[llvm] [AArch64][SVE] Avoid movprfx by reusing register for _UNDEF pseudos. (PR #166926)

[Paul Walker via llvm-commits]

================
@@ -1123,24 +1123,83 @@ unsigned AArch64RegisterInfo::getRegPressureLimit(const TargetRegisterClass *RC,
   }
 }
 
-// FORM_TRANSPOSED_REG_TUPLE nodes are created to improve register allocation
-// where a consecutive multi-vector tuple is constructed from the same indices
-// of multiple strided loads. This may still result in unnecessary copies
-// between the loads and the tuple. Here we try to return a hint to assign the
-// contiguous ZPRMulReg starting at the same register as the first operand of
-// the pseudo, which should be a subregister of the first strided load.
+// We add regalloc hints for different cases:
+// * Choosing a better destination operand for predicated SVE instructions
+//   where the inactive lanes are undef, by choosing a register that is not
+//   unique to the other operands of the instruction.
 //
-// For example, if the first strided load has been assigned $z16_z20_z24_z28
-// and the operands of the pseudo are each accessing subregister zsub2, we
-// should look through through Order to find a contiguous register which
-// begins with $z24 (i.e. $z24_z25_z26_z27).
+// * Improve register allocation for SME multi-vector instructions where we can
+//   benefit from the strided- and contiguous register multi-vector tuples.
 //
+//   Here FORM_TRANSPOSED_REG_TUPLE nodes are created to improve register
+//   allocation where a consecutive multi-vector tuple is constructed from the
+//   same indices of multiple strided loads. This may still result in
+//   unnecessary copies between the loads and the tuple. Here we try to return a
+//   hint to assign the contiguous ZPRMulReg starting at the same register as
+//   the first operand of the pseudo, which should be a subregister of the first
+//   strided load.
+//
+//   For example, if the first strided load has been assigned $z16_z20_z24_z28
+//   and the operands of the pseudo are each accessing subregister zsub2, we
+//   should look through through Order to find a contiguous register which
+//   begins with $z24 (i.e. $z24_z25_z26_z27).
 bool AArch64RegisterInfo::getRegAllocationHints(
     Register VirtReg, ArrayRef<MCPhysReg> Order,
     SmallVectorImpl<MCPhysReg> &Hints, const MachineFunction &MF,
     const VirtRegMap *VRM, const LiveRegMatrix *Matrix) const {
-
   auto &ST = MF.getSubtarget<AArch64Subtarget>();
+  const AArch64InstrInfo *TII =
+      MF.getSubtarget<AArch64Subtarget>().getInstrInfo();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+
+  // For predicated SVE instructions where the inactive lanes are undef,
+  // pick a destination register that is not unique to avoid introducing
+  // a movprfx to copy a unique register to the destination operand.
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paulwalker-arm wrote:

Up to you but you could just stop after "....avoid introducing a movprfx."

https://github.com/llvm/llvm-project/pull/166926