[RISCV] Begin moving post-isel vector peepholes to a MF pass (#70342)

We currently have three postprocess peephole optimisations for vector
pseudos:

1) Masked pseudo with all ones mask -> unmasked pseudo
2) Merge vmerge pseudo into operand pseudo's mask
3) vmerge pseudo with all ones mask -> vmv.v.v pseudo

This patch aims to move these peepholes out of SelectionDAG and into a
separate RISCVFoldMasks MachineFunction pass.

There are a few motivations for doing this:

* The current SelectionDAG implementation operates on MachineSDNodes,
which are essentially MachineInstrs but require a bunch of logic to
reason about chain and glue operands. The RISCVII::has*Op helper
functions also don't exactly line up with the SDNode operands. Mutating
these pseudos and their operands in place becomes a good bit easier at
the MachineInstr level. For example, we would no longer need to check
for cycles in the DAG during performCombineVMergeAndVOps.

* Although it's further down the line, moving this code out of
SelectionDAG allows it to be reused by GlobalISel later on.

* In performCombineVMergeAndVOps, it may be possible to commute the
operands to enable folding in more cases (see
test/CodeGen/RISCV/rvv/vmadd-vp.ll). There is existing machinery to
commute operands in TII::commuteInstruction, but it's implemented on
MachineInstrs.

The pass runs straight after ISel, before any of the other machine SSA
optimization passes run. This is so that dead-mi-elimination can mop up
any vmsets that are no longer used (but if preferred we could try and
erase them from inside RISCVFoldMasks itself). This also means that
these peepholes are no longer run at codegen -O0, so this patch isn't
strictly NFC.

Only the performVMergeToVMv peephole is refactored in this patch, the
remaining two would be implemented later. And as noted by @preames, it
should be possible to move doPeepholeSExtW out of SelectionDAG as well.

1 files changed, 0 insertions, 36 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
index 94d1994..c2cac99 100644
--- a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
@@ -3685,40 +3685,6 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
   return true;
 }
 
-// Transform (VMERGE_VVM_<LMUL> false, false, true, allones, vl, sew) to
-// (VMV_V_V_<LMUL> false, true, vl, sew). It may decrease uses of VMSET.
-bool RISCVDAGToDAGISel::performVMergeToVMv(SDNode *N) {
-#define CASE_VMERGE_TO_VMV(lmul)                                               \
-  case RISCV::PseudoVMERGE_VVM_##lmul:                                    \
-    NewOpc = RISCV::PseudoVMV_V_V_##lmul;                                 \
-    break;
-  unsigned NewOpc;
-  switch (N->getMachineOpcode()) {
-  default:
-    llvm_unreachable("Expected VMERGE_VVM_<LMUL> instruction.");
-  CASE_VMERGE_TO_VMV(MF8)
-  CASE_VMERGE_TO_VMV(MF4)
-  CASE_VMERGE_TO_VMV(MF2)
-  CASE_VMERGE_TO_VMV(M1)
-  CASE_VMERGE_TO_VMV(M2)
-  CASE_VMERGE_TO_VMV(M4)
-  CASE_VMERGE_TO_VMV(M8)
-  }
-
-  if (!usesAllOnesMask(N, /* MaskOpIdx */ 3))
-    return false;
-
-  SDLoc DL(N);
-  SDValue PolicyOp =
-    CurDAG->getTargetConstant(/*TUMU*/ 0, DL, Subtarget->getXLenVT());
-  SDNode *Result = CurDAG->getMachineNode(
-      NewOpc, DL, N->getValueType(0),
-      {N->getOperand(1), N->getOperand(2), N->getOperand(4), N->getOperand(5),
-       PolicyOp});
-  ReplaceUses(N, Result);
-  return true;
-}
-
 bool RISCVDAGToDAGISel::doPeepholeMergeVVMFold() {
   bool MadeChange = false;
   SelectionDAG::allnodes_iterator Position = CurDAG->allnodes_end();
@@ -3730,8 +3696,6 @@ bool RISCVDAGToDAGISel::doPeepholeMergeVVMFold() {
 
     if (IsVMerge(N) || IsVMv(N))
       MadeChange |= performCombineVMergeAndVOps(N);
-    if (IsVMerge(N) && N->getOperand(0) == N->getOperand(1))
-      MadeChange |= performVMergeToVMv(N);
   }
   return MadeChange;
 }