[llvm] [RISCV] Introduce local peephole to reduce VLs based on demanded VL (PR #104689)

[Luke Lau via llvm-commits]

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@@ -404,33 +476,17 @@ bool RISCVVectorPeephole::foldVMV_V_V(MachineInstr &MI) {
       SrcPassthru.getReg() != Passthru.getReg())
     return false;
 
-  // Because Src and MI have the same passthru, we can use either AVL as long as
-  // it's the smaller of the two.
-  //
-  // (src pt, ..., vl=5)       x x x x x|. . .
-  // (vmv.v.v pt, src, vl=3)   x x x|. . . . .
-  // ->
-  // (src pt, ..., vl=3)       x x x|. . . . .
-  //
-  // (src pt, ..., vl=3)       x x x|. . . . .
-  // (vmv.v.v pt, src, vl=6)   x x x . . .|. .
-  // ->
-  // (src pt, ..., vl=3)       x x x|. . . . .
+  // Src VL will have already been reduced if legal (see tryToReduceVL),
+  // so we don't need to handle a smaller source VL here.  However, the
+  // user's VL may be larger
   MachineOperand &SrcVL = Src->getOperand(RISCVII::getVLOpNum(Src->getDesc()));
   const MachineOperand *MinVL = getKnownMinVL(&MI.getOperand(3), &SrcVL);
-  if (!MinVL)
-    return false;
-
-  bool VLChanged = !MinVL->isIdenticalTo(SrcVL);
-  bool ActiveElementsAffectResult = RISCVII::activeElementsAffectResult(
-      TII->get(RISCV::getRVVMCOpcode(Src->getOpcode())).TSFlags);
-
-  if (VLChanged && (ActiveElementsAffectResult || Src->mayRaiseFPException()))
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lukel97 wrote:

Very satisfying to see this generalized. Nit, could you update the comment above `foldVMV_V_V` to reflect that Src's VL doesn't change anymore and that instead we check for MI.VL >= Src.VL

https://github.com/llvm/llvm-project/pull/104689