[PATCH] D20993: [lit] Add support for PGO profile and code coverage collection

[Matthias Braun via llvm-commits]

MatzeB added a comment.

In http://reviews.llvm.org/D20993#449116, @vsk wrote:

> In http://reviews.llvm.org/D20993#449092, @MatzeB wrote:
>
> > This adds a bigger bunch of code to lit, I wonder if it is necessary:
> >
> > - I assume the necessity for the logic inside lit is just the fact that the profile data overwrites each other after running a command. Is setting LLVM_PROFILE_FILE with a '%p' placeholder not enough to avoid this?
>
>
> Using PID substitution gets close to solving the problem of having overwritten profiles, but not all the way. On 32-bit systems PID wraparound would pose a real problem. In this patch I include the hash of the test command to minimize loss of profiles.


This is a really annoying problem to have... Maybe we should find a solution within the profile infrastructure itself (can we add another flag that adds a unique suffix to the filename if it already exists?) so not every user of the profiling infrastructure has to jump through the same hoops (I've done a similar dance in the test-suite profile support).

> 

> 

> > The final profile data merging can always be done outside of llvm-lit afterwards.

> 

> 

> On a practical level, I don't think this is possible. Raw profiles are too large. Turning off the cleanup step and running check-llvm produces over half a terrabyte of data. There are a few ways to address this without touching lit:

> 

> 1. Reduce the size of raw profiles. This would make the compiler runtime larger and more complex.

> 2. Run a monitor process that does the merging/cleanup. I don't think that approach has any advantages compared to modifying lit.

> 

>   Alternatively, we could do in-place raw profile merging in the compiler runtime. I don't think that's preferable because we'd have to introduce a lot of complexity to compiler-rt (e.g portable mandatory file locking).


Ah, so that seems harder to solve without an external driver merging the profile data. I'm still not a big fan of injecting this stuff into the core of lit, but we may have no other choice today.

> 

> 

> > Or are there other reasons why you need to intercept the opt/llc/etc. calls?

> 

> 

> Intercepting the calls makes it possible to generate 'unique' hashes for the profiles, see my comment above.

> 

> > - The lit tests don't strike me as the most typical llvm usage (they are often designed to test corner cases of the compiler), wouldn't it be better to gather profiles by compiling the llvm test-suite or something similar?

> 

> 

> It's true that the profiles gathered from check-llvm aren't ideal for PGO. But, they are perfect for measuring code coverage of the llvm codebase. Moreover because lit is portable, other projects can use these changes to gather coverage reports with their tests.


Oh indeed I did not think about code coverage.

> (IIRC the test-suite also uses lit?)


The test-suite uses lit to execute the benchmarks. I already added profiling support for that (see the TEST_SUITE_PROFILE_GENERATE/TEST_SUITE_PROFILE_USE variants). This is however about running the benchmarks. It does not help you when you want to generate profile data from the clang executable that compiled the benchmarks as that is only done by make (or ninja) and not lit.


http://reviews.llvm.org/D20993