[llvm] [X86] Refuse machine outlining under Windows CFI (PR #225263)

[Demetrios Chiuratto Agourakis via llvm-commits]

https://github.com/agourakis82 updated https://github.com/llvm/llvm-project/pull/225263

>From 408278dbb4388ea402141d80bf4e4da8ee3cd18f Mon Sep 17 00:00:00 2001
From: Demetrios Chiuratto Agourakis <demetrios at agourakis.med.br>
Date: Tue, 22 Sep 2026 01:54:10 +0000
Subject: [PATCH] [X86] Refuse machine outlining under Windows CFI

AArch64 already declines to outline from functions when
usesWindowsCFI() is true because the outliner does not yet generate or
preserve Windows unwind info. X86 lacked the same fence, allowing the
machine outliner to rewrite WinEH/SEH-bearing code and emit illegal
.seh_* sequences around funclets (#213862).

Mirror the AArch64 safety check in X86InstrInfo::isFunctionSafeToOutlineFrom.

Fixes #213862
---
 llvm/lib/Target/X86/X86InstrInfo.cpp          |  7 ++++
 .../X86/machine-outliner-windows-cfi.ll       | 36 +++++++++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 llvm/test/CodeGen/X86/machine-outliner-windows-cfi.ll

diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index db11719b86cbeb1..65451223b409eda 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -10798,6 +10798,13 @@ bool X86InstrInfo::isFunctionSafeToOutlineFrom(
   if (!OutlineFromLinkOnceODRs && F.hasLinkOnceODRLinkage())
     return false;
 
+  // FIXME: Teach the outliner to generate/handle Windows unwind info.
+  // Without this fence, outlining from WinEH/SEH-bearing code can corrupt
+  // .seh_* frame state (llvm/llvm-project#213862). AArch64 already refuses
+  // outlining when usesWindowsCFI() is true.
+  if (MF.getTarget().getMCAsmInfo().usesWindowsCFI())
+    return false;
+
   // This function is viable for outlining, so return true.
   return true;
 }
diff --git a/llvm/test/CodeGen/X86/machine-outliner-windows-cfi.ll b/llvm/test/CodeGen/X86/machine-outliner-windows-cfi.ll
new file mode 100644
index 000000000000000..5ca540daf272969
--- /dev/null
+++ b/llvm/test/CodeGen/X86/machine-outliner-windows-cfi.ll
@@ -0,0 +1,36 @@
+; RUN: llc -enable-machine-outliner -mtriple=x86_64-pc-windows-msvc < %s | FileCheck %s
+; RUN: llc -enable-machine-outliner -mtriple=x86_64-apple-darwin < %s | FileCheck %s --check-prefix=DARWIN
+
+; X86 previously outlined from Windows-CFI functions, which can corrupt .seh_*
+; frame state around WinEH/funclets (#213862). Mirror AArch64 and refuse
+; outlining when usesWindowsCFI() is true.
+
+define i32 @f1(i32 %a, i32 %b) nounwind {
+entry:
+  %a2 = mul i32 %a, %b
+  %a3 = add i32 %a2, %b
+  %a4 = mul i32 %a3, %a2
+  %a5 = add i32 %a4, %a3
+  ret i32 %a5
+}
+
+define i32 @f2(i32 %a, i32 %b) nounwind {
+entry:
+  %a2 = mul i32 %a, %b
+  %a3 = add i32 %a2, %b
+  %a4 = mul i32 %a3, %a2
+  %a5 = add i32 %a4, %a3
+  ret i32 %a5
+}
+
+; CHECK-LABEL: f1:
+; CHECK-NOT: OUTLINED_FUNCTION
+; CHECK: imull
+; CHECK-LABEL: f2:
+; CHECK-NOT: OUTLINED_FUNCTION
+; CHECK: imull
+
+; DARWIN-LABEL: _f1:
+; DARWIN: OUTLINED_FUNCTION
+; DARWIN-LABEL: _f2:
+; DARWIN: OUTLINED_FUNCTION