[PATCH] Allow the register scavenger to spill multiple registers

[Hal Finkel]

----- Original Message -----
> From: "Richard Sandiford" <rdsandiford at googlemail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>, "Jakob Stoklund Olesen" <stoklund at 2pi.dk>
> Sent: Tuesday, March 26, 2013 6:40:05 AM
> Subject: Re: [PATCH] Allow the register scavenger to spill multiple registers
> 
> Hal Finkel <hfinkel-uKDnm65ik8M at public.gmane.org> writes:
> > Please review the attached patch which lets the register scavenger
> > make
> > use of multiple spill slots in order to guarantee that it will be
> > able
> > to provide multiple registers simultaneously. I intend to use this
> > capability in the PowerPC backend to handle spills where we need
> > two
> > registers (one for indexing and one for transfer) without keeping
> > an
> > extra reserved register.
> 
> This might have been discussed already, sorry, but: this patch seems
> to
> change the behaviour for targets that use something other than a
> frame
> index as scavenging spill space.  (E.g. Thumb 1 uses r12, MIPS16 uses
> t0.)
> Those targets never called setScavengingFrameIndex() and instead left
> the ScavengingFrameIndex as -1.  They used saveScavengerRegister to
> do
> the saving and restoring.

Ack, you're right! I'll reorder the check later today to allow the old behavior.

Thanks!

 -Hal

> 
> Targets that don't register a frame index now hit the assert:
> 
>   assert(SI < Scavenged.size() &&
>          "Scavenger slots are live, unable to scavenge another
>          register!");
> 
> before saveScavengerRegister() is called.
> 
> FWIW, a simple hack like calling addScavengingFrameIndex(-1) doesn't
> work
> because:
> 
> @@ -631,9 +633,11 @@ void
> PEI::calculateFrameObjectOffsets(MachineFunction &Fn) {
>    // stack pointer.
>    if (RS && (!TFI.hasFP(Fn) || RegInfo->needsStackRealignment(Fn) ||
>               !RegInfo->useFPForScavengingIndex(Fn))) {
> -    int SFI = RS->getScavengingFrameIndex();
> -    if (SFI >= 0)
> -      AdjustStackOffset(MFI, SFI, StackGrowsDown, Offset, MaxAlign);
> +    SmallVector<int, 2> SFIs;
> +    RS->getScavengingFrameIndices(SFIs);
> +    for (SmallVector<int, 2>::iterator I = SFIs.begin(),
> +         IE = SFIs.end(); I != IE; ++I)
> +      AdjustStackOffset(MFI, *I, StackGrowsDown, Offset, MaxAlign);
>    }
>  
>    if (!TFI.targetHandlesStackFrameRounding()) {
> 
> assumes every index is valid.
> 
> Richard
>