the conditional, which, for CFI purposes requires to generate a type check/jump table.
> >>>>>>
> >>>>>> I was wondering if there was a way to mitigate this limitation.
> >>>>>>
> >>>>>> a.c
> >>>>>> =============================
> >>>>>> typedef int (*fptr_t) (void);
> >>>>>> fptr_t get_fptr();
> >>>>>> int main(int argc, char *argv[])
> >>>>>> {
> >>>>>> fptr_t fp = get_fptr();
> >>>>>> return fp();
> >>>>>> }
> >>>>>>
> >>>>>>
> >>>>>> b.c
> >>>>>> =============================
> >>>>>> typedef int (*fptr_t) (void);
> >>>>>> int foo(void) { return 11; }
> >>>>>> int bar(void) { return 22; }
> >>>>>>
> >>>>>> static fptr_t fptr = bar;
> >>>>>> static int i = 53;
> >>>>>>
> >>>>>> fptr_t get_fptr(void)
> >>>>>> {
> >>>>>> if (i >= 0)
> >>>>>>   fptr = foo;
> >>>>>> else
> >>>>>>   fptr = bar;
> >>>>>>
> >>>>>> return fptr;
> >>>>>> }
> >>>>>>
> >>>>>> _______________________________________________
> >>>>>> LLVM Developers mailing list
> >>>>>> [hidden email]
> >>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>>>
> >>>>>
> >>>>>
> >>>>> --
> >>>>> Teresa Johnson |   Software Engineer |      [hidden email] |   408-460-2413
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> --
> >>>> Teresa Johnson |    Software Engineer |      [hidden email] |   408-460-2413
> >>>
> >>>
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> [hidden email]
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>>
> >>>
> >>>
> >>>
> >>> --
> >>> --
> >>> Peter
> >>
> >>
> >>
> >>
> >> --
> >> --
> >> Peter
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > [hidden email]
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


------------------------------

Message: 5
Date: Tue, 01 May 2018 10:46:33 -0700
From: Chris Bieneman via llvm-dev <[hidden email]>
To: Siddharth Shankar Swain <[hidden email]>
Cc: llvm-dev <[hidden email]>
Subject: Re: [llvm-dev] Disabling Exception in LLVM
Message-ID: <[hidden email]>
Content-Type: text/plain; charset="utf-8"

LLVM does not allow the use of exceptions in our code. We do not allow throwing or catching them. That does not mean you cannot compile the code with exceptions enabled, it just means we don't use them.

Clang is a full C++ compiler. Even though LLVM & Clang do not use exceptions in their implementation, Clang does support compiling C++ code that uses exceptions.

Does this answer your question?
-Chris

> On May 1, 2018, at 10:38 AM, Siddharth Shankar Swain <[hidden email]> wrote:
>
> Hi Chris,
> Thanks for answering, Can u clarify on this comment mentioned in https://github.com/Z3Prover/z3/issues/861 <https://github.com/Z3Prover/z3/issues/861> .
>
>   cplusplus no exception support · Issue #861 · Z3Prover/z3 · GitHub
> LLVM's source code does not use exceptions for performance reasons and so is compiled by default with -fno-exceptions. When using LLVM's libraries via it's C++ interface it is important to match how LLVM was built (i.e. do not mix code built with and without exception support).
> The Clang compiler which is built on top of LLVM is also compiled without exceptions by default. However the built Clang can compile C++ code with or without exceptions (using the -fno-exceptions). flag. Said another way Clang's implementation doesn't use exceptions but Clang itself can compile C++ code with or without exception support.
>
> Can anyone clarify on this comment ?
>
> Thanks,
> Siddharth
>
> On Tue, May 1, 2018 at 11:02 PM, Chris Bieneman <[hidden email] <mailto:[hidden email]>> wrote:
> Siddharth,
>
> I'm not sure what coding standards you refer to when you say "some C++ coding standard". This question is answered in the LLVM Coding Standards document here:
>
> https://www.llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions <https://www.llvm.org/docs/CodingStandards.html#do-not-use-rtti-or-exceptions>
>
> As such LLVM's coding standards prohibit the use of exceptions and RTTI.
>
> -Chris
>
>> On May 1, 2018, at 10:02 AM, Siddharth Shankar Swain via llvm-dev <[hidden email] <mailto:[hidden email]>> wrote:
>>
>> Hi all,
>> Can anyone explain why exceptions are disabled in LLVM, even if some C++ coding standard tells to use exceptions ?
>> Thanks,
>> Siddharth
>> _______________________________________________
>> LLVM Developers mailing list
>> [hidden email] <mailto:[hidden email]>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>
>

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Message: 6
Date: Tue, 01 May 2018 17:48:57 +0000
From: Teresa Johnson via llvm-dev <[hidden email]>
To: David Blaikie <[hidden email]>
Cc: llvm-dev <[hidden email]>, Davide Italiano
        <[hidden email]>, David Li <[hidden email]>
Subject: Re: [llvm-dev] RFC: LLVM Assembly format for ThinLTO Summary
Message-ID:
        <CAAe5K+Vm3cpY+VqNVU_rC6Jtms=4HzMbgJvxraNF=[hidden email]>
Content-Type: text/plain; charset="utf-8"

Hi David,
Thanks for the comments, replies below.
Teresa

On Mon, Apr 30, 2018 at 11:52 AM David Blaikie <[hidden email]> wrote:

> Hi Teresa,
>
> Awesome to see - looking forward to it!
>
> On Tue, Apr 24, 2018 at 7:44 AM Teresa Johnson via llvm-dev <
> [hidden email]> wrote:
>
>> Hi everyone,
>>
>> I started working on a long-standing request to have the summary dumped
>> in a readable format to text, and specifically to emit to LLVM assembly.
>> Proposal below, please let me know your thoughts.
>>
>> Thanks,
>> Teresa
>>
>>
>>
>>
>>
>>
>> *RFC: LLVM Assembly format for ThinLTO
>> Summary========================================Background-----------------ThinLTO
>> operates on small summaries computed during the compile step (i.e. with “-c
>> -flto=thin”), which are then analyzed and updated during the Thin Link
>> stage, and utilized to perform IR updates during the post-link ThinLTO
>> backends. The summaries are emitted as LLVM Bitcode, however, not currently
>> in the LLVM assembly.There are two ways to generate a bitcode file
>> containing summary records for a module: 1. Compile with “clang -c
>> -flto=thin”*
>>
>
> As an aside - I seem to recall that at least internally at Google some
> kind of summary-only bitcode files are used (so that the whole bitcode file
> (especially in builds with debug info) doesn't have to be shipped to the
> node doing the summary merging). How are those summary-only files
> produced? Is that upstream? Or done in a more low-level way (like an
> objcopy, llvm-* tool invocation done as a post-processing step, etc)?
>

This is done upstream, under a special clang option that can be given in
addition to -flto=thin, so that the compile step emits both the full
IR+summary (for the distributed backends) as well as a minimized bitcode
file with summary (for the thin link). Note that the distributed backends
don't actually need the summary with the IR (as it gets all the info it
needs from the combined summary index written out by the thin link), so we
could theoretically improve this to suppress the summary write for that
first file under that option.


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>> * 1. Build from LLVM assembly using “opt -module-summary”Either of these
>> will result in the ModuleSummaryIndex analysis pass (which builds the
>> summary index in memory for a module) to be added to the pipeline just
>> before bitcode emission.Additionally, a combined index is created by
>> merging all the per-module indexes during the Thin Link, which is
>> optionally emitted as a bitcode file.Currently, the only way to view these
>> records is via “llvm-bcanalyzer -dump”, then manually decoding the raw
>> bitcode dumps.Relatedly, there is YAML reader/writer support for CFI
>> related summary fields (-wholeprogramdevirt-read-summary and
>> -wholeprogramdevirt-write-summary). Last summer, GSOC student Charles
>> Saternos implemented support to dump the summary in YAML from llvm-lto2
>> (D34080), including the rest of the summary fields (D34063), however, there
>> was pushback on the related RFC for dumping via YAML or another format
>> rather than emitting as LLVM assembly.Goals: 1. Define LLVM assembly format
>> for summary index2. Define interaction between parsing of summary from LLVM
>> assembly and synthesis of new summary index from IR.3. Implement printing
>> and parsing of summary index LLVM assemblyProposed LLVM Assembly
>> Format----------------------------------------------There are several top
>> level data structures within the ModuleSummaryIndex: 1.
>> ModulePathStringTable: Holds the paths to the modules summarized in the
>> index (only one entry for per-module indexes and multiple in the combined
>> index), along with their hashes (for incremental builds and global
>> promotion).2. GlobalValueMap: A map from global value GUIDs to the
>> corresponding function/variable/alias summary (or summaries for the
>> combined index and weak linkage).3. CFI-related data structures (TypeIdMap,
>> CfiFunctionDefs, and CfiFunctionDecls)I have a WIP patch to AsmWriter.cpp
>> to print the ModuleSummaryIndex that I was using to play with the format.
>> It currently prints 1 and 2 above. I’ve left the CFI related summary data
>> structures as a TODO for now, until the format is at least conceptually
>> agreed, but from looking at those I don’t see an issue with using the same
>> format (with a note/question for Peter on CFI type test representation
>> below).I modeled the proposed format on metadata, with a few key
>> differences noted below. Like metadata, I propose enumerating the entries
>> with the SlotTracker, and prefixing them with a special character. Avoiding
>> characters already used in some fashion (i.e. “!” for metadata and “#” for
>> attributes), I initially have chosen “^”. Open to suggestions
>> though.Consider the following example:extern void foo();int X;int bar() {
>>  foo();  return X;}void barAlias() __attribute__ ((alias ("bar")));int
>> main() {  barAlias();  return bar();}The proposed format has one entry per
>> ModulePathStringTable entry and one per GlobalValueMap/GUID, and looks
>> like:^0 = module: {path: testA.o, hash: 5487197307045666224}^1 = gv: {guid:
>> 1881667236089500162, name: X, summaries: {variable: {module: ^0, flags:
>> {linkage: common, notEligibleToImport: 0, live: 0, dsoLocal: 1}}}}^2 = gv:
>> {guid: 6699318081062747564, name: foo}^3 = gv: {guid: 15822663052811949562,
>> name: main, summaries: {function: {module: ^0, flags: {linkage: extern,
>> notEligibleToImport: 1, live: 0, dsoLocal: 1}, insts: 5, funcFlags:
>> {readNone: 0, readOnly: 0, noRecurse: 0, returnDoesNotAlias: 0}, calls:
>> {{callee: ^5, hotness: unknown}, {callee: ^4, hotness: unknown}}}}}^4 = gv:
>> {guid: 16434608426314478903, name: bar, summaries: {function: {module: ^0,
>> flags: {linkage: extern, notEligibleToImport: 1, live: 0, dsoLocal: 1},
>> insts: 3, funcFlags: {readNone: 0, readOnly: 0, noRecurse: 0,
>> returnDoesNotAlias: 0}, calls: {{callee: ^2, hotness: unknown}}, refs:
>> {^1}}}}^5 = gv: {guid: 18040127437030252312, name: barAlias, summaries:
>> {alias: {module: ^0, flags: {linkage: extern, notEligibleToImport: 0, live:
>> 0, dsoLocal: 1}, aliasee: ^4}}}*
>>
>
> Syntax seems pretty good to me!
>

Great!


>
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>> *Like metadata, the fields are tagged (currently using lower camel case,
>> maybe upper camel case would be preferable).The proposed format has a
>> structure that reflects the data structures in the summary index. For
>> example, consider the entry “^4”. This corresponds to the function “bar”.
>> The entry for that GUID in the GlobalValueMap contains a list of summaries.
>> For per-module summaries such as this, there will be at most one summary
>> (with no summary list for an external function like “foo”). In the combined
>> summary there may be multiple, e.g. in the case of linkonce_odr functions
>> which have definitions in multiple modules. The summary list for bar (“^4”)
>> contains a FunctionSummary, so the summary is tagged “function:”. The
>> FunctionSummary contains both a flags structure (inherited from the base
>> GlobalValueSummary class), and a funcFlags structure (specific to
>> FunctionSummary). It therefore contains a brace-enclosed list of flag
>> tags/values for each.Where a global value summary references another global
>> value summary (e.g. via a call list, reference list, or aliasee), the entry
>> is referenced by its slot. E.g. the alias “barAlias” (“^5”) references its
>> aliasee “bar” as “^4”.Note that in comparison metadata assembly entries
>> tend to be much more decomposed since many metadata fields are themselves
>> metadata (so then entries tend to be shorter with references to other
>> metadata nodes).Currently, I am emitting the summary entries at the end,
>> after the metadata nodes. Note that the ModuleSummaryIndex is not currently
>> referenced from the Module, and isn’t currently created when parsing the
>> Module IR bitcode (there is a separate derived class for reading the
>> ModuleSummaryIndex from bitcode). This is because they are not currently
>> used at the same time. However, in the future there is no reason why we
>> couldn’t tag the global values in the Module’s LLVM assembly with the
>> corresponding summary entry if the ModuleSummaryIndex is available when
>> printing the Module in the assembly writer. I.e. we could do the following
>> for “main” from the above example when printing the IR definition (note the
>> “^3” at the end):define  dso_local i32 @main() #0 !dbg !17 ^3 {For CFI data
>> structures, the format would be similar. It appears that TypeIds are
>> referred to by string name in the top level TypeIdMap (std::map indexed by
>> std::string type identifier), whereas they are referenced by GUID within
>> the FunctionSummary class (i.e. the TypeTests vector and the VFuncId
>> structure). For the LLVM assembly I think there should be a top level entry
>> for each TypeIdMap, which lists both the type identifier string and its
>> GUID (followed by its associated information stored in the map), and the
>> TypeTests/VFuncId references on the FunctionSummary entries can reference
>> it by summary slot number. I.e. something like:^1 = typeid: {guid: 12345,
>> identifier: name_of_type, …^2 = gv: {... {function: {.... typeTests: {^1,
>> …Peter - is that correct and does that sound ok?Issues when Parsing of
>> Summaries from
>> Assembly--------------------------------------------------------------------When
>> reading an LLVM assembly file containing module summary entries, a
>> ModuleSummaryIndex will be created from the entries.Things to consider are
>> the behavior when: - Invoked with “opt -module-summary” (which currently
>> builds a new summary index from the IR). Options:*
>>
>
>>
>> * 1. recompute summary and throw away summary in the assembly file*
>>
>
> What happens currently if you run `opt -module-summary` on a bitcode file
> that already contains a summary? I feel like the behavior should be the
> same when run on a textual IR file containing a summary, probably?
>

We rebuild the summary. Note that this in part is due to the fact mentioned
above that we have separate readers for the Module IR and the summary. The
opt tool does not even read the summary if present. We currently only read
the summary during the thin link (when building the combined index for
analysis), and in the distributed backends where we read the combined
summary index file emitted for that file by the distributed thin link.


>
>>
>>
>>
>>
>>
>> * 1. ignore -module-summary and build the summary from the LLVM
>> assembly2. give an error3. compare the two summaries (one created from the
>> assembly and the new one created by the analysis phase from the IR), and
>> error if they are different.My opinion is to do a),  so that the behavior
>> using -module-summary doesn’t change. We also need a way to force building
>> of a fresh module summary for cases where the user has modified the LLVM
>> assembly of the IR (see below). - How to handle older LLVM assembly files
>> that don’t contain new summary fields. Options:*
>>
>
> Same thoughts would apply here for "what do we do in the bitcode case" -
> with the option to not support old/difficult textual IR. If there are
> easy/obvious defaults, I'd say it's probably worth baking those in (&
> baking them in even for the existing fields we know about, to make it
> easier to write more terse test cases that don't have to
> verbosily/redundantly specify lots of default values?) to the
> parsing/loading logic?
>

So we do emit an index version in the bitcode, and auto-upgrade in a
conservative manner anything that wasn't emitted prior. We could presumably
serialize out the version number and handle auto-upgrading from textual
assembly the same way (as the version is bumped beyond the current version
at least). If we want to allow omission of some fields for test simplicity,
we could do a similar thing and apply conservative values where possible
for omitted fields (e.g. the flags). That seems fine to me, in which case I
don't think we need a version number. Although this has implications for
the validator, see below.


>
>>
>>
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>> * 1. Force the LLVM assembly file to be recreated with a new summary.
>> I.e. “opt -module-summary -o - | llvm-dis”.2. Auto-upgrade, by silently
>> creating conservative values for the new summary entries.I lean towards b)
>> (when possible) for user-friendliness and to reduce required churn on test
>> inputs. - How to handle partial or incorrect LLVM assembly summary entries.
>> How to handle partial summaries depends in part on how we answer the prior
>> question about auto-upgrading. I think the best option like there is to
>> handle it automatically when possible. However, I do think we should error
>> on glaring errors like obviously missing information. For example, when
>> there is summary data in the LLVM assembly, but summary entries are missing
>> for some global values. E.g. if the user modified the assembly to add a
>> function but forgot to add a corresponding summary entry. We could still
>> have subtle issues (e.g. user adds a new call but forgets to update the
>> caller’s summary call list), but it will be harder to detect those.*
>>
>
> I'd be OK with the summary being validated by the IR validator (same way
> other properties of IR are validated & even simple things like if you use
> the wrong IR type to refer to an IR value, you get a parse error, etc) -
> which, I realize, would make it feel like the textual summary was entirely
> redundant
>

It is redundant when the IR is also available, which relates to Peter and
others' objections to serializing this back in. An issue with validation
would be if we allowed omission of some fields and/or auto-upgrading as
discussed above. The applied conservative values might very well not match
the recomputed values. But as I mentioned here we may just want to validate
for glaring errors like required info - i.e. I think we should require that
every GV has an associated summary entry.

(except in cases of standalone summaries - which I imagine will be the
> common case in tests, because the summary processing should be tested in
> isolation (except for testing things like this validation logic itself,
> etc)).
>

Yes, I suspect the biggest usage in tests would be a standalone combined
summary file that we can use to test the application of the thin link
optimizations on a single IR file in the LTO backend pipeline. I.e the
input to the test would be one module IR assembly file (no summary) and one
combined index assembly file, it would run just the ThinLTO backend
pipeline, and check the resulting IR via llvm-dis to ensure the
optimization is applied effectively.



> - Dave
>
>
>>
>>
>> --
>> Teresa Johnson |  Software Engineer |  [hidden email] |
>> 408-460-2413 <(408)%20460-2413>
>> _______________________________________________
>> LLVM Developers mailing list
>> [hidden email]
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>

--
Teresa Johnson |  Software Engineer |  [hidden email] |  408-460-2413
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