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

[Craig Topper via llvm-commits]

================
@@ -58,11 +70,62 @@ class RISCVFoldMasks : public MachineFunctionPass {
 
 } // namespace
 
-char RISCVFoldMasks::ID = 0;
+char RISCVVectorPeephole::ID = 0;
+
+INITIALIZE_PASS(RISCVVectorPeephole, 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 RISCVVectorPeephole::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();
----------------
topperc wrote:

Do we need to check that the shift amount isn't out of bounds?

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