[all-commits] [llvm/llvm-project] ae8b56: [MemProf] Disable hot hints by default (#124338)

[Teresa Johnson via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ae8b560899c00d1792fd87ec591558cd91add2ef
      https://github.com/llvm/llvm-project/commit/ae8b560899c00d1792fd87ec591558cd91add2ef
  Author: Teresa Johnson <tejohnson at google.com>
  Date:   2025-01-24 (Fri, 24 Jan 2025)

  Changed paths:
    M llvm/lib/Analysis/MemoryProfileInfo.cpp
    M llvm/test/Transforms/PGOProfile/memprof.ll
    M llvm/test/Transforms/PGOProfile/memprof_loop_unroll.ll
    M llvm/unittests/Analysis/MemoryProfileInfoTest.cpp

  Log Message:
  -----------
  [MemProf] Disable hot hints by default (#124338)

By default we were marking some contexts as hot, and adding hot hints to
unambiguously hot allocations. However, there is not yet support for
cloning to expose hot allocation contexts, and none is planned for the
forseeable future.

While we convert hot contexts to notcold contexts during the cloning
step, their existence was greatly limiting the context trimming
performed when we add the MemProf profile to the IR. This change simply
disables the generation of hot contexts / hints by default, as few
allocations were unambiguously hot.

A subsequent change will address the issue when hot hints are optionally
enabled. See PR124219 for details.

This change resulted in significant overhead reductions for a large
target:
~48% reduction in the per-module ThinLTO bitcode summary sizes
~72% reduction in the distributed ThinLTO bitcode combined summary sizes
~68% reduction in thin link time
~34% reduction in thin link peak memory



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