[RISCV] Teach instruction selection to elide sext.w in some cases.

If a sext_inreg is up for isel, and all its users are W instructions,
we can skip emitting the sext_inreg. This helpful if the producing
instruction can't become a W instruction.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D108966

Author: topperc

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -1536,6 +1536,7 @@ bool RISCVDAGToDAGISel::selectZExti32(SDValue N, SDValue &Val) {
 bool RISCVDAGToDAGISel::hasAllNBitUsers(SDNode *Node, unsigned Bits) const {
   assert((Node->getOpcode() == ISD::ADD || Node->getOpcode() == ISD::SUB ||
           Node->getOpcode() == ISD::MUL || Node->getOpcode() == ISD::SHL ||
+          Node->getOpcode() == ISD::SIGN_EXTEND_INREG ||
           isa<ConstantSDNode>(Node)) &&
          "Unexpected opcode");
 

llvm/lib/Target/RISCV/RISCVInstrInfo.td

@@ -1262,10 +1262,18 @@ class overflowingbinopw<SDPatternOperator operator>
   return hasAllWUsers(Node);
 }]>;
 
+def sexti32_allwusers : PatFrag<(ops node:$src),
+                                (sext_inreg node:$src, i32), [{
+  return hasAllWUsers(Node);
+}]>;
+
 let Predicates = [IsRV64] in {
 
 /// sext and zext
 
+// Sign extend is not needed if all users are W instructions.
+def : Pat<(sexti32_allwusers GPR:$rs1), (XLenVT GPR:$rs1)>;
+
 def : Pat<(sext_inreg GPR:$rs1, i32), (ADDIW GPR:$rs1, 0)>;
 
 /// ALU operations

llvm/test/CodeGen/RISCV/rv64zba.ll

@@ -376,7 +376,6 @@ define i64 @sh3adduw_2(i64 %0, i64 %1) {
 ; to remove the sext_inreg because it has multiple uses. The ashr will use the
 ; sext_inreg to become sraiw. This leaves the sext_inreg only used by the shl.
 ; If the shl is selected as sllw, we don't need the sext_inreg.
-; FIXME: We should not emit a sext.w.
 define i64 @sh2add_extra_sext(i32 %x, i32 %y, i32 %z) {
 ; RV64I-LABEL: sh2add_extra_sext:
 ; RV64I:       # %bb.0:
@@ -390,17 +389,15 @@ define i64 @sh2add_extra_sext(i32 %x, i32 %y, i32 %z) {
 ; RV64B-LABEL: sh2add_extra_sext:
 ; RV64B:       # %bb.0:
 ; RV64B-NEXT:    sh2add a0, a0, a1
-; RV64B-NEXT:    sext.w a1, a0
-; RV64B-NEXT:    sllw a1, a2, a1
+; RV64B-NEXT:    sllw a1, a2, a0
 ; RV64B-NEXT:    sraiw a0, a0, 2
 ; RV64B-NEXT:    mul a0, a1, a0
 ; RV64B-NEXT:    ret
 ;
 ; RV64ZBA-LABEL: sh2add_extra_sext:
 ; RV64ZBA:       # %bb.0:
 ; RV64ZBA-NEXT:    sh2add a0, a0, a1
-; RV64ZBA-NEXT:    sext.w a1, a0
-; RV64ZBA-NEXT:    sllw a1, a2, a1
+; RV64ZBA-NEXT:    sllw a1, a2, a0
 ; RV64ZBA-NEXT:    sraiw a0, a0, 2
 ; RV64ZBA-NEXT:    mul a0, a1, a0
 ; RV64ZBA-NEXT:    ret