[RISCV] Use a DAG combine to prune pointless vrgather.vi (#135392)

If the vrgather.vi is preceeded by a vmv.v.x which writes a superset of
the lanes writen by the vrgather, and the vrgather has no passthru, then
the vrgather has no semantic effect.

This is the start of a mini-series of patches around rewriting
vrgather.vi/vx preceeded by vmv.v.x, vfmf.v.f, vmv.s.x, etc... Starting
with the simplest, but also lowest impact.

One point I'd like a second oppinion on is the out of bounds semenatic
change. As far as I can tell, all the indices are in bounds by
construction. The doc change is as much as I couldn't figure out how to
test the alternative as anything else.

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -19709,6 +19709,19 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
     if (SDValue V = combineToVCPOP(N, DAG, Subtarget))
       return V;
     break;
+  case RISCVISD::VRGATHER_VX_VL: {
+    // Drop a redundant vrgather_vx.
+    // Note this assumes that out of bounds indices produce poison
+    // and can thus be replaced without having to prove them inbounds..
+    SDValue Src = N->getOperand(0);
+    SDValue Passthru = N->getOperand(2);
+    SDValue VL = N->getOperand(4);
+    // TODO: Handle fmv.v.f?
+    if (Src.getOpcode() == RISCVISD::VMV_V_X_VL && Passthru.isUndef() &&
+        VL == Src.getOperand(2))
+      return Src;
+    break;
+  }
   }
 
   return SDValue();

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -388,7 +388,8 @@ enum NodeType : unsigned {
   VMSET_VL,
 
   // Matches the semantics of vrgather.vx and vrgather.vv with extra operands
-  // for passthru and VL. Operands are (src, index, mask, passthru, vl).
+  // for passthru and VL, except that out of bound indices result in a poison
+  // result not zero.  Operands are (src, index, mask, passthru, vl).
   VRGATHER_VX_VL,
   VRGATHER_VV_VL,
   VRGATHEREI16_VV_VL,

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shuffle-int.ll

@@ -1346,10 +1346,9 @@ define <4 x i16> @vmerge_2(<4 x i16> %x) {
 define <4 x i16> @vmerge_3(<4 x i16> %x) {
 ; CHECK-LABEL: vmerge_3:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    vsetivli zero, 4, e16, mf2, ta, mu
+; CHECK-NEXT:    vsetivli zero, 4, e16, mf2, ta, ma
 ; CHECK-NEXT:    vmv.v.i v0, 6
-; CHECK-NEXT:    vmv.v.i v9, 5
-; CHECK-NEXT:    vrgather.vi v8, v9, 1, v0.t
+; CHECK-NEXT:    vmerge.vim v8, v8, 5, v0
 ; CHECK-NEXT:    ret
    %s = shufflevector <4 x i16> %x, <4 x i16> <i16 poison, i16 5, i16 poison, i16 poison>, <4 x i32> <i32 0, i32 5, i32 5, i32 3>
    ret <4 x i16> %s