[FunctionSpecialization] Preserve call counts of specialized functions #157768
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A function that has been specialized will have its function entry counts preserved as follows: * Each specialization's count is the sum of each call site's basic block's number of entries as computed by `BlockFrequencyInfo`. * The original function's count will be decreased by the counts of its specializations. Tracking issue: llvm#147390
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@llvm/pr-subscribers-function-specialization @llvm/pr-subscribers-llvm-transforms Author: Alan Zhao (alanzhao1) ChangesA function that has been specialized will have its function entry counts preserved as follows:
Tracking issue: #147390 Full diff: https://github.com/llvm/llvm-project/pull/157768.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/IPO/FunctionSpecialization.cpp b/llvm/lib/Transforms/IPO/FunctionSpecialization.cpp
index a459a9eddbcfc..324723c7942ab 100644
--- a/llvm/lib/Transforms/IPO/FunctionSpecialization.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionSpecialization.cpp
@@ -784,9 +784,25 @@ bool FunctionSpecializer::run() {
// Update the known call sites to call the clone.
for (CallBase *Call : S.CallSites) {
+ Function *Clone = S.Clone;
LLVM_DEBUG(dbgs() << "FnSpecialization: Redirecting " << *Call
- << " to call " << S.Clone->getName() << "\n");
+ << " to call " << Clone->getName() << "\n");
Call->setCalledFunction(S.Clone);
+ if (std::optional<uint64_t> Count =
+ GetBFI(*Call->getFunction())
+ .getBlockProfileCount(Call->getParent())) {
+ uint64_t CallCount = *Count + Clone->getEntryCount()->getCount();
+ Clone->setEntryCount(CallCount);
+ if (std::optional<llvm::Function::ProfileCount> MaybeOriginalCount =
+ S.F->getEntryCount()) {
+ uint64_t OriginalCount = MaybeOriginalCount->getCount();
+ if (OriginalCount > CallCount) {
+ S.F->setEntryCount(OriginalCount - CallCount);
+ } else {
+ S.F->setEntryCount(0);
+ }
+ }
+ }
}
Clones.push_back(S.Clone);
@@ -1043,6 +1059,9 @@ Function *FunctionSpecializer::createSpecialization(Function *F,
// clone must.
Clone->setLinkage(GlobalValue::InternalLinkage);
+ if (F->getEntryCount())
+ Clone->setEntryCount(0);
+
// Initialize the lattice state of the arguments of the function clone,
// marking the argument on which we specialized the function constant
// with the given value.
diff --git a/llvm/test/Transforms/FunctionSpecialization/profile-counts.ll b/llvm/test/Transforms/FunctionSpecialization/profile-counts.ll
new file mode 100644
index 0000000000000..4a2ad4ff9fe90
--- /dev/null
+++ b/llvm/test/Transforms/FunctionSpecialization/profile-counts.ll
@@ -0,0 +1,52 @@
+; RUN: opt -passes="ipsccp<func-spec>" -force-specialization -S < %s | FileCheck %s
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+
+@A = external dso_local constant i32, align 4
+@B = external dso_local constant i32, align 4
+
+; CHECK: define dso_local i32 @bar(i32 %x, i32 %y, ptr %z) !prof ![[BAR_PROF:[0-9]]] {
+define dso_local i32 @bar(i32 %x, i32 %y, ptr %z) !prof !0 {
+entry:
+ %tobool = icmp ne i32 %x, 0
+; CHECK: br i1 %tobool, label %if.then, label %if.else, !prof ![[BRANCH_PROF:[0-9]]]
+ br i1 %tobool, label %if.then, label %if.else, !prof !1
+
+if.then:
+; CHECK: if.then:
+; CHECK: call i32 @foo.specialized.1(i32 %x, ptr @A)
+ %call = call i32 @foo(i32 %x, ptr @A)
+ br label %return
+
+if.else:
+; CHECK: if.else:
+; CHECK: call i32 @foo.specialized.2(i32 %y, ptr @B)
+ %call1 = call i32 @foo(i32 %y, ptr @B)
+ br label %return
+
+; CHECK: return:
+; CHECK: %call2 = call i32 @foo(i32 %x, ptr %z)
+return:
+ %retval.0 = phi i32 [ %call, %if.then ], [ %call1, %if.else ]
+ %call2 = call i32 @foo(i32 %x, ptr %z);
+ %add = add i32 %retval.0, %call2
+ ret i32 %add
+}
+
+; CHECK: define internal i32 @foo(i32 %x, ptr %b) !prof ![[FOO_UNSPEC_PROF:[0-9]]]
+; CHECK: define internal i32 @foo.specialized.1(i32 %x, ptr %b) !prof ![[FOO_SPEC_1_PROF:[0-9]]]
+; CHECK: define internal i32 @foo.specialized.2(i32 %x, ptr %b) !prof ![[FOO_SPEC_2_PROF:[0-9]]]
+define internal i32 @foo(i32 %x, ptr %b) !prof !2 {
+entry:
+ %0 = load i32, ptr %b, align 4
+ %add = add nsw i32 %x, %0
+ ret i32 %add
+}
+
+; CHECK: ![[BAR_PROF]] = !{!"function_entry_count", i64 1000}
+; CHECK: ![[BRANCH_PROF]] = !{!"branch_weights", i32 1, i32 3}
+; CHECK: ![[FOO_UNSPEC_PROF]] = !{!"function_entry_count", i64 234}
+; CHECK: ![[FOO_SPEC_1_PROF]] = !{!"function_entry_count", i64 250}
+; CHECK: ![[FOO_SPEC_2_PROF]] = !{!"function_entry_count", i64 750}
+!0 = !{!"function_entry_count", i64 1000}
+!1 = !{!"branch_weights", i32 1, i32 3}
+!2 = !{!"function_entry_count", i64 1234}
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alanzhao1
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Sep 10, 2025
The previous fix in llvm#157768 had a bug; instead of subtracting the original function's call count per call site of a specialization, we were subtracting the running total of the specialization's calls. Tracking issue: llvm#147390
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@alanzhao1 this is causing clang crashes when building clang itself. Reduced test case: https://gcc.godbolt.org/z/h5qjdW4c9 |
alanzhao1
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Sep 21, 2025
… to zero We were hitting an assert discovered in #157768 (comment)
Should be fixed by 7d748a9 |
llvm-sync bot
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Sep 21, 2025
…count equal to zero We were hitting an assert discovered in llvm/llvm-project#157768 (comment)
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A function that has been specialized will have its function entry counts preserved as follows:
BlockFrequencyInfo.Tracking issue: #147390