[llvm] [RISCV] Convert AVLs with vlenb to VLMAX where possible (PR #97800)

[Craig Topper via llvm-commits]

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@@ -62,6 +69,54 @@ char RISCVFoldMasks::ID = 0;
 
 INITIALIZE_PASS(RISCVFoldMasks, DEBUG_TYPE, "RISC-V Fold Masks", false, false)
 
+// If an AVL is a VLENB that's possibly scaled to be equal to VLMAX, convert it
+// to the VLMAX sentinel value.
+bool RISCVFoldMasks::convertToVLMAX(MachineInstr &MI) const {
+  if (!RISCVII::hasVLOp(MI.getDesc().TSFlags) ||
+      !RISCVII::hasSEWOp(MI.getDesc().TSFlags))
+    return false;
+  MachineOperand &VL = MI.getOperand(RISCVII::getVLOpNum(MI.getDesc()));
+  if (!VL.isReg())
+    return false;
+  MachineInstr *Def = MRI->getVRegDef(VL.getReg());
+  if (!Def)
+    return false;
+
+  // Fixed-point value, denominator=8
+  unsigned ScaleFixed = 8;
+  // Check if the VLENB was potentially scaled with slli/srli
+  if (Def->getOpcode() == RISCV::SLLI) {
+    ScaleFixed <<= Def->getOperand(2).getImm();
+    Def = MRI->getVRegDef(Def->getOperand(1).getReg());
+  } else if (Def->getOpcode() == RISCV::SRLI) {
+    ScaleFixed >>= Def->getOperand(2).getImm();
+    Def = MRI->getVRegDef(Def->getOperand(1).getReg());
+  }
+
+  if (!Def || Def->getOpcode() != RISCV::PseudoReadVLENB)
+    return false;
+
+  auto LMUL = RISCVVType::decodeVLMUL(RISCVII::getLMul(MI.getDesc().TSFlags));
+  // Fixed-point value, denominator=8
+  unsigned LMULFixed = LMUL.second ? (8 / LMUL.first) : 8 * LMUL.first;
+  unsigned SEW =
+      1 << MI.getOperand(RISCVII::getSEWOpNum(MI.getDesc())).getImm();
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topperc wrote:

Does this need to account for mask instructions having an SEW operand of 0?

https://github.com/llvm/llvm-project/pull/97800