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[PGO] Fix incorrect count threshold calculation when 0% cutoff #117359

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ken-matsui merged 1 commit into from
Feb 18, 2025
Merged

[PGO] Fix incorrect count threshold calculation when 0% cutoff #117359

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9 changes: 9 additions & 0 deletions llvm/lib/ProfileData/ProfileSummaryBuilder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -77,9 +77,18 @@ static const uint32_t DefaultCutoffsData[] = {
const ArrayRef<uint32_t> ProfileSummaryBuilder::DefaultCutoffs =
DefaultCutoffsData;

// An entry for the 0th percentile to correctly calculate hot/cold count
// thresholds when -profile-summary-cutoff-hot/cold is 0. If the hot cutoff is
// 0, no sample counts are treated as hot. If the cold cutoff is 0, all sample
// counts are treated as cold. Assumes there is no UINT64_MAX sample counts.
Comment on lines +80 to +83
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@ellishg ellishg Dec 5, 2024

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Is this a typo? Or am I misunderstanding these flags?

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// An entry for the 0th percentile to correctly calculate hot/cold count
// thresholds when -profile-summary-cutoff-hot/cold is 0. If the hot cutoff is
// 0, no sample counts are treated as hot. If the cold cutoff is 0, all sample
// counts are treated as cold. Assumes there is no UINT64_MAX sample counts.
// An entry for the 0th percentile to correctly calculate hot/cold count
// thresholds when -profile-summary-cutoff-hot/cold is 0. If the hot cutoff is
// 0, all sample counts are treated as hot. If the cold cutoff is 0, no sample
// counts are treated as cold. Assumes there is no UINT64_MAX sample counts.

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@ken-matsui ken-matsui Dec 9, 2024

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Actually, it is not a typo.

  • -profile-summary-cutoff-hot: A count is hot if it exceeds the minimum count to reach this percentile of total counts.
  • -profile-summary-cutoff-cold: A count is cold if it is below the minimum count to reach this percentile of total counts.

When cutoff is 0, the minimum count is UINT64_MAX. So, when a count exceeds UINT64_MAX, it is hot, meaning no counts are hot. Similarly, when a count is below UINT64_MAX, it is cold, meaning all counts are cold.

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@ellishg ellishg Feb 13, 2025

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It seems like this can be accomplished with -profile-summary-cutoff-hot=1000000. A function will be hot if its count is larger than 100% of functions, which is impossible. So no functions are hot. Likewise, -profile-summary-cutoff-cold=1000000 should make all functions cold. Why do we need a special case for these? With this patch, how do I specify -profile-summary-cutoff-hot to make all functions hot?

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@ken-matsui ken-matsui Feb 17, 2025

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No, -profile-summary-cutoff-hot=1000000 (unimplemented) doesn't mean that a function will be hot if its count is larger than 100% of functions. Instead, a function will be hot if its count is larger than a minimum count required to become 100% of functions, i.e., always.

With this patch, how do I specify -profile-summary-cutoff-hot to make all functions hot?

This patch isn't resolving the problem you are referring to. That requires a different patch to handle -profile-summary-cutoff-hot=1000000. This patch resolves an issue that 0% cutoff (to make all functions NOT hot) isn't supported.

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@ellishg ellishg Feb 18, 2025

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Thanks for the explanation and the clarification! LGTM

static const ProfileSummaryEntry ZeroCutoffEntry = {0, UINT64_MAX, 0};

const ProfileSummaryEntry &
ProfileSummaryBuilder::getEntryForPercentile(const SummaryEntryVector &DS,
uint64_t Percentile) {
if (Percentile == 0)
return ZeroCutoffEntry;

auto It = partition_point(DS, [=](const ProfileSummaryEntry &Entry) {
return Entry.Cutoff < Percentile;
});
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8 changes: 8 additions & 0 deletions llvm/test/Analysis/ProfileSummary/basic.ll
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Expand Up @@ -2,12 +2,16 @@
; RUN: opt < %s -disable-output -profile-summary-hot-count=500 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-HOT
; RUN: opt < %s -disable-output -profile-summary-cold-count=0 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-COLD
; RUN: opt < %s -disable-output -profile-summary-cold-count=200 -profile-summary-hot-count=1000 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=OVERRIDE-BOTH
; RUN: opt < %s -disable-output -profile-summary-cutoff-hot=0 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=HOT-CUTOFF-0
; RUN: opt < %s -disable-output -profile-summary-cutoff-cold=0 -profile-summary-hot-count=18446744073709551615 -passes=print-profile-summary -S 2>&1 | FileCheck %s -check-prefixes=COLD-CUTOFF-0

define void @f1() !prof !20 {
; CHECK-LABEL: f1 :hot
; OVERRIDE-HOT-LABEL: f1
; OVERRIDE-COLD-LABEL: f1 :hot
; OVERRIDE-BOTH-LABEL: f1
; HOT-CUTOFF-0-LABEL: f1{{$}}
; COLD-CUTOFF-0-LABEL: f1 :cold

ret void
}
Expand All @@ -17,6 +21,8 @@ define void @f2() !prof !21 {
; OVERRIDE-HOT-LABEL: f2 :cold
; OVERRIDE-COLD-LABEL: f2
; OVERRIDE-BOTH-LABEL: f2
; HOT-CUTOFF-0-LABEL: f2 :cold
; COLD-CUTOFF-0-LABEL: f2 :cold

ret void
}
Expand All @@ -26,6 +32,8 @@ define void @f3() !prof !22 {
; OVERRIDE-HOT-LABEL: f3
; OVERRIDE-COLD-LABEL: f3
; OVERRIDE-BOTH-LABEL: f3
; HOT-CUTOFF-0-LABEL: f3{{$}}
; COLD-CUTOFF-0-LABEL: f3 :cold

ret void
}
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