DAG: Fold bitcast of scalar_to_vector to anyext (#122660)

scalar_to_vector is difficult to make appear and test,
but I found one case where this makes an observable difference.
It fires more often than this in the test suite, but most of them
have no net result in the final code. This helps reduce regressions
in a future commit.

Author: arsenm

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -16012,6 +16012,14 @@ SDValue DAGCombiner::visitBITCAST(SDNode *N) {
     if (SDValue CombineLD = CombineConsecutiveLoads(N0.getNode(), VT))
       return CombineLD;
 
+  // int_vt (bitcast (vec_vt (scalar_to_vector elt_vt:x)))
+  //   => int_vt (any_extend elt_vt:x)
+  if (N0.getOpcode() == ISD::SCALAR_TO_VECTOR && VT.isScalarInteger()) {
+    SDValue SrcScalar = N0.getOperand(0);
+    if (SrcScalar.getValueType().isScalarInteger())
+      return DAG.getNode(ISD::ANY_EXTEND, SDLoc(N), VT, SrcScalar);
+  }
+
   // Remove double bitcasts from shuffles - this is often a legacy of
   // XformToShuffleWithZero being used to combine bitmaskings (of
   // float vectors bitcast to integer vectors) into shuffles.

llvm/test/CodeGen/AMDGPU/scalar_to_vector.ll

@@ -332,3 +332,33 @@ define amdgpu_kernel void @scalar_to_vector_test6(ptr addrspace(1) %out, i8 zero
   store <2 x half> %bc, ptr addrspace(1) %out
   ret void
 }
+
+; bitcast (scalar_to_vector x) -> any_extend x
+define i64 @bitcast_combine_scalar_to_vector_v4i16(i16 %arg) {
+; SI-LABEL: bitcast_combine_scalar_to_vector_v4i16:
+; SI:       ; %bb.0:
+; SI-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
+; SI-NEXT:    v_and_b32_e32 v1, 0xffff, v0
+; SI-NEXT:    v_and_b32_e32 v2, 0xff00, v0
+; SI-NEXT:    v_bfe_u32 v0, v0, 8, 8
+; SI-NEXT:    v_or_b32_e32 v2, v0, v2
+; SI-NEXT:    v_lshlrev_b32_e32 v3, 16, v2
+; SI-NEXT:    v_or_b32_e32 v0, v1, v3
+; SI-NEXT:    v_or_b32_e32 v1, v2, v3
+; SI-NEXT:    s_setpc_b64 s[30:31]
+;
+; GFX89-LABEL: bitcast_combine_scalar_to_vector_v4i16:
+; GFX89:       ; %bb.0:
+; GFX89-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
+; GFX89-NEXT:    v_and_b32_e32 v1, 0xffffff00, v0
+; GFX89-NEXT:    v_or_b32_sdwa v1, v0, v1 dst_sel:DWORD dst_unused:UNUSED_PAD src0_sel:BYTE_1 src1_sel:DWORD
+; GFX89-NEXT:    v_lshlrev_b32_e32 v2, 16, v1
+; GFX89-NEXT:    v_or_b32_sdwa v1, v1, v2 dst_sel:DWORD dst_unused:UNUSED_PAD src0_sel:WORD_0 src1_sel:DWORD
+; GFX89-NEXT:    v_or_b32_sdwa v0, v0, v2 dst_sel:DWORD dst_unused:UNUSED_PAD src0_sel:WORD_0 src1_sel:DWORD
+; GFX89-NEXT:    s_setpc_b64 s[30:31]
+  %arg.cast = bitcast i16 %arg to <2 x i8>
+  %tmp1 = shufflevector <2 x i8> %arg.cast, <2 x i8> poison, <8 x i32> <i32 0, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1>
+  %tmp2 = shufflevector <8 x i8> %tmp1, <8 x i8> poison, <8 x i32> <i32 0, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
+  %cast = bitcast <8 x i8> %tmp2 to i64
+  ret i64 %cast
+}