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

[Dehao Chen via llvm-dev]

If we use sum instead of geomean, then the diff will be 5.59%

Yes, most of the size growth are from small applications, with one
exception: h264, which has significant amount of loops to unroll/vectorize.

Dehao

On Thu, Oct 27, 2016 at 1:18 PM, Xinliang David Li <davidxl at google.com>
wrote:

> The large percentages are from those tiny benchmarks. If you look at
> omnetpp (0.52%), and xalanc (1.46%), the increase is small. To get a better
> average increase, you can sum up total debug_line size before and after and
> compute percentage accordingly.
>
> David
>
> On Thu, Oct 27, 2016 at 1:11 PM, Dehao Chen <dehao at google.com> wrote:
>
>> The impact to debug_line is actually not small. I only implemented the
>> part 1 (encoding duplication factor) for loop unrolling and loop
>> vectorization. The debug_line size overhead for "-O2 -g1" binary of speccpu
>> C/C++ benchmarks:
>>
>> 433.milc 23.59%
>> 444.namd 6.25%
>> 447.dealII 8.43%
>> 450.soplex 2.41%
>> 453.povray 5.40%
>> 470.lbm 0.00%
>> 482.sphinx3 7.10%
>> 400.perlbench 2.77%
>> 401.bzip2 9.62%
>> 403.gcc 2.67%
>> 429.mcf 9.54%
>> 445.gobmk 7.40%
>> 456.hmmer 9.79%
>> 458.sjeng 9.98%
>> 462.libquantum 10.90%
>> 464.h264ref 30.21%
>> 471.omnetpp 0.52%
>> 473.astar 5.67%
>> 483.xalancbmk 1.46%
>> mean 7.86%
>> Dehao
>>
>> On Thu, Oct 27, 2016 at 11:55 AM, Xinliang David Li <davidxl at google.com>
>> wrote:
>>
>>> Do you have an estimate of the debug_line size increase? I guess it will
>>> be small.
>>>
>>> David
>>>
>>> On Thu, Oct 27, 2016 at 11:39 AM, Dehao Chen <dehao at google.com> wrote:
>>>
>>>> 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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