[PATCH] D131471: [RISCV] Fold (sub constant, (setcc x, y, eq/neq)) -> (add constant - 1, (setcc x, y, neq/eq))

[Liao Chunyu via Phabricator via llvm-commits]

liaolucy created this revision.
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(setcc x, y, eq/neq) are seqz, snez that set rd = 0/1.

addi is used to process immediate, which can save instructions for load immediate.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D131471

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/setcc-logic.ll


Index: llvm/test/CodeGen/RISCV/setcc-logic.ll
===================================================================
--- llvm/test/CodeGen/RISCV/setcc-logic.ll
+++ llvm/test/CodeGen/RISCV/setcc-logic.ll
@@ -118,3 +118,24 @@
   %r = and i1 %a, %b
   ret i1 %r
 }
+
+define i32 @bar(i32 %n) {
+; RV32I-LABEL: bar:
+; RV32I:       # %bb.0: # %entry
+; RV32I-NEXT:    addi a0, a0, -9
+; RV32I-NEXT:    snez a0, a0
+; RV32I-NEXT:    addi a0, a0, 1
+; RV32I-NEXT:    ret
+;
+; RV64I-LABEL: bar:
+; RV64I:       # %bb.0: # %entry
+; RV64I-NEXT:    sext.w a0, a0
+; RV64I-NEXT:    addi a0, a0, -9
+; RV64I-NEXT:    snez a0, a0
+; RV64I-NEXT:    addi a0, a0, 1
+; RV64I-NEXT:    ret
+entry:
+  %cmp = icmp eq i32 %n, 9
+  %a = select i1 %cmp, i32 1, i32 2
+  ret i32 %a
+}
Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -8264,10 +8264,32 @@
 }
 
 static SDValue performSUBCombine(SDNode *N, SelectionDAG &DAG) {
-  // fold (sub x, (select lhs, rhs, cc, 0, y)) ->
-  //      (select lhs, rhs, cc, x, (sub x, y))
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
+
+  // Prefer to make this 'add 0/1' rather than 'sub 0/1'
+  // sub constant(!0), 0/1 -> add constant - 1, 1/0
+  // NODE: constant == 0, No redundant instructions are generated.
+  // (sub constant, (setcc x, y, eq/neq)) ->
+  // (add constant - 1, (setcc x, y, neq/eq))
+  if (isa<ConstantSDNode>(N0) && N1.getOpcode() == ISD::SETCC) {
+    auto *Nnz0 = cast<ConstantSDNode>(N0);
+    const auto *CC = cast<CondCodeSDNode>(N1->getOperand(2));
+    ISD::CondCode CCVal = CC->get();
+    if (!Nnz0->isZero() && (CCVal == ISD::SETEQ || CCVal == ISD::SETNE)) {
+      EVT VT = N->getValueType(0);
+      int64_t ImmVal = cast<ConstantSDNode>(N0)->getSExtValue() - 1;
+      SDValue NewN0 = DAG.getConstant(ImmVal, SDLoc(N), VT);
+      SDValue CCInverse =
+          DAG.getCondCode(ISD::getSetCCInverse(CCVal, N0.getValueType()));
+      SDValue NewN1 = DAG.getNode(ISD::SETCC, SDLoc(N), VT, N1->getOperand(0),
+                                  N1->getOperand(1), CCInverse);
+      return DAG.getNode(ISD::ADD, SDLoc(N), VT, NewN0, NewN1);
+    }
+  }
+
+  // fold (sub x, (select lhs, rhs, cc, 0, y)) ->
+  //      (select lhs, rhs, cc, x, (sub x, y))
   return combineSelectAndUse(N, N1, N0, DAG, /*AllOnes*/ false);
 }
 


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