[PATCH] D140206: [RISCV] Ommit SRA in case of setlt or setge with zero constant

[Elena Lepilkina via Phabricator via llvm-commits]

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Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D140206

Files:
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/branch_zero.ll


Index: llvm/test/CodeGen/RISCV/branch_zero.ll
===================================================================
--- llvm/test/CodeGen/RISCV/branch_zero.ll
+++ llvm/test/CodeGen/RISCV/branch_zero.ll
@@ -8,7 +8,6 @@
 ; CHECK-NEXT:  .LBB0_1: # %for.body
 ; CHECK-NEXT:    # =>This Inner Loop Header: Depth=1
 ; CHECK-NEXT:    slli a0, a0, 48
-; CHECK-NEXT:    srai a0, a0, 48
 ; CHECK-NEXT:    bltz a0, .LBB0_4
 ; CHECK-NEXT:  # %bb.2: # %while.cond.preheader.i
 ; CHECK-NEXT:    # in Loop: Header=BB0_1 Depth=1
Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -9527,6 +9527,19 @@
 static bool combine_CC(SDValue &LHS, SDValue &RHS, SDValue &CC, const SDLoc &DL,
                        SelectionDAG &DAG, const RISCVSubtarget &Subtarget) {
   ISD::CondCode CCVal = cast<CondCodeSDNode>(CC)->get();
+
+  // As far as arithmetic right shift always saves the sign,
+  // shift can be omitted.
+  // Fold setlt (sra X, N), 0 -> setlt X, 0 and
+  // setge (sra X, N), 0 -> setge X, 0
+  if (auto *RHSConst = dyn_cast<ConstantSDNode>(RHS.getNode())) {
+    if ((CCVal == ISD::SETGE || CCVal == ISD::SETLT) &&
+        LHS.getOpcode() == ISD::SRA && RHSConst->isZero()) {
+      LHS = LHS.getOperand(0);
+      return true;
+    }
+  }
+
   if (!ISD::isIntEqualitySetCC(CCVal))
     return false;
 


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