[PATCH] Allow the register scavenger to spill multiple registers

[Richard Sandiford]

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.

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