[win][x64] Unwind v2: treat SetFrame differently from StackAlloc

dpaoliello · dpaoliello · commit 9a3819255141 · 2025-08-21T09:42:42.000-07:00
* SetFrame does not imply that there was a stack allocation.
* `mov` is used in the epilog to set the frame back, not to deallocate. It is also optional.
* If the frame is set back in the epilog, then it must be done before deallocation and register popping.
diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
@@ -22,6 +22,7 @@ class MCSection;
 
 /// MCExpr that represents the epilog unwind code in an unwind table.
 class MCUnwindV2EpilogTargetExpr final : public MCTargetExpr {
+  StringRef FunctionName;
   const MCSymbol *FunctionEnd;
   const MCSymbol *UnwindV2Start;
   const MCSymbol *EpilogEnd;
@@ -30,17 +31,19 @@ class MCUnwindV2EpilogTargetExpr final : public MCTargetExpr {
 
   MCUnwindV2EpilogTargetExpr(const WinEH::FrameInfo &FrameInfo,
                              const WinEH::FrameInfo::Epilog &Epilog,
-                             uint8_t EpilogSize_)
-      : FunctionEnd(FrameInfo.FuncletOrFuncEnd),
+                             uint8_t EpilogSize_, StringRef FunctionName_)
+      : FunctionName(FunctionName_), FunctionEnd(FrameInfo.FuncletOrFuncEnd),
         UnwindV2Start(Epilog.UnwindV2Start), EpilogEnd(Epilog.End),
         EpilogSize(EpilogSize_), Loc(Epilog.Loc) {}
 
 public:
   static MCUnwindV2EpilogTargetExpr *
   create(const WinEH::FrameInfo &FrameInfo,
          const WinEH::FrameInfo::Epilog &Epilog, uint8_t EpilogSize_,
+         StringRef FunctionName_,
          MCContext &Ctx) {
-    return new (Ctx) MCUnwindV2EpilogTargetExpr(FrameInfo, Epilog, EpilogSize_);
+    return new (Ctx) MCUnwindV2EpilogTargetExpr(FrameInfo, Epilog, EpilogSize_,
+                                                 FunctionName_);
   }
 
   void printImpl(raw_ostream &OS, const MCAsmInfo *MAI) const override {
@@ -337,7 +340,7 @@ static void EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
       // between the start of the epilog and the end of the function until
       // layout has been completed.
       auto *MCE = MCUnwindV2EpilogTargetExpr::create(*info, Epilog.second,
-                                                     EpilogSize, context);
+                                                     EpilogSize, info->Function->getName(), context);
       OS->addFixup(MCE, FK_Data_2);
       OS->appendContents(2, 0);
     }
@@ -383,14 +386,16 @@ bool MCUnwindV2EpilogTargetExpr::evaluateAsRelocatableImpl(
   auto Offset = GetOptionalAbsDifference(*Asm, FunctionEnd, UnwindV2Start);
   if (!Offset) {
     Asm->getContext().reportError(
-        Loc, "Failed to evaluate epilog offset for Unwind v2");
+        Loc, "Failed to evaluate epilog offset for Unwind v2 in " + FunctionName);
     return false;
   }
   assert(*Offset > 0);
   constexpr uint16_t MaxEpilogOffset = 0x0fff;
   if (*Offset > MaxEpilogOffset) {
+    dbgs() << "Unwindv2: " << UnwindV2Start->getSection().getName();
+    dbgs() << "FunctionEnd: " << FunctionEnd->getSection().getName();
     Asm->getContext().reportError(Loc,
-                                  "Epilog offset is too large for Unwind v2");
+                                  "Epilog offset is too large (0x" + Twine::utohexstr(*Offset) + ") for Unwind v2 in " + FunctionName);
     return false;
   }
 
diff --git a/llvm/lib/Target/X86/X86WinEHUnwindV2.cpp b/llvm/lib/Target/X86/X86WinEHUnwindV2.cpp
@@ -90,10 +90,15 @@ DebugLoc findDebugLoc(const MachineBasicBlock &MBB) {
 }
 
 bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
+  // noreturn functions don't have epilogs.
+  Function &F = MF.getFunction();
+  if (F.doesNotReturn())
+    return false;
+
   WinX64EHUnwindV2Mode Mode =
       ForceMode.getNumOccurrences()
           ? static_cast<WinX64EHUnwindV2Mode>(ForceMode.getValue())
-          : MF.getFunction().getParent()->getWinX64EHUnwindV2Mode();
+          : F.getParent()->getWinX64EHUnwindV2Mode();
 
   if (Mode == WinX64EHUnwindV2Mode::Disabled)
     return false;
@@ -105,6 +110,7 @@ bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
   // Prolog information.
   SmallVector<int64_t> PushedRegs;
   bool HasStackAlloc = false;
+  bool HasSetFrame = false;
   unsigned ApproximatePrologCodeCount = 0;
 
   // Requested changes.
@@ -130,15 +136,20 @@ bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
         break;
 
       case X86::SEH_StackAlloc:
-      case X86::SEH_SetFrame:
         if (State != FunctionState::InProlog)
-          llvm_unreachable("SEH_StackAlloc or SEH_SetFrame outside of prolog");
+          llvm_unreachable("SEH_StackAlloc outside of prolog");
         // Assume a large alloc...
-        ApproximatePrologCodeCount +=
-            (MI.getOpcode() == X86::SEH_StackAlloc) ? 3 : 1;
+        ApproximatePrologCodeCount += 3;
         HasStackAlloc = true;
         break;
 
+      case X86::SEH_SetFrame:
+        if (State != FunctionState::InProlog)
+          llvm_unreachable("SEH_SetFrame outside of prolog");
+        ApproximatePrologCodeCount++;
+        HasSetFrame = true;
+        break;
+
       case X86::SEH_SaveReg:
       case X86::SEH_SaveXMM:
         if (State != FunctionState::InProlog)
@@ -190,8 +201,30 @@ bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
         State = FunctionState::FinishedEpilog;
         break;
 
-      case X86::LEA64r:
       case X86::MOV64rr:
+        if (State == FunctionState::InEpilog) {
+          // If the prolog contains a stack allocation, then the first
+          // instruction in the epilog must be to adjust the stack pointer.
+          if (!HasSetFrame)
+            return rejectCurrentFunctionInternalError(
+                MF, Mode,
+                "The epilog is setting frame back, but prolog did not set it");
+          if (PoppedRegCount > 0)
+            return rejectCurrentFunctionInternalError(
+                MF, Mode,
+                "The epilog is setting the frame back after popping "
+                "registers");
+          if (HasStackDealloc)
+            return rejectCurrentFunctionInternalError(
+                MF, Mode,
+                "Cannot set the frame back after the stack "
+                "allocation has been deallocated");
+        } else if (State == FunctionState::FinishedEpilog)
+          return rejectCurrentFunctionInternalError(
+              MF, Mode, "Unexpected mov instruction after the epilog");
+        break;
+
+      case X86::LEA64r:
       case X86::ADD64ri32:
         if (State == FunctionState::InEpilog) {
           // If the prolog contains a stack allocation, then the first
@@ -211,8 +244,7 @@ bool X86WinEHUnwindV2::runOnMachineFunction(MachineFunction &MF) {
           HasStackDealloc = true;
         } else if (State == FunctionState::FinishedEpilog)
           return rejectCurrentFunctionInternalError(
-              MF, Mode,
-              "Unexpected lea, mov or add instruction after the epilog");
+              MF, Mode, "Unexpected lea or add instruction after the epilog");
         break;
 
       case X86::POP64r:
diff --git a/llvm/test/CodeGen/X86/win64-eh-unwindv2-errors.mir b/llvm/test/CodeGen/X86/win64-eh-unwindv2-errors.mir
@@ -106,7 +106,7 @@ body:             |
 # RUN:    -x86-wineh-unwindv2-force-mode=1 |  FileCheck %s \
 # RUN:    --check-prefix=BESTEFFORT
 # DEALLOC-AFTER-EPILOG: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'dealloc_after_epilog':
-# DEALLOC-AFTER-EPILOG-SAME: Unexpected lea, mov or add instruction after the epilog
+# DEALLOC-AFTER-EPILOG-SAME: Unexpected lea or add instruction after the epilog
 
 --- |
   define dso_local void @dealloc_after_epilog() local_unnamed_addr {
@@ -161,6 +161,135 @@ body:             |
     RET64
 ...
 
+;--- mov_no_setframe.mir
+# RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - \
+# RUN:    %t/mov_no_setframe.mir -run-pass=x86-wineh-unwindv2 2>&1 | \
+# RUN:    FileCheck %s --check-prefix=MOV-NO-SETFRAME
+# RUN: llc -mtriple=x86_64-pc-windows-msvc -o - %t/mov_no_setframe.mir \
+# RUN:    -run-pass=x86-wineh-unwindv2 -x86-wineh-unwindv2-force-mode=1 | \
+# RUN:    FileCheck %s --check-prefix=BESTEFFORT
+# MOV-NO-SETFRAME: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'mov_no_setframe':
+# MOV-NO-SETFRAME-SAME: The epilog is setting frame back, but prolog did not set it
+
+--- |
+  define dso_local void @mov_no_setframe() local_unnamed_addr {
+  entry:
+    ret void
+  }
+  !llvm.module.flags = !{!0}
+  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
+...
+---
+name:            mov_no_setframe
+body:             |
+  bb.0.entry:
+    frame-setup SEH_EndPrologue
+    SEH_BeginEpilogue
+    $rsp = MOV64rr $rbp
+    SEH_EndEpilogue
+    RET64
+...
+
+;--- mov_after_epilog.mir
+# RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - \
+# RUN:    %t/mov_after_epilog.mir -run-pass=x86-wineh-unwindv2 2>&1 | \
+# RUN:    FileCheck %s --check-prefix=MOV-AFTER-EPILOG
+# RUN: llc -mtriple=x86_64-pc-windows-msvc -o - \
+# RUN:    %t/mov_after_epilog.mir -run-pass=x86-wineh-unwindv2 \
+# RUN:    -x86-wineh-unwindv2-force-mode=1 |  FileCheck %s \
+# RUN:    --check-prefix=BESTEFFORT
+# MOV-AFTER-EPILOG: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'mov_after_epilog':
+# MOV-AFTER-EPILOG-SAME: Unexpected mov instruction after the epilog
+
+--- |
+  define dso_local void @mov_after_epilog() local_unnamed_addr {
+  entry:
+    ret void
+  }
+  !llvm.module.flags = !{!0}
+  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
+...
+---
+name:            mov_after_epilog
+body:             |
+  bb.0.entry:
+    $rbp = MOV64rr $rsp
+    frame-setup SEH_SetFrame 52, 0
+    frame-setup SEH_EndPrologue
+    SEH_BeginEpilogue
+    SEH_EndEpilogue
+    $rsp = MOV64rr $rbp
+    RET64
+...
+
+;--- pop_before_mov.mir
+# RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - \
+# RUN:    %t/pop_before_mov.mir -run-pass=x86-wineh-unwindv2 2>&1 | \
+# RUN:    FileCheck %s --check-prefix=POP-BEFORE-MOV
+# RUN: llc -mtriple=x86_64-pc-windows-msvc -o - %t/pop_before_mov.mir \
+# RUN:    -run-pass=x86-wineh-unwindv2 -x86-wineh-unwindv2-force-mode=1 | \
+# RUN:    FileCheck %s --check-prefix=BESTEFFORT
+# POP-BEFORE-MOV: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'pop_before_mov':
+# POP-BEFORE-MOV-SAME: The epilog is setting the frame back after popping registers
+
+--- |
+  define dso_local void @pop_before_mov() local_unnamed_addr {
+  entry:
+    ret void
+  }
+  !llvm.module.flags = !{!0}
+  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
+...
+---
+name:            pop_before_mov
+body:             |
+  bb.0.entry:
+    frame-setup PUSH64r killed $rdi, implicit-def $rsp, implicit $rsp
+    frame-setup SEH_PushReg 55
+    $rbp = MOV64rr $rsp
+    frame-setup SEH_SetFrame 52, 0
+    frame-setup SEH_EndPrologue
+    SEH_BeginEpilogue
+    $rdi = frame-destroy POP64r implicit-def $rsp, implicit $rsp
+    $rsp = MOV64rr $rbp
+    SEH_EndEpilogue
+    RET64
+...
+
+;--- mov_after_dealloc.mir
+# RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - \
+# RUN:    %t/mov_after_dealloc.mir -run-pass=x86-wineh-unwindv2 2>&1 | \
+# RUN:    FileCheck %s --check-prefix=MOV-AFTER-DEALLOC
+# RUN: llc -mtriple=x86_64-pc-windows-msvc -o - %t/mov_after_dealloc.mir \
+# RUN:    -run-pass=x86-wineh-unwindv2 -x86-wineh-unwindv2-force-mode=1 | \
+# RUN:    FileCheck %s --check-prefix=BESTEFFORT
+# MOV-AFTER-DEALLOC: LLVM ERROR: Windows x64 Unwind v2 is required, but LLVM has generated incompatible code in function 'mov_after_dealloc':
+# MOV-AFTER-DEALLOC-SAME: Cannot set the frame back after the stack allocation has been deallocated
+
+--- |
+  define dso_local void @mov_after_dealloc() local_unnamed_addr {
+  entry:
+    ret void
+  }
+  !llvm.module.flags = !{!0}
+  !0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
+...
+---
+name:            mov_after_dealloc
+body:             |
+  bb.0.entry:
+    $rbp = MOV64rr $rsp
+    frame-setup SEH_SetFrame 52, 0
+    $rsp = frame-setup SUB64ri32 $rsp, 40, implicit-def dead $eflags
+    frame-setup SEH_StackAlloc 40
+    frame-setup SEH_EndPrologue
+    SEH_BeginEpilogue
+    $rsp = frame-destroy ADD64ri32 $rsp, 40, implicit-def dead $eflags
+    $rsp = MOV64rr $rbp
+    SEH_EndEpilogue
+    RET64
+...
+
 ;--- too_many_pops.mir
 # RUN: not --crash llc -mtriple=x86_64-pc-windows-msvc -o - %t/too_many_pops.mir \
 # RUN:    -run-pass=x86-wineh-unwindv2 2>&1 | FileCheck %s \
diff --git a/llvm/test/CodeGen/X86/win64-eh-unwindv2.ll b/llvm/test/CodeGen/X86/win64-eh-unwindv2.ll
@@ -171,9 +171,28 @@ define dso_local void @large_aligned_alloc() align 16 {
 ; CHECK-NEXT:   retq
 ; CHECK-NEXT:   .seh_endproc
 
+define dso_local void @set_frame_only() local_unnamed_addr {
+  tail call i64 @llvm.x86.flags.read.u64()
+  ret void
+}
+
+; CHECK-LABEL:  set_frame_only:
+; CHECK:        .seh_unwindversion 2
+; CHECK:        .seh_pushreg %rbp
+; CHECK:        .seh_setframe %rbp, 0
+; CHECK:        .seh_endprologue
+; CHECK-NOT:    .seh_endproc
+; CHECK:        .seh_startepilogue
+; CHECK-NEXT:   .seh_unwindv2start
+; CHECK-NEXT:   popq    %rbp
+; CHECK-NEXT:   .seh_endepilogue
+; CHECK-NEXT:   retq
+; CHECK-NEXT:   .seh_endproc
+
+declare i64 @llvm.x86.flags.read.u64()
 declare void @a() local_unnamed_addr
 declare i32 @b() local_unnamed_addr
 declare i32 @c(i32) local_unnamed_addr
 
 !llvm.module.flags = !{!0}
-!0 = !{i32 1, !"winx64-eh-unwindv2", i32 1}
+!0 = !{i32 1, !"winx64-eh-unwindv2", i32 2}
