[llvm-dev] (RFC) Encoding code duplication factor in discriminator

[Hal Finkel via llvm-dev]

Hi Dehao,

This is definitely an important problem, thanks for writing this up!

There is a related problem that I think we can address at the same time: When we multiversion code, for example when we use runtime checks to enable the creation of a vectorized loop while retaining the scalar loop, and then we collect profiling data, we should be able to recover the relative running time of the different versions of the loop. In the future, I suspect we'll end up with transformations that produce even more versions of loops (Intel's compiler, for example, has a useful pragma that allows the user to request loop specializations for a specified set of trip counts).

I'd like to have a scheme where the source location + discriminator can be mapped to information about the relevant loop version so that our profiling tools can display that information usefully to the user, and so that our optimizations can make useful choices (i.e. don't bother vectorizing a loop when the scalar loop is run a lot but the vectorized version almost never runs).

In short, I think that differentiating these different regions using the descriminator seems like a natural choice, but "And we do not need to introduce new building blocks to debug info" might not save us all that much in the long run. To keep information on what regions correspond to what optimizations, we may need to do that. That's not a bad thing, and I'd rather we solve this in a way that is extensible. Plus, this might make it easier to use fewer bits, thus helping the overall impact on the size of the debug sections.

Thanks again,
Hal

----- Original Message -----
> From: "Dehao Chen via llvm-dev" <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Cc: "Xinliang David Li" <davidxl at google.com>
> Sent: Thursday, October 27, 2016 1:39:15 PM
> Subject: [llvm-dev] (RFC) Encoding code duplication factor in discriminator
> 
> Motivation:
> Many optimizations duplicate code. E.g. loop unroller duplicates the
> loop body, GVN duplicates computation, etc. The duplicated code will
> share the same debug info with the original code. For SamplePGO, the
> debug info is used to present the profile. Code duplication will
> affect profile accuracy. Taking loop unrolling for example:
> 
> 
> #1 foo();
> #2 for (i = 0; i < N; i++) {
> 
> #3 bar();
> #4 }
> 
> 
> If N is 8 during runtime, a reasonable profile will look like:
> 
> 
> #1: 10
> #3: 80
> 
> 
> 
> But if the compiler unrolls the loop by a factor of 4, the callsite
> to bar() is duplicated 4 times and the profile will look like:
> 
> 
> #1: 10
> #3: 20
> 
> 
> The sample count for #3 is 20 because all 4 callsites to bar() are
> sampled 20 times each, and they shared the same debug loc (#3) so
> that 20 will be attributed to #3 (If one debugloc is mapped by
> multiple instructions, the max sample count of these instructions is
> used as debugloc's sample count).
> 
> 
> When loading this profile into compiler, it will think the loop trip
> count is 2 instead of 8.
> 
> 
> Proposal:
> When compiler duplicates code, it encodes the duplication info in the
> debug info. As the duplication is not interesting to debugger, I
> propose to encode this as part of the discriminator.
> 
> 
> There are 2 types of code duplication:
> 
> 
> 1. duplicated code are guaranteed to have the same execution count
> (e.g. loop unroll and loop vectorize). We can record the duplication
> factor, for the above example "4" is recorded in the discriminator.
> 2. duplicated code that may have different execution count (e.g. loop
> peel and gvn). For a same debugloc, a unique number is assigned to
> each copy and encoded in the discriminator.
> 
> 
> Assume that the discriminator is uint32. The traditional
> discriminator is less than 256, let's take 8 bit for it. For
> duplication factor (type 1 duplication), we assume the maximum
> unroll_factor * vectorize_factor is less than 256, thus 8 bit for
> it. For unique number(type 2 duplication), we assume code is at most
> duplicated 32 times, thus 5 bit for it. Overall, we still have 11
> free bits left in the discriminator encoding.
> 
> 
> Let's take the original source as an example, after loop unrolling
> and peeling, the code may looks like:
> 
> 
> for (i = 0; i < N & 3; i+= 4) {
> foo(); // discriminator: 0x40
> foo(); // discriminator: 0x40
> foo(); // discriminator: 0x40
> foo(); // discriminator: 0x40
> }
> if (i++ < N) {
> foo(); // discriminator: 0x100
> if (i++ < N) {
> foo(); // discriminator: 0x200
> if (i++ < N) {
> foo(); // discriminator: 0x300
> }
> }
> }
> 
> 
> The cost of this change would be increased debug_line size because:
> 1. we are recording more discriminators 2. discriminators become
> larger and will take more ULEB128 encoding.
> 
> 
> The benefit is that the sample pgo profile can accurately represent
> the code execution frequency. And we do not need to introduce new
> building blocks to debug info.
> 
> 
> Comments?
> 
> 
> Thanks,
> Dehao
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> 

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory