[PATCH] D92695: [X86ISelLowering] don't emit frame pointers for eflags intrinsics.

[Reid Kleckner via Phabricator via llvm-commits]

rnk added inline comments.


================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:25975-25976
-      // sequence.
-      MachineFrameInfo &MFI = DAG.getMachineFunction().getFrameInfo();
-      MFI.setHasCopyImplyingStackAdjustment(true);
       // Don't do anything here, we will expand these intrinsics out later
----------------
pengfei wrote:
> rnk wrote:
> > pengfei wrote:
> > > craig.topper wrote:
> > > > pengfei wrote:
> > > > > How about address Craig and Reid's concerns by adding the check conditions like above?
> > > > emitPrologue calls setUsesRedZone. That would occur after this code runs. This is too early to call getUsesRedZone.
> > > Maybe we can check for `-mno-red-zone` only given Linux kernel is built with it:
> > > ```
> > > MachineFunction &MF = DAG.getMachineFunction();
> > > if (!MF.getFunction().hasFnAttribute(Attribute::NoRedZone))
> > >   MF.getFrameInfo().setHasCopyImplyingStackAdjustment(true);
> > > ```
> > I would try to tackle this the other way around: usage of readflags/writeflags should prevent PEI from enabling the redzone optimization.
> > 
> > I think what we really want here is to tweak X86FrameLowering::hasFP.
> > If WinCFI is being used, where we have per-instruction accurate CFI, we need to force the use of a frame pointer.
> > For Linux, this flag should prevent the use of a red zone later in X86FrameLowering::emitProlog. We don't need to force FP usage if WinCFI is not in use.
> We can't check `hasWinCFI` here due to the same reason as `getUsesRedZone`, I think we can check `isOSWindows` instead.
> Regarding red zone, I think it is a great idea. So the code can be
> ```
> --- a/llvm/lib/Target/X86/X86ISelLowering.cpp
> +++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
> @@ -27148,8 +27148,11 @@ static SDValue LowerINTRINSIC_W_CHAIN(SDValue Op, const X86Subtarget &Subtarget,
>      case llvm::Intrinsic::x86_flags_write_u64: {
>        // We need a frame pointer because this will get lowered to a PUSH/POP
>        // sequence.
> -      MachineFrameInfo &MFI = DAG.getMachineFunction().getFrameInfo();
> -      MFI.setHasCopyImplyingStackAdjustment(true);
> +      MachineFunction &MF = DAG.getMachineFunction();
> +      if (Subtarget.getTargetTriple().isOSWindows())
> +        MF.getFrameInfo().setHasCopyImplyingStackAdjustment(true);
> +      else
> +        MF.getFunction().addFnAttr(Attribute::NoRedZone);
>        // Don't do anything here, we will expand these intrinsics out later
>        // during FinalizeISel in EmitInstrWithCustomInserter.
>        return Op;
> ```
> I checked it with latest xmain. No test will be affected.
Right, I didn't mean that this code should check if CFI has been emitted, but whether WinCFI should be used. I believe this information is carried on MCAsmInfo. The proposed triple check is incorrect, I don't think we need to force FP usage on 32-bit Windows, it's really specific to Win64. Please use the wincfi check, it is more self-documenting.


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  https://reviews.llvm.org/D92695/new/

https://reviews.llvm.org/D92695