}
case Instruction::Shl: {
// (shl X, C1) & C2 == 0 iff (X & C2 >>u C1) == 0
- auto KZF = [BitWidth](const APInt &KnownZero, unsigned ShiftAmt) {
- return (KnownZero << ShiftAmt) |
+ bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
+ auto KZF = [BitWidth, NSW](const APInt &KnownZero, unsigned ShiftAmt) {
+ APInt KZResult = (KnownZero << ShiftAmt) |
APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
+ // If this shift has "nsw" keyword, then the result is either a poison
+ // value or has the same sign bit as the first operand.
+ if (NSW && KnownZero.isNegative())
+ KZResult.setBit(BitWidth - 1);
+ return KZResult;
};
- auto KOF = [BitWidth](const APInt &KnownOne, unsigned ShiftAmt) {
- return KnownOne << ShiftAmt;
+ auto KOF = [BitWidth, NSW](const APInt &KnownOne, unsigned ShiftAmt) {
+ APInt KOResult = KnownOne << ShiftAmt;
+ if (NSW && KnownOne.isNegative())
+ KOResult.setBit(BitWidth - 1);
+ return KOResult;
};
computeKnownBitsFromShiftOperator(I, KnownZero, KnownOne,
--- /dev/null
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; Result of left shifting a non-negative integer
+; with nsw flag should also be non-negative
+define i1 @test_shift_nonnegative(i32 %a) {
+; CHECK-LABEL: @test_shift_nonnegative(
+; CHECK: ret i1 true
+ %b = lshr i32 %a, 2
+ %shift = shl nsw i32 %b, 3
+ %cmp = icmp sge i32 %shift, 0
+ ret i1 %cmp
+}
+
+; Result of left shifting a negative integer with
+; nsw flag should also be negative
+define i1 @test_shift_negative(i32 %a, i32 %b) {
+; CHECK-LABEL: @test_shift_negative(
+; CHECK: ret i1 true
+ %c = or i32 %a, -2147483648
+ %d = and i32 %b, 7
+ %shift = shl nsw i32 %c, %d
+ %cmp = icmp slt i32 %shift, 0
+ ret i1 %cmp
+}