Commit 988a5334 authored by Reid Kleckner's avatar Reid Kleckner
Browse files

[Win64] Ensure all stack frames are 8 byte aligned 

The unwind info format requires that all adjustments are 8 byte aligned,
and the bottom three bits are masked out. Most Win64 calling conventions
have 32 bytes of shadow stack space for spilling parameters, and I
believe that constructing these fixed stack objects had the side effect
of ensuring an alignment of 8. However, the Intel regcall convention
does not have this shadow space, so when using that convention, it was
possible to make a 4 byte stack frame, which was impossible to describe
with unwind info.

Fixes pr48867
parent c8fc5c03

llvm/lib/Target/X86/X86FrameLowering.cpp

+12 −4
Original line number Diff line number Diff line
@@ -3516,13 +3516,21 @@ void X86FrameLowering::processFunctionBeforeFrameFinalized( 
  // emitPrologue if it gets called and emits CFI.

  MF.setHasWinCFI(false);



  // If we are using Windows x64 CFI, ensure that the stack is always 8 byte

  // aligned. The format doesn't support misaligned stack adjustments.

  if (MF.getTarget().getMCAsmInfo()->usesWindowsCFI())

    MF.getFrameInfo().ensureMaxAlignment(Align(SlotSize));



  // If this function isn't doing Win64-style C++ EH, we don't need to do

  // anything.

  const Function &F = MF.getFunction();

  if (!STI.is64Bit() || !MF.hasEHFunclets() ||

      classifyEHPersonality(F.getPersonalityFn()) != EHPersonality::MSVC_CXX)

    return;

  if (STI.is64Bit() && MF.hasEHFunclets() &&

      classifyEHPersonality(MF.getFunction().getPersonalityFn()) ==

          EHPersonality::MSVC_CXX) {

    adjustFrameForMsvcCxxEh(MF);

  }

}



void X86FrameLowering::adjustFrameForMsvcCxxEh(MachineFunction &MF) const {

  // Win64 C++ EH needs to allocate the UnwindHelp object at some fixed offset

  // relative to RSP after the prologue.  Find the offset of the last fixed

  // object, so that we can allocate a slot immediately following it. If there

llvm/lib/Target/X86/X86FrameLowering.h

+2 −0
Original line number Diff line number Diff line
@@ -224,6 +224,8 @@ private: 
                                       const DebugLoc &DL, uint64_t Offset,

                                       uint64_t Align) const;



  void adjustFrameForMsvcCxxEh(MachineFunction &MF) const;



  /// Aligns the stack pointer by ANDing it with -MaxAlign.

  void BuildStackAlignAND(MachineBasicBlock &MBB,

                          MachineBasicBlock::iterator MBBI, const DebugLoc &DL,

llvm/test/CodeGen/X86/win64_regcall.ll

0 → 100644
+12 −0
Original line number Diff line number Diff line 
; RUN: llc < %s -mtriple=x86_64-windows-msvc | FileCheck %s



define dso_local x86_regcallcc void @ensure_align() local_unnamed_addr #0 {

entry:

  %b = alloca i32, align 4

  call void asm sideeffect "nopl $0", "=*m,~{dirflag},~{fpsr},~{flags}"(i32* nonnull %b)

  ret void

}



; CHECK-LABEL: ensure_align: # @ensure_align

; CHECK: .seh_stackalloc 8

; CHECK: .seh_endprologue