[AArch64] Fold spills of COPY of WZR/XZR

Summary:
In AArch64InstrInfo::foldMemoryOperandImpl, catch more cases where the
COPY being spilled is copying from WZR/XZR, but the source register is
not in the COPY destination register's regclass.

For example, when spilling:

  %vreg0 = COPY %XZR ; %vreg0:GPR64common

without this change, the code in TargetInstrInfo::foldMemoryOperand()
and canFoldCopy() that normally handles cases like this would fail to
optimize since %XZR is not in GPR64common.  So the spill code generated
would be:

  %vreg0 = COPY %XZR
  STR %vreg

instead of the new code generated:

  STR %XZR

Reviewers: qcolombet, MatzeB

Subscribers: mcrosier, aemerson, t.p.northover, llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D26976

llvm-svn: 288176

1 files changed, 25 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index 465137f2750..7a3f39df659 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -2598,6 +2598,31 @@ MachineInstr *AArch64InstrInfo::foldMemoryOperandImpl(
     }
   }
 
+  // Handle the case where a WZR/XZR copy is being spilled but the destination
+  // register class doesn't contain WZR/XZR.  For example:
+  //
+  //   %vreg0<def> = COPY %XZR; GPR64common:%vreg0
+  //
+  // In this case we can still safely fold away the COPY and generate the
+  // following spill code:
+  //
+  //   STRXui %XZR, <fi#0>
+  //
+  if (MI.isFullCopy() && Ops.size() == 1 && Ops[0] == 0) {
+    MachineBasicBlock &MBB = *MI.getParent();
+    const MachineOperand &SrcMO = MI.getOperand(1);
+    unsigned SrcReg = SrcMO.getReg();
+    if (SrcReg == AArch64::WZR || SrcReg == AArch64::XZR) {
+      const TargetRegisterInfo &TRI = getRegisterInfo();
+      const TargetRegisterClass &RC = SrcReg == AArch64::WZR
+                                          ? AArch64::GPR32RegClass
+                                          : AArch64::GPR64RegClass;
+      storeRegToStackSlot(MBB, InsertPt, SrcReg, SrcMO.isKill(), FrameIndex,
+                          &RC, &TRI);
+      return &*--InsertPt;
+    }
+  }
+
   // Cannot fold.
   return nullptr;
 }