From 51038cad3d1742efade9be2f0f6df9c0cb25da74 Mon Sep 17 00:00:00 2001
From: Leonard Chan <leonardchan@google.com>
Date: Wed, 14 May 2025 13:41:23 -0700
Subject: [PATCH] [llvm][CFI] Do not canonicalize COFF functions in a comdat

COFF requires that a function exists with the same name as a comdat. Not
having this key function results in `LLVM ERROR: Associative COMDAT
symbol '...' does not exist.` CFI by default will attempt to
canonicalize a function by appending `.cfi` to its name which allows
external references to refer to the new canonical alias, but it does not
change the comdat name. We can just change the comdat name since symbol
resolution has already happened before LTO.
---
 llvm/lib/Transforms/IPO/LowerTypeTests.cpp    | 16 ++++++++-
 .../LowerTypeTests/cfi-coff-comdat-rename.ll  | 35 +++++++++++++++++++
 2 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/Transforms/LowerTypeTests/cfi-coff-comdat-rename.ll

diff --git a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
index d855647095550..d615c10c23030 100644
--- a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -1715,8 +1715,22 @@ void LowerTypeTestsModule::buildBitSetsFromFunctionsNative(
           F->getValueType(), 0, F->getLinkage(), "", CombinedGlobalElemPtr, &M);
       FAlias->setVisibility(F->getVisibility());
       FAlias->takeName(F);
-      if (FAlias->hasName())
+      if (FAlias->hasName()) {
         F->setName(FAlias->getName() + ".cfi");
+        // For COFF we should also rename the comdat if this function also
+        // happens to be the key function. Even if the comdat name changes, this
+        // should still be fine since comdat and symbol resolution happens
+        // before LTO, so all symbols which would prevail have been selected.
+        if (F->hasComdat() && ObjectFormat == Triple::COFF &&
+            F->getComdat()->getName() == FAlias->getName()) {
+          Comdat *OldComdat = F->getComdat();
+          Comdat *NewComdat = M.getOrInsertComdat(F->getName());
+          for (GlobalObject &GO : M.global_objects()) {
+            if (GO.getComdat() == OldComdat)
+              GO.setComdat(NewComdat);
+          }
+        }
+      }
       replaceCfiUses(F, FAlias, IsJumpTableCanonical);
       if (!F->hasLocalLinkage())
         F->setVisibility(GlobalVariable::HiddenVisibility);
diff --git a/llvm/test/Transforms/LowerTypeTests/cfi-coff-comdat-rename.ll b/llvm/test/Transforms/LowerTypeTests/cfi-coff-comdat-rename.ll
new file mode 100644
index 0000000000000..aa56a3abf00fc
--- /dev/null
+++ b/llvm/test/Transforms/LowerTypeTests/cfi-coff-comdat-rename.ll
@@ -0,0 +1,35 @@
+; RUN: opt -S -passes=lowertypetests %s | FileCheck %s
+
+;; This is a check to assert we don't crash with:
+;;
+;;   LLVM ERROR: Associative COMDAT symbol '...' does not exist.
+;;
+;; So this just needs to exit normally.
+; RUN: opt -S -passes=lowertypetests %s | llc -asm-verbose=false
+
+target datalayout = "e-p:64:64"
+target triple = "x86_64-pc-windows-msvc"
+
+@a = global [2 x ptr] [ptr @f1, ptr @f2]
+
+; CHECK: $f1.cfi = comdat any
+$f1 = comdat any
+
+; CHECK: @f1.cfi() comdat !type !0
+define void @f1() comdat !type !0 {
+  ret void
+}
+
+; CHECK: @f2.cfi() comdat($f1.cfi) !type !0
+define void @f2() comdat($f1) !type !0 {
+  ret void
+}
+
+declare i1 @llvm.type.test(ptr %ptr, metadata %bitset) nounwind readnone
+
+define i1 @foo(ptr %p) {
+  %x = call i1 @llvm.type.test(ptr %p, metadata !"typeid1")
+  ret i1 %x
+}
+
+!0 = !{i32 0, !"typeid1"}