[PATCH] D127080: [DAGCombiner][RISCV] Improve computeKnownBits for (smin X, C) where C is negative.
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[DAGCombiner][RISCV] Improve computeKnownBits for (smin X, C) where C is negative.
If C is negative, the result of the smin must also be
negative, so all sign bits of the result are 1.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D127080
Files:
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
llvm/test/CodeGen/RISCV/min-max.ll
Index: llvm/test/CodeGen/RISCV/min-max.ll
===================================================================
--- llvm/test/CodeGen/RISCV/min-max.ll
+++ llvm/test/CodeGen/RISCV/min-max.ll
@@ -638,3 +638,22 @@
%c = call i32 @llvm.smax.i32(i32 %a, i32 10)
ret i32 %c
}
+
+define signext i32 @smin_i32_pos_constant(i32 signext %a) {
+; NOZBB-LABEL: smin_i32_pos_constant:
+; NOZBB: # %bb.0:
+; NOZBB-NEXT: li a1, -10
+; NOZBB-NEXT: blt a0, a1, .LBB25_2
+; NOZBB-NEXT: # %bb.1:
+; NOZBB-NEXT: li a0, -10
+; NOZBB-NEXT: .LBB25_2:
+; NOZBB-NEXT: ret
+;
+; ZBB-LABEL: smin_i32_pos_constant:
+; ZBB: # %bb.0:
+; ZBB-NEXT: li a1, -10
+; ZBB-NEXT: min a0, a0, a1
+; ZBB-NEXT: ret
+ %c = call i32 @llvm.smin.i32(i32 %a, i32 -10)
+ ret i32 %c
+}
Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3699,8 +3699,8 @@
}
// For SMAX, if CstLow is non-negative we know the result will be
// non-negative and thus all sign bits are 0.
- // TODO: There's an equivalent of this for smin with negative constant for
- // known ones.
+ // For SMIN, if CstHigh is negative we know the result will be
+ // negative and thus all sign bits are 1.
if (IsMax && CstLow) {
const APInt &ValueLow = CstLow->getAPIntValue();
if (ValueLow.isNonNegative()) {
@@ -3710,6 +3710,15 @@
}
}
+ if (!IsMax && CstHigh) {
+ const APInt &ValueHigh = CstHigh->getAPIntValue();
+ if (ValueHigh.isNegative()) {
+ unsigned SignBits = ComputeNumSignBits(Op.getOperand(0), Depth + 1);
+ Known.One.setHighBits(std::min(SignBits, ValueHigh.getNumSignBits()));
+ break;
+ }
+ }
+
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
if (IsMax)
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