https://github.com/mtrofin created
https://github.com/llvm/llvm-project/pull/145578
None
>From 1a15250e8b8cbdbb17613641fbd6003b4a901606 Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtro...@google.com>
Date: Tue, 24 Jun 2025 09:50:40 -0700
Subject: [PATCH] [pgo] add means to specify "unknown" MD_prof
---
llvm/include/llvm/IR/ProfDataUtils.h | 12 +++++++++
llvm/lib/IR/ProfDataUtils.cpp | 22 ++++++++++++++++
llvm/lib/IR/Verifier.cpp | 3 +++
llvm/test/Bitcode/branch-weight-unknown.ll | 30 ++++++++++++++++++++++
4 files changed, 67 insertions(+)
create mode 100644 llvm/test/Bitcode/branch-weight-unknown.ll
diff --git a/llvm/include/llvm/IR/ProfDataUtils.h
b/llvm/include/llvm/IR/ProfDataUtils.h
index 8e8d069b836f1..59a5d99f09512 100644
--- a/llvm/include/llvm/IR/ProfDataUtils.h
+++ b/llvm/include/llvm/IR/ProfDataUtils.h
@@ -133,6 +133,18 @@ LLVM_ABI bool extractProfTotalWeight(const Instruction &I,
LLVM_ABI void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights,
bool IsExpected);
+/// Specify that the branch weights for this terminator cannot be known at
+/// compile time. This should only be called by passes, and never as a default
+/// behavior in e.g. MDBuilder. The goal is to use this info to validate passes
+/// do not accidentally drop profile info, and this API is called in cases
where
+/// the pass explicitly cannot provide that info. Defaulting it in would hide
+/// bugs where the pass forgets to transfer over or otherwise specify profile
+/// info.
+LLVM_ABI void setExplicitlyUnknownBranchWeights(Instruction &I);
+
+LLVM_ABI bool isExplicitlyUnknownBranchWeightsMetadata(const MDNode& MD);
+LLVM_ABI bool hasExplicitlyUnknownBranchWeights(const Instruction &I);
+
/// Scaling the profile data attached to 'I' using the ratio of S/T.
LLVM_ABI void scaleProfData(Instruction &I, uint64_t S, uint64_t T);
diff --git a/llvm/lib/IR/ProfDataUtils.cpp b/llvm/lib/IR/ProfDataUtils.cpp
index 21524eb840539..6eb7d437764aa 100644
--- a/llvm/lib/IR/ProfDataUtils.cpp
+++ b/llvm/lib/IR/ProfDataUtils.cpp
@@ -44,6 +44,9 @@ constexpr unsigned MinBWOps = 3;
// the minimum number of operands for MD_prof nodes with value profiles
constexpr unsigned MinVPOps = 5;
+const char* UnknownBranchWeightsMarker = "unknown";
+
+
// We may want to add support for other MD_prof types, so provide an
abstraction
// for checking the metadata type.
bool isTargetMD(const MDNode *ProfData, const char *Name, unsigned MinOps) {
@@ -232,6 +235,25 @@ bool extractProfTotalWeight(const Instruction &I, uint64_t
&TotalVal) {
return extractProfTotalWeight(I.getMetadata(LLVMContext::MD_prof), TotalVal);
}
+void setExplicitlyUnknownBranchWeights(Instruction &I) {
+ MDBuilder MDB(I.getContext());
+ I.setMetadata(LLVMContext::MD_prof,
+ MDNode::get(I.getContext(),
+ MDB.createString(UnknownBranchWeightsMarker)));
+}
+
+bool isExplicitlyUnknownBranchWeightsMetadata(const MDNode &MD) {
+ if (MD.getNumOperands() != 1)
+ return false;
+ return MD.getOperand(0).equalsStr(UnknownBranchWeightsMarker);
+}
+
+bool hasExplicitlyUnknownBranchWeights(const Instruction &I) {
+ auto *MD = I.getMetadata(LLVMContext::MD_prof);
+ if (!MD) return false;
+ return isExplicitlyUnknownBranchWeightsMetadata(*MD);
+}
+
void setBranchWeights(Instruction &I, ArrayRef<uint32_t> Weights,
bool IsExpected) {
MDBuilder MDB(I.getContext());
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
index ae95e3e2bff8d..0ffe4ac257da5 100644
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -4964,6 +4964,9 @@ void Verifier::visitDereferenceableMetadata(Instruction&
I, MDNode* MD) {
}
void Verifier::visitProfMetadata(Instruction &I, MDNode *MD) {
+ if (isExplicitlyUnknownBranchWeightsMetadata(*MD))
+ return;
+
Check(MD->getNumOperands() >= 2,
"!prof annotations should have no less than 2 operands", MD);
diff --git a/llvm/test/Bitcode/branch-weight-unknown.ll
b/llvm/test/Bitcode/branch-weight-unknown.ll
new file mode 100644
index 0000000000000..921be1ff5da97
--- /dev/null
+++ b/llvm/test/Bitcode/branch-weight-unknown.ll
@@ -0,0 +1,30 @@
+; Test branch weight unknown validation
+
+; RUN: split-file %s %t
+; RUN: opt -passes=verify %t/correct.ll --disable-output
+; RUN: not opt -passes=verify %t/incorrect.ll --disable-output
+; RUN: not opt -passes=verify %t/on_function.ll --disable-output
+
+;--- correct.ll
+define void @correct(i32 %a) {
+ %c = icmp eq i32 %a, 0
+ br i1 %c, label %yes, label %no, !prof !0
+yes:
+ ret void
+no:
+ ret void
+}
+
+!0 = !{!"unknown"}
+
+;--- incorrect.ll
+define void @correct(i32 %a) {
+ %c = icmp eq i32 %a, 0
+ br i1 %c, label %yes, label %no, !prof !0
+yes:
+ ret void
+no:
+ ret void
+}
+
+!0 = !{!"unknown", i32 12, i32 67}
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