[all-commits] [llvm/llvm-project] d1b376: [FuncSpec] Rework the discardment logic for unprof...

[Alexandros Lamprineas via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d1b376fd7bf73bca557f3c174d4c129ed4d45ae5
      https://github.com/llvm/llvm-project/commit/d1b376fd7bf73bca557f3c174d4c129ed4d45ae5
  Author: Alexandros Lamprineas <alexandros.lamprineas at arm.com>
  Date:   2023-08-09 (Wed, 09 Aug 2023)

  Changed paths:
    M llvm/include/llvm/Transforms/IPO/FunctionSpecialization.h
    M llvm/lib/Transforms/IPO/FunctionSpecialization.cpp
    M llvm/unittests/Transforms/IPO/FunctionSpecializationTest.cpp

  Log Message:
  -----------
  [FuncSpec] Rework the discardment logic for unprofitable specializations.

Currently we make an arbitrary comparison between codesize and latency
in order to decide whether to keep a specialization or not. Sometimes
the latency savings are biased in favor of loops because of imprecise
block frequencies, therefore this metric contains a lot of noise. This
patch tries to address the problem as follows:

* Reject specializations whose codesize savings are less than X% of
  the original function size.
* Reject specializations whose latency savings are less than Y% of
  the original function size.
* Reject specializations whose inlining bonus is less than Z% of
  the original function size.

I am not saying this is super precise, but at least X, Y and Z are
configurable, allowing us to tweak the cost model. Moreover, it lets
us prioritize codesize over latency, which is a less noisy metric.

I am also increasing the minimum size a function should have to be
considered a candidate for specialization. Initially the cost of
a function was calculated as

  CodeMetrics::NumInsts * InlineConstants::getInstrCost()

which later in D150464 was altered into CodeMetrics::NumInsts since
the metric is supposed to model TargetTransformInfo::TCK_CodeSize.
However, we omitted adjusting MinFunctionSize in that commit.

Differential Revision: https://reviews.llvm.org/D157123