[llvm] [SanitizerCoverage] Don't create comdat for unnamed functions (PR #228959)
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Sun Oct 4 17:28:19 PDT 2026
https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/228959
MergeFunctions::mergeTwoFunctions may turn identical functions into a
private unnamed function, which trips `assert(F.hasName())`. Without
assertions, all unnamed functions share an empty-name IR comdat (which
yields no ELF section group per MCContext::getELFSection).
Fix #228820: Just drop comdat for unnamed functions. SanitizerCoverage
then retains the metadata sections via `llvm.used`. It's acceptable to
have redundat metadata sections with discarded associated text sections.
Improve comments at https://reviews.llvm.org/D97430 modified code.
>From bbd4d0528c919bf2232af7a628d61d4144fa0ded Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Sun, 4 Oct 2026 15:21:13 -0700
Subject: [PATCH] [SanitizerCoverage] Don't create comdat for unnamed functions
MergeFunctions::mergeTwoFunctions may turn identical functions into a
private unnamed function, which trips `assert(F.hasName())`. Without
assertions, all unnamed functions share an empty-name IR comdat (which
yields no ELF section group per MCContext::getELFSection).
Fix #228820: Just drop comdat for unnamed functions. SanitizerCoverage
then retains the metadata sections via `llvm.used`. It's acceptable to
have redundat metadata sections with discarded associated text sections.
Improve comments at https://reviews.llvm.org/D97430 modified code.
---
.../Instrumentation/SanitizerCoverage.cpp | 18 ++++++-------
llvm/lib/Transforms/Utils/Instrumentation.cpp | 5 +++-
.../SanitizerCoverage/interposable-symbol.ll | 27 ++++++++++++++-----
3 files changed, 32 insertions(+), 18 deletions(-)
diff --git a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
index f2e2441c4b1bc84..9e59f60d3d9265c 100644
--- a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
+++ b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
@@ -775,7 +775,12 @@ GlobalVariable *ModuleSanitizerCoverage::CreateFunctionLocalArrayInSection(
*CurModule, ArrayTy, false, GlobalVariable::PrivateLinkage,
Constant::getNullValue(ArrayTy), "__sancov_gen_");
- // noipa doesn't affect linkage, so it is ignored when deciding on a comdat.
+ // sancov_pcs parallels the other arrays, so they must be retained or
+ // discarded together. Put them in F's comdat to tie them to F for linker GC
+ // benefit. Outside ELF (nodeduplicate), a new comdat for an interposable F
+ // could prevail over a strong definition (COFF: the weak external becomes a
+ // COMDAT definition; Wasm: comdats deduplicate first). noipa doesn't affect
+ // symbol resolution.
if (TargetTriple.supportsCOMDAT() &&
(F.hasComdat() || TargetTriple.isOSBinFormatELF() ||
!F.isInterposable(/*CheckNoIPA=*/false)))
@@ -784,15 +789,8 @@ GlobalVariable *ModuleSanitizerCoverage::CreateFunctionLocalArrayInSection(
Array->setSection(getSectionName(Section));
Array->setAlignment(Align(DL->getTypeStoreSize(Ty).getFixedValue()));
- // sancov_pcs parallels the other metadata section(s). Optimizers (e.g.
- // GlobalOpt/ConstantMerge) may not discard sancov_pcs and the other
- // section(s) as a unit, so we conservatively retain all unconditionally in
- // the compiler.
- //
- // With comdat (COFF/ELF), the linker can guarantee the associated sections
- // will be retained or discarded as a unit, so llvm.compiler.used is
- // sufficient. Otherwise, conservatively make all of them retained by the
- // linker.
+ // Optimizers (e.g. GlobalOpt/ConstantMerge) may not discard the arrays as a
+ // unit, so retain them in the compiler; without a comdat, in the linker too.
if (Array->hasComdat())
GlobalsToAppendToCompilerUsed.push_back(Array);
else
diff --git a/llvm/lib/Transforms/Utils/Instrumentation.cpp b/llvm/lib/Transforms/Utils/Instrumentation.cpp
index 3e43b85a11a3cc1..5f71703725ba11f 100644
--- a/llvm/lib/Transforms/Utils/Instrumentation.cpp
+++ b/llvm/lib/Transforms/Utils/Instrumentation.cpp
@@ -96,7 +96,10 @@ GlobalVariable *llvm::createPrivateGlobalForString(Module &M, StringRef Str,
Comdat *llvm::getOrCreateFunctionComdat(Function &F, Triple &T) {
if (auto Comdat = F.getComdat())
return Comdat;
- assert(F.hasName());
+ // Unnamed functions would share a single empty-name comdat, which has little
+ // linker garbage collection benefit.
+ if (!F.hasName())
+ return nullptr;
Module *M = F.getParent();
// Make a new comdat for the function. Use the "no duplicates" selection kind
diff --git a/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll b/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll
index 9131d4bf27dba87..64b381c68fd5936 100644
--- a/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll
+++ b/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll
@@ -7,6 +7,8 @@ $WeakComdat = comdat any
define void @Vanilla() {
entry:
+ call i32 @0()
+ call i32 @1()
ret void
}
@@ -43,19 +45,30 @@ entry:
ret void
}
+define private i32 @0() {
+ ret i32 0
+}
+
+define i32 @1() {
+ ret i32 1
+}
; CHECK: $Vanilla = comdat nodeduplicate
; ELF: $LinkOnceOdr = comdat nodeduplicate
; COFF: $LinkOnceOdr = comdat any
; CHECK: $WeakComdat = comdat any
; CHECK: $NoIPA = comdat nodeduplicate
-; CHECK: @__sancov_gen_ = private global [1 x i32] zeroinitializer, section {{.*}}, comdat($Vanilla), align 4{{$}}
-; CHECK-NEXT: @__sancov_gen_.1 = private global [1 x i32] zeroinitializer, section {{.*}}, align 4{{$}}
-; CHECK-NEXT: @__sancov_gen_.2 = private global [1 x i32] zeroinitializer, section {{.*}}, align 4{{$}}
-; CHECK-NEXT: @__sancov_gen_.3 = private global [1 x i32] zeroinitializer, section {{.*}}, comdat($LinkOnceOdr), align 4{{$}}
-; CHECK-NEXT: @__sancov_gen_.4 = private global [1 x i32] zeroinitializer, section {{.*}}, comdat($WeakOdr), align 4{{$}}
-; CHECK-NEXT: @__sancov_gen_.5 = private global [1 x i32] zeroinitializer, section {{.*}}, comdat($WeakComdat), align 4{{$}}
-; CHECK-NEXT: @__sancov_gen_.6 = private global [1 x i32] zeroinitializer, section {{.*}}, comdat($NoIPA), align 4{{$}}
+; CHECK: @__sancov_gen_ = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($Vanilla), align 4{{$}}
+; ELF-NEXT: @__sancov_gen_.1 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($LinkOnce), align 4{{$}}
+; ELF-NEXT: @__sancov_gen_.2 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($Weak), align 4{{$}}
+; COFF-NEXT: @__sancov_gen_.1 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", align 4{{$}}
+; COFF-NEXT: @__sancov_gen_.2 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", align 4{{$}}
+; CHECK-NEXT: @__sancov_gen_.3 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($LinkOnceOdr), align 4{{$}}
+; CHECK-NEXT: @__sancov_gen_.4 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($WeakOdr), align 4{{$}}
+; CHECK-NEXT: @__sancov_gen_.5 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($WeakComdat), align 4{{$}}
+; CHECK-NEXT: @__sancov_gen_.6 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", comdat($NoIPA), align 4{{$}}
+; CHECK-NEXT: @__sancov_gen_.7 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", align 4{{$}}
+; CHECK-NEXT: @__sancov_gen_.8 = private global [1 x i32] zeroinitializer, section "{{[^"]*}}", align 4{{$}}
; CHECK: define void @Vanilla() comdat {
; ELF: define linkonce void @LinkOnce() comdat {
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