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

[Nick Desaulniers via Phabricator via llvm-commits]

nickdesaulniers marked an inline comment as done.
nickdesaulniers 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
----------------
rnk wrote:
> 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.
I think the red zone concerns are handled correctly already...

The only caller of `X86MachineFunctionInfo::setUsesRedZone` is guarded by a call to `X86FrameLowering::has128ByteRedZone`, which checks for the noredzone fn attr:


```
return Is64Bit && !IsWin64CC && !Fn.hasFnAttribute(Attribute::NoRedZone);
```


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

https://reviews.llvm.org/D92695