[sancov] Use comdats when one already exists (#131929)

This code avoids adding comdat groups to interposable linkage types
(weak, linkonce (non-ODR)) to avoid changing semantics, since comdat
elimination happens before weak/strong prevailaing symbol resolution.
However, if the function is already in a comdat, we can add to the group
without changing the semantics of the linked program.

Fixes an issue uncovered in PR #126240

llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

diff --git a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
index 398f83e..c5100de 100644
--- a/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
+++ b/llvm/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
@@ -745,7 +745,7 @@
       Constant::getNullValue(ArrayTy), "__sancov_gen_");
 
   if (TargetTriple.supportsCOMDAT() &&
-      (TargetTriple.isOSBinFormatELF() || !F.isInterposable()))
+      (F.hasComdat() || TargetTriple.isOSBinFormatELF() || !F.isInterposable()))
     if (auto Comdat = getOrCreateFunctionComdat(F, TargetTriple))
       Array->setComdat(Comdat);
   Array->setSection(getSectionName(Section));

llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll

diff --git a/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll b/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll
index 29a7562..e8b2309 100644
--- a/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll
+++ b/llvm/test/Instrumentation/SanitizerCoverage/interposable-symbol.ll
@@ -2,6 +2,8 @@
 ; RUN: opt < %s -passes='module(sancov-module)' -sanitizer-coverage-level=3 -sanitizer-coverage-trace-pc-guard -mtriple x86_64-linux-gnu -S | FileCheck %s --check-prefixes=CHECK,ELF
 ; RUN: opt < %s -passes='module(sancov-module)' -sanitizer-coverage-level=3 -sanitizer-coverage-trace-pc-guard -mtriple x86_64-windows-msvc -S | FileCheck %s --check-prefixes=CHECK,COFF
 
+$WeakComdat = comdat any
+
 define void @Vanilla() {
 entry:
   ret void
@@ -29,14 +31,22 @@
   ret void
 }
 
+define weak void @WeakComdat() comdat {
+entry:
+  ret void
+}
+
+
 ; CHECK:      $Vanilla = comdat nodeduplicate
 ; ELF:        $LinkOnceOdr = comdat nodeduplicate
 ; COFF:       $LinkOnceOdr = comdat any
+; CHECK:      $WeakComdat = comdat any
 ; CHECK:      @__sancov_gen_ = private global [1 x i32] zeroinitializer, section {{.*}}, comdat($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: define void @Vanilla() comdat {
 ; ELF:   define linkonce void @LinkOnce() comdat {
@@ -46,3 +56,4 @@
 ; CHECK: declare extern_weak void @ExternWeak()
 ; CHECK: define linkonce_odr void @LinkOnceOdr() comdat {
 ; CHECK: define weak_odr void @WeakOdr() comdat {
+; CHECK: define weak void @WeakComdat() comdat {