[ARM] Don't reserve R12 on Thumb1 as an emergency spill slot.

The current implementation of ThumbRegisterInfo::saveScavengerRegister
is bad for two reasons: one, it's buggy, and two, it blocks using R12
for other optimizations.  So this patch gets rid of it, and adds the
necessary support for using an ordinary emergency spill slot on Thumb1.

(Specifically, I think saveScavengerRegister was broken by r305625, and
nobody noticed for two years because the codepath is almost never used.
The new code will also probably not be used much, but it now has better
tests, and if we fail to emit a necessary emergency spill slot we get a
reasonable error message instead of a miscompile.)

A rough outline of the changes in the patch:

1. Gets rid of ThumbRegisterInfo::saveScavengerRegister.
2. Modifies ARMFrameLowering::determineCalleeSaves to allocate an
emergency spill slot for Thumb1.
3. Implements useFPForScavengingIndex, so the emergency spill slot isn't
placed at a negative offset from FP on Thumb1.
4. Modifies the heuristics for allocating an emergency spill slot to
support Thumb1.  This includes fixing ExtraCSSpill so we don't try to
use "lr" as a substitute for allocating an emergency spill slot.
5. Allocates a base pointer in more cases, so the emergency spill slot
is always accessible.
6. Modifies ARMFrameLowering::ResolveFrameIndexReference to compute the
right offset in the new cases where we're forcing a base pointer.
7. Ensures we never generate a load or store with an offset outside of
its frame object.  This makes the heuristics more straightforward.
8. Changes Thumb1 prologue and epilogue emission so it never uses
register scavenging.

Some of the changes to the emergency spill slot heuristics in
determineCalleeSaves affect ARM/Thumb2; hopefully, they should allow
the compiler to avoid allocating an emergency spill slot in cases
where it isn't necessary. The rest of the changes should only affect
Thumb1.

Differential Revision: https://reviews.llvm.org/D63677

llvm-svn: 364490

1 files changed, 11 insertions, 57 deletions

diff --git a/llvm/lib/Target/ARM/ThumbRegisterInfo.cpp b/llvm/lib/Target/ARM/ThumbRegisterInfo.cpp
index e485c79d3f3..a96417ffce4 100644
--- a/llvm/lib/Target/ARM/ThumbRegisterInfo.cpp
+++ b/llvm/lib/Target/ARM/ThumbRegisterInfo.cpp
@@ -446,63 +446,6 @@ void ThumbRegisterInfo::resolveFrameIndex(MachineInstr &MI, unsigned BaseReg,
   (void)Done;
 }
 
-/// saveScavengerRegister - Spill the register so it can be used by the
-/// register scavenger. Return true.
-bool ThumbRegisterInfo::saveScavengerRegister(
-    MachineBasicBlock &MBB, MachineBasicBlock::iterator I,
-    MachineBasicBlock::iterator &UseMI, const TargetRegisterClass *RC,
-    unsigned Reg) const {
-
-  const ARMSubtarget &STI = MBB.getParent()->getSubtarget<ARMSubtarget>();
-  if (!STI.isThumb1Only())
-    return ARMBaseRegisterInfo::saveScavengerRegister(MBB, I, UseMI, RC, Reg);
-
-  // Thumb1 can't use the emergency spill slot on the stack because
-  // ldr/str immediate offsets must be positive, and if we're referencing
-  // off the frame pointer (if, for example, there are alloca() calls in
-  // the function, the offset will be negative. Use R12 instead since that's
-  // a call clobbered register that we know won't be used in Thumb1 mode.
-  const TargetInstrInfo &TII = *STI.getInstrInfo();
-  DebugLoc DL;
-  BuildMI(MBB, I, DL, TII.get(ARM::tMOVr))
-      .addReg(ARM::R12, RegState::Define)
-      .addReg(Reg, RegState::Kill)
-      .add(predOps(ARMCC::AL));
-
-  // The UseMI is where we would like to restore the register. If there's
-  // interference with R12 before then, however, we'll need to restore it
-  // before that instead and adjust the UseMI.
-  bool done = false;
-  for (MachineBasicBlock::iterator II = I; !done && II != UseMI ; ++II) {
-    if (II->isDebugInstr())
-      continue;
-    // If this instruction affects R12, adjust our restore point.
-    for (unsigned i = 0, e = II->getNumOperands(); i != e; ++i) {
-      const MachineOperand &MO = II->getOperand(i);
-      if (MO.isRegMask() && MO.clobbersPhysReg(ARM::R12)) {
-        UseMI = II;
-        done = true;
-        break;
-      }
-      if (!MO.isReg() || MO.isUndef() || !MO.getReg() ||
-          TargetRegisterInfo::isVirtualRegister(MO.getReg()))
-        continue;
-      if (MO.getReg() == ARM::R12) {
-        UseMI = II;
-        done = true;
-        break;
-      }
-    }
-  }
-  // Restore the register from R12
-  BuildMI(MBB, UseMI, DL, TII.get(ARM::tMOVr))
-      .addReg(Reg, RegState::Define)
-      .addReg(ARM::R12, RegState::Kill)
-      .add(predOps(ARMCC::AL));
-
-  return true;
-}
-
 void ThumbRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
                                             int SPAdj, unsigned FIOperandNum,
                                             RegScavenger *RS) const {
@@ -618,3 +561,14 @@ void ThumbRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator II,
   if (MI.isPredicable())
     MIB.add(predOps(ARMCC::AL));
 }
+
+bool
+ThumbRegisterInfo::useFPForScavengingIndex(const MachineFunction &MF) const {
+  if (MF.getSubtarget<ARMSubtarget>().isThumb1Only()) {
+    // For Thumb1, the emergency spill slot must be some small positive
+    // offset from the base/stack pointer.
+    return false;
+  }
+  // For Thumb2, put the emergency spill slot next to FP.
+  return true;
+}