[llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

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[llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev
I've started looking at the state of code coverage recently; we figured
LLVM itself would be a good test to figure out how mature it is, so I
gave it a shot.  My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I
didn't try).  If you link with binutils ld, the program will generate
broken profile information.  Apparently, the linked binary is missing
the __llvm_prf_names section.  This took me half a day to figure out.  
This issue isn't documented anywhere, and the only error message I got
was "Assertion `!Key.empty()' failed." from llvm-cov.

2. The generated binaries are big and slow.  Comparing to a build
without coverage, llc becomes 8x larger overall (text section becomes
roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to
250 seconds.

3. The generated profile information takes up a lot of space: llc
generates a 90MB profraw file.

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the
profiles generated by "make check", it takes 50GB of memory to process
about 1.5GB of profiles.  Is it supposed to use that much?

5. Using prepare-code-coverage-artifact.py generates "warning: 229
functions have mismatched data".  I'm not sure what's causing this... I
guess it has something to do with merging the profile data for multiple
binaries?  The error message is not very helpful.

5. The HTML output highlights the semicolon after a break or return
statement in some switch statements in red.  (For example,
LowerADDC_ADDE_SUBC_SUBE in ARMISelLowering.cpp.)  Not really important,
but annoying.

6. On the bright side, when it works, the generated coverage information
is precise and easy to read.

-Eli

--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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Re: [llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev
Hi Eli,

Thanks for sharing your experience. I'd very much like to fix the problems you encountered.

On Jun 16, 2017, at 6:08 PM, Friedman, Eli <[hidden email]> wrote:

I've started looking at the state of code coverage recently; we figured LLVM itself would be a good test to figure out how mature it is, so I gave it a shot.

You may already be aware of this, but for readers who are not, there is a public bot which produces coverage reports for llvm roughly twice a day. You can find it by visiting llvm.org and clicking on the "llvm-cov" link within the "Useful Links" box (in the "Dev. Resources" section). Coverage is gathered by running check-{llvm,clang,polly,lld} and the 'nightly' test suite.

My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.

I expect llvm-cov to print out "Failed to load coverage: <reason>" in this situation. There was some work done to tighten up error reporting in ProfileData and its clients in r270020. If your host toolchain does have these changes, please file a bug, and I'll have it fixed.

I was not aware of the issue with the binutils linker. We do have some end-to-end, runtime tests in compiler-rt which use this linker, so this type of failure is surprising. I've CC'd David Li, who has some experience working with this linker, in case he has any insight about the issue.

If you are using a relatively up-to-date host toolchain, I'll add a note to our docs suggesting that users use gold when compiling with coverage enabled.

2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.

The binary size increase comes from coverage mapping data, counter increment instrumentation, and profiling metadata.

The coverage mapping section is highly compressible, but exploiting the compressibility has proven to be tricky. I filed: llvm.org/PR33499.

Coverage makes use of frontend-based instrumentation, which is much less efficient than the IR-based kind. If we can find a way to map counters inserted by IR PGO to AST nodes, we could improve the situation. I filed: llvm.org/PR33500.

We can reduce testing time by *not* instrumented basic tools like count, not, FileCheck etc. I filed: llvm.org/PR33501.

3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.

I don't have any ideas about how to fix this. You can decrease the space overhead for raw profiles by altering LLVM_PROFILE_MERGE_POOL_SIZE from 4 to a lower value.

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?

By default, llvm-profdata uses hardware_concurrency() to determine the number of threads to use to merge profiles. You can change the default by passing -j/--num-threads to llvm-profdata. I'm open to changing the 'prep' script to use -j4 or something like that.

5. Using prepare-code-coverage-artifact.py generates "warning: 229 functions have mismatched data".  I'm not sure what's causing this... I guess it has something to do with merging the profile data for multiple binaries?  The error message is not very helpful.

This is unexpected. I'll try to reproduce this, and I'll fix the diagnostic along the way. I filed: llvm.org/PR33502.

5. The HTML output highlights the semicolon after a break or return statement in some switch statements in red.  (For example, LowerADDC_ADDE_SUBC_SUBE in ARMISelLowering.cpp.)  Not really important, but annoying.

I'm sure I'm sitting on a bug report about this already, but unfortunately haven't had the time to get around to it.

6. On the bright side, when it works, the generated coverage information is precise and easy to read.

Good to hear.

vedant


-Eli

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Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project



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Re: [llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev
In reply to this post by Tim Northover via llvm-dev


On Fri, Jun 16, 2017 at 6:08 PM, Friedman, Eli via llvm-dev <[hidden email]> wrote:
I've started looking at the state of code coverage recently; we figured LLVM itself would be a good test to figure out how mature it is, so I gave it a shot.  My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.



I believe the gnu-ld bug is https://sourceware.org/bugzilla/show_bug.cgi?id=19161 which is fixed in version 2.26.

 
2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.

Over last couple of years, the instrumentation and coverage data overhead has reduced greatly.  FE based instrumentation in general has larger overhead than IR based instrumentation, but the coverage testing currently only works with FE instrumentation. 
 

3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.

This looks like in the normal range of raw profile size.


David
 

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?

5. Using prepare-code-coverage-artifact.py generates "warning: 229 functions have mismatched data".  I'm not sure what's causing this... I guess it has something to do with merging the profile data for multiple binaries?  The error message is not very helpful.

5. The HTML output highlights the semicolon after a break or return statement in some switch statements in red.  (For example, LowerADDC_ADDE_SUBC_SUBE in ARMISelLowering.cpp.)  Not really important, but annoying.

6. On the bright side, when it works, the generated coverage information is precise and easy to read.

-Eli

--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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Re: [llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev
In reply to this post by Tim Northover via llvm-dev
On 6/18/2017 3:51 PM, Vedant Kumar wrote:
My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.

I expect llvm-cov to print out "Failed to load coverage: <reason>" in this situation. There was some work done to tighten up error reporting in ProfileData and its clients in r270020. If your host toolchain does have these changes, please file a bug, and I'll have it fixed.

Host toolchain is trunk clang... but using system binutils (which is 2.24 on my Ubuntu 14.04 system... and apparently that's too old per David Li's response).  Anyway, filed https://bugs.llvm.org/show_bug.cgi?id=33517 .


2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.

The binary size increase comes from coverage mapping data, counter increment instrumentation, and profiling metadata.

The coverage mapping section is highly compressible, but exploiting the compressibility has proven to be tricky. I filed: llvm.org/PR33499.

If I'm cross-compiling for a target where the space matters, can I rid of the data for the copy on the device using "strip -R __llvm_covmap" or something like that, then use llvm-cov on the original?

Coverage makes use of frontend-based instrumentation, which is much less efficient than the IR-based kind. If we can find a way to map counters inserted by IR PGO to AST nodes, we could improve the situation. I filed: llvm.org/PR33500.

This would be nice... but I assume it's hard. :)


We can reduce testing time by *not* instrumented basic tools like count, not, FileCheck etc. I filed: llvm.org/PR33501.

3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.

I don't have any ideas about how to fix this. You can decrease the space overhead for raw profiles by altering LLVM_PROFILE_MERGE_POOL_SIZE from 4 to a lower value.

Disk space is cheap, but the I/O takes a long time.  I guess it's specifically bad for LLVM's "make check", maybe not so bad for other cases.

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?

By default, llvm-profdata uses hardware_concurrency() to determine the number of threads to use to merge profiles. You can change the default by passing -j/--num-threads to llvm-profdata. I'm open to changing the 'prep' script to use -j4 or something like that.


Oh, so it's using a couple gigabytes per thread multiplied by 24 cores?  Okay, now I'm not so worried. :)

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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Re: [llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev

On Jun 19, 2017, at 4:32 PM, Friedman, Eli <[hidden email]> wrote:

On 6/18/2017 3:51 PM, Vedant Kumar wrote:
My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.

I expect llvm-cov to print out "Failed to load coverage: <reason>" in this situation. There was some work done to tighten up error reporting in ProfileData and its clients in r270020. If your host toolchain does have these changes, please file a bug, and I'll have it fixed.

Host toolchain is trunk clang... but using system binutils (which is 2.24 on my Ubuntu 14.04 system... and apparently that's too old per David Li's response).  Anyway, filed https://bugs.llvm.org/show_bug.cgi?id=33517 .

I've updated the clang docs re: 'Source based code coverage' to reflect this issue. I've also tightened up our error reporting a bit so we fail earlier with something better than an assertion message (r305765, r305767).

2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.

The binary size increase comes from coverage mapping data, counter increment instrumentation, and profiling metadata.

The coverage mapping section is highly compressible, but exploiting the compressibility has proven to be tricky. I filed: llvm.org/PR33499.

If I'm cross-compiling for a target where the space matters, can I rid of the data for the copy on the device using "strip -R __llvm_covmap" or something like that, then use llvm-cov on the original?

I haven't tried this but I expect it to work. Instrumented programs don't reference the __llvm_covmap section.

Coverage makes use of frontend-based instrumentation, which is much less efficient than the IR-based kind. If we can find a way to map counters inserted by IR PGO to AST nodes, we could improve the situation. I filed: llvm.org/PR33500.

This would be nice... but I assume it's hard. :)

It seems like it is. At a high level, you'd need some way to associate the counters placed by IR PGO instrumentation to the counters that clang expects to see while walking an AST. I don't have a concrete design for this in mind.

We can reduce testing time by *not* instrumented basic tools like count, not, FileCheck etc. I filed: llvm.org/PR33501.

3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.

I don't have any ideas about how to fix this. You can decrease the space overhead for raw profiles by altering LLVM_PROFILE_MERGE_POOL_SIZE from 4 to a lower value.

Disk space is cheap, but the I/O takes a long time.  I guess it's specifically bad for LLVM's "make check", maybe not so bad for other cases.

You can speed up "make check" a bit by using non-instrumented versions of count, not, FileCheck, etc.

vedant

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?

By default, llvm-profdata uses hardware_concurrency() to determine the number of threads to use to merge profiles. You can change the default by passing -j/--num-threads to llvm-profdata. I'm open to changing the 'prep' script to use -j4 or something like that.


Oh, so it's using a couple gigabytes per thread multiplied by 24 cores?  Okay, now I'm not so worried. :)

-Eli

-- 
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Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project


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Re: [llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev

On Jun 19, 2017, at 7:29 PM, Vedant Kumar <[hidden email]> wrote:


On Jun 19, 2017, at 4:32 PM, Friedman, Eli <[hidden email]> wrote:

On 6/18/2017 3:51 PM, Vedant Kumar wrote:
My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.

I expect llvm-cov to print out "Failed to load coverage: <reason>" in this situation. There was some work done to tighten up error reporting in ProfileData and its clients in r270020. If your host toolchain does have these changes, please file a bug, and I'll have it fixed.

Host toolchain is trunk clang... but using system binutils (which is 2.24 on my Ubuntu 14.04 system... and apparently that's too old per David Li's response).  Anyway, filed https://bugs.llvm.org/show_bug.cgi?id=33517 .

I've updated the clang docs re: 'Source based code coverage' to reflect this issue. I've also tightened up our error reporting a bit so we fail earlier with something better than an assertion message (r305765, r305767).

2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.

The binary size increase comes from coverage mapping data, counter increment instrumentation, and profiling metadata.

The coverage mapping section is highly compressible, but exploiting the compressibility has proven to be tricky. I filed: llvm.org/PR33499.

If I'm cross-compiling for a target where the space matters, can I rid of the data for the copy on the device using "strip -R __llvm_covmap" or something like that, then use llvm-cov on the original?

I haven't tried this but I expect it to work. Instrumented programs don't reference the __llvm_covmap section.

Coverage makes use of frontend-based instrumentation, which is much less efficient than the IR-based kind. If we can find a way to map counters inserted by IR PGO to AST nodes, we could improve the situation. I filed: llvm.org/PR33500.

This would be nice... but I assume it's hard. :)

It seems like it is. At a high level, you'd need some way to associate the counters placed by IR PGO instrumentation to the counters that clang expects to see while walking an AST. I don't have a concrete design for this in mind.

We can reduce testing time by *not* instrumented basic tools like count, not, FileCheck etc. I filed: llvm.org/PR33501.

3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.

I don't have any ideas about how to fix this. You can decrease the space overhead for raw profiles by altering LLVM_PROFILE_MERGE_POOL_SIZE from 4 to a lower value.

Disk space is cheap, but the I/O takes a long time.  I guess it's specifically bad for LLVM's "make check", maybe not so bad for other cases.

You can speed up "make check" a bit by using non-instrumented versions of count, not, FileCheck, etc.

Ah, sorry for mentioning this twice.

On another note, I'm looking into the "N mismatched functions" warnings issue, and suspect that it happens when there are conflicting definitions of the same function in different binaries. The issue doesn't seem to occur when using profiles from just one binary to generate a report for that binary. I'll dig into this a bit more and update PR33502.

vedant


vedant

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?

By default, llvm-profdata uses hardware_concurrency() to determine the number of threads to use to merge profiles. You can change the default by passing -j/--num-threads to llvm-profdata. I'm open to changing the 'prep' script to use -j4 or something like that.


Oh, so it's using a couple gigabytes per thread multiplied by 24 cores?  Okay, now I'm not so worried. :)

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project



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Re: [llvm-dev] My experience using -DLLVM_BUILD_INSTRUMENTED_COVERAGE to generate coverage

Tim Northover via llvm-dev


On Mon, Jun 19, 2017 at 7:36 PM, Vedant Kumar <[hidden email]> wrote:

On Jun 19, 2017, at 7:29 PM, Vedant Kumar <[hidden email]> wrote:


On Jun 19, 2017, at 4:32 PM, Friedman, Eli <[hidden email]> wrote:

On 6/18/2017 3:51 PM, Vedant Kumar wrote:
My experience:

1. You have to specify -DLLVM_USE_LINKER=gold (or maybe lld works; I didn't try).  If you link with binutils ld, the program will generate broken profile information.  Apparently, the linked binary is missing the __llvm_prf_names section.  This took me half a day to figure out.  This issue isn't documented anywhere, and the only error message I got was "Assertion `!Key.empty()' failed." from llvm-cov.

I expect llvm-cov to print out "Failed to load coverage: <reason>" in this situation. There was some work done to tighten up error reporting in ProfileData and its clients in r270020. If your host toolchain does have these changes, please file a bug, and I'll have it fixed.

Host toolchain is trunk clang... but using system binutils (which is 2.24 on my Ubuntu 14.04 system... and apparently that's too old per David Li's response).  Anyway, filed https://bugs.llvm.org/show_bug.cgi?id=33517 .

I've updated the clang docs re: 'Source based code coverage' to reflect this issue. I've also tightened up our error reporting a bit so we fail earlier with something better than an assertion message (r305765, r305767).

2. The generated binaries are big and slow.  Comparing to a build without coverage, llc becomes 8x larger overall (text section becomes roughly 2x larger).  And check-llvm-codegen-arm goes from 3 seconds to 250 seconds.

The binary size increase comes from coverage mapping data, counter increment instrumentation, and profiling metadata.

The coverage mapping section is highly compressible, but exploiting the compressibility has proven to be tricky. I filed: llvm.org/PR33499.

If I'm cross-compiling for a target where the space matters, can I rid of the data for the copy on the device using "strip -R __llvm_covmap" or something like that, then use llvm-cov on the original?

I haven't tried this but I expect it to work. Instrumented programs don't reference the __llvm_covmap section.



Right. The user can also use objcopy -only-section=__llvm_covmap <in> <out> to copy the covmap section into a smaller file, and feed that later to the coverage tool.

David
 
Coverage makes use of frontend-based instrumentation, which is much less efficient than the IR-based kind. If we can find a way to map counters inserted by IR PGO to AST nodes, we could improve the situation. I filed: llvm.org/PR33500.

This would be nice... but I assume it's hard. :)

It seems like it is. At a high level, you'd need some way to associate the counters placed by IR PGO instrumentation to the counters that clang expects to see while walking an AST. I don't have a concrete design for this in mind.

We can reduce testing time by *not* instrumented basic tools like count, not, FileCheck etc. I filed: llvm.org/PR33501.

3. The generated profile information takes up a lot of space: llc generates a 90MB profraw file.

I don't have any ideas about how to fix this. You can decrease the space overhead for raw profiles by altering LLVM_PROFILE_MERGE_POOL_SIZE from 4 to a lower value.

Disk space is cheap, but the I/O takes a long time.  I guess it's specifically bad for LLVM's "make check", maybe not so bad for other cases.

You can speed up "make check" a bit by using non-instrumented versions of count, not, FileCheck, etc.

Ah, sorry for mentioning this twice.

On another note, I'm looking into the "N mismatched functions" warnings issue, and suspect that it happens when there are conflicting definitions of the same function in different binaries. The issue doesn't seem to occur when using profiles from just one binary to generate a report for that binary. I'll dig into this a bit more and update PR33502.

vedant


vedant

4. When prepare-code-coverage-artifact.py invokes llvm-profdata for the profiles generated by "make check", it takes 50GB of memory to process about 1.5GB of profiles.  Is it supposed to use that much?

By default, llvm-profdata uses hardware_concurrency() to determine the number of threads to use to merge profiles. You can change the default by passing -j/--num-threads to llvm-profdata. I'm open to changing the 'prep' script to use -j4 or something like that.


Oh, so it's using a couple gigabytes per thread multiplied by 24 cores?  Okay, now I'm not so worried. :)

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project




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