[llvm] [PGO][profcheck] ignore explicitly cold functions (PR #151778)
Mircea Trofin via llvm-commits
llvm-commits at lists.llvm.org
Fri Aug 1 15:59:28 PDT 2025
https://github.com/mtrofin updated https://github.com/llvm/llvm-project/pull/151778
>From b46b859686919a7f9f5221aabe5cc567cdddc0c0 Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtrofin at google.com>
Date: Fri, 1 Aug 2025 15:30:23 -0700
Subject: [PATCH] [PGO][profcheck] ignore explicitly cold functions
---
llvm/lib/Transforms/Utils/ProfileVerify.cpp | 19 ++++++++++++-
.../PGOProfile/prof-verify-as-needed.ll | 28 +++++++++++++++----
.../PGOProfile/prof-verify-existing.ll | 8 ++++--
.../PGOProfile/prof-verify-known-cold.ll | 15 ++++++++++
.../test/Transforms/PGOProfile/prof-verify.ll | 7 +++--
5 files changed, 65 insertions(+), 12 deletions(-)
create mode 100644 llvm/test/Transforms/PGOProfile/prof-verify-known-cold.ll
diff --git a/llvm/lib/Transforms/Utils/ProfileVerify.cpp b/llvm/lib/Transforms/Utils/ProfileVerify.cpp
index b972132eb8c42..a58e2d7f044da 100644
--- a/llvm/lib/Transforms/Utils/ProfileVerify.cpp
+++ b/llvm/lib/Transforms/Utils/ProfileVerify.cpp
@@ -20,8 +20,12 @@
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/Support/BranchProbability.h"
+#include "llvm/Support/CommandLine.h"
using namespace llvm;
+static cl::opt<int64_t>
+ DefaultFunctionEntryCount("profcheck-default-fct-entry-count",
+ cl::init(1000));
namespace {
class ProfileInjector {
Function &F;
@@ -63,6 +67,17 @@ bool ProfileInjector::inject() {
// will get the same BPI it does if the injector wasn't running.
auto &BPI = FAM.getResult<BranchProbabilityAnalysis>(F);
+ // Inject a function count if there's none. It's reasonable for a pass to
+ // want to clear the MD_prof of a function with zero entry count. From a
+ // metadata economy perspective, if a profile comes empty for a function, it's
+ // cheaper to assign it the 0-entry count explicitly than to mark every branch
+ // as cold.
+ if (!F.getEntryCount(true))
+ F.setEntryCount(DefaultFunctionEntryCount);
+ // If there is an entry count that's 0, then don't bother injecting. We won't
+ // verify these either.
+ if (F.getEntryCount(true)->getCount() == 0)
+ return false;
bool Changed = false;
for (auto &BB : F) {
auto *Term = getTerminatorBenefitingFromMDProf(BB);
@@ -119,11 +134,13 @@ PreservedAnalyses ProfileInjectorPass::run(Function &F,
PreservedAnalyses ProfileVerifierPass::run(Function &F,
FunctionAnalysisManager &FAM) {
+ if (!F.getEntryCount(true) || F.getEntryCount(true)->getCount() == 0)
+ return PreservedAnalyses::all();
for (const auto &BB : F)
if (const auto *Term =
ProfileInjector::getTerminatorBenefitingFromMDProf(BB))
if (!Term->getMetadata(LLVMContext::MD_prof))
F.getContext().emitError("Profile verification failed");
- return PreservedAnalyses::none();
+ return PreservedAnalyses::all();
}
diff --git a/llvm/test/Transforms/PGOProfile/prof-verify-as-needed.ll b/llvm/test/Transforms/PGOProfile/prof-verify-as-needed.ll
index 07e1f2d3c6127..322a055f6ebe9 100644
--- a/llvm/test/Transforms/PGOProfile/prof-verify-as-needed.ll
+++ b/llvm/test/Transforms/PGOProfile/prof-verify-as-needed.ll
@@ -1,6 +1,6 @@
; Test that prof-inject only injects missing metadata
-; RUN: opt -passes=prof-inject %s -S -o - | FileCheck %s
+; RUN: opt -passes=prof-inject -profcheck-default-fct-entry-count=10 %s -S -o - | FileCheck %s
define void @foo(i32 %i) {
%c = icmp eq i32 %i, 0
@@ -13,8 +13,26 @@ no:
ret void
}
+define void @cold(i32 %i) !prof !1 {
+ %c = icmp eq i32 %i, 0
+ br i1 %c, label %yes, label %no
+yes:
+ br i1 %c, label %yes2, label %no
+yes2:
+ ret void
+no:
+ ret void
+}
!0 = !{!"branch_weights", i32 1, i32 2}
-; CHECK: br i1 %c, label %yes, label %no, !prof !0
-; CHECK: br i1 %c, label %yes2, label %no, !prof !1
-; CHECK: !0 = !{!"branch_weights", i32 1, i32 2}
-; CHECK: !1 = !{!"branch_weights", i32 3, i32 5}
+!1 = !{!"function_entry_count", i32 0}
+
+; CHECK-LABEL: @foo
+; CHECK: br i1 %c, label %yes, label %no, !prof !1
+; CHECK: br i1 %c, label %yes2, label %no, !prof !2
+; CHECK-LABEL: @cold
+; CHECK: br i1 %c, label %yes, label %no{{$}}
+; CHECK: br i1 %c, label %yes2, label %no{{$}}
+; CHECK: !0 = !{!"function_entry_count", i64 10}
+; CHECK: !1 = !{!"branch_weights", i32 1, i32 2}
+; CHECK: !2 = !{!"branch_weights", i32 3, i32 5}
+; CHECK: !3 = !{!"function_entry_count", i32 0}
diff --git a/llvm/test/Transforms/PGOProfile/prof-verify-existing.ll b/llvm/test/Transforms/PGOProfile/prof-verify-existing.ll
index ea4f0f9f1dadf..3945e23f9a5b6 100644
--- a/llvm/test/Transforms/PGOProfile/prof-verify-existing.ll
+++ b/llvm/test/Transforms/PGOProfile/prof-verify-existing.ll
@@ -16,6 +16,8 @@ no:
!0 = !{!"branch_weights", i32 1, i32 2}
!1 = !{!"unknown"}
-; CHECK: br i1 %c, label %yes, label %no, !prof !0
-; CHECK: !0 = !{!"branch_weights", i32 1, i32 2}
-; CHECK: !1 = !{!"unknown"}
+; CHECK: define void @foo(i32 %i) !prof !0
+; CHECK: br i1 %c, label %yes, label %no, !prof !1
+; CHECK: !0 = !{!"function_entry_count", i64 1000}
+; CHECK: !1 = !{!"branch_weights", i32 1, i32 2}
+; CHECK: !2 = !{!"unknown"}
diff --git a/llvm/test/Transforms/PGOProfile/prof-verify-known-cold.ll b/llvm/test/Transforms/PGOProfile/prof-verify-known-cold.ll
new file mode 100644
index 0000000000000..7875300006761
--- /dev/null
+++ b/llvm/test/Transforms/PGOProfile/prof-verify-known-cold.ll
@@ -0,0 +1,15 @@
+; Test prof-verify for functions explicitly marked as cold
+
+; RUN: opt -passes=prof-inject,prof-verify %s -o - 2>&1 | FileCheck %s
+
+define void @foo(i32 %i) !prof !0 {
+ %c = icmp eq i32 %i, 0
+ br i1 %c, label %yes, label %no
+yes:
+ ret void
+no:
+ ret void
+}
+!0 = !{!"function_entry_count", i32 0}
+
+; CHECK-NOT: Profile verification failed
diff --git a/llvm/test/Transforms/PGOProfile/prof-verify.ll b/llvm/test/Transforms/PGOProfile/prof-verify.ll
index 3d984d88ffffb..213cbe5ba3a5f 100644
--- a/llvm/test/Transforms/PGOProfile/prof-verify.ll
+++ b/llvm/test/Transforms/PGOProfile/prof-verify.ll
@@ -5,7 +5,7 @@
; RUN: opt -passes=prof-inject,prof-verify %s --disable-output
; RUN: opt -enable-profcheck %s -S -o - | FileCheck %s --check-prefix=INJECT
-define void @foo(i32 %i) {
+define void @foo(i32 %i) !prof !0 {
%c = icmp eq i32 %i, 0
br i1 %c, label %yes, label %no
yes:
@@ -13,8 +13,9 @@ yes:
no:
ret void
}
+!0 = !{!"function_entry_count", i32 1}
-; INJECT: br i1 %c, label %yes, label %no, !prof !0
-; INJECT: !0 = !{!"branch_weights", i32 3, i32 5}
+; INJECT: br i1 %c, label %yes, label %no, !prof !1
+; INJECT: !1 = !{!"branch_weights", i32 3, i32 5}
; VERIFY: Profile verification failed
\ No newline at end of file
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