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The patch improves ValueTracking on left shift with nsw flag.
Summary: The patch fixes PR28946. Reviewers: majnemer, sanjoy Differential Revision: http://reviews.llvm.org/D23296 From: Li Huang llvm-svn: 279684
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@ -795,6 +795,14 @@ static void computeKnownBitsFromShiftOperator(
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computeKnownBits(I->getOperand(0), KnownZero, KnownOne, Depth + 1, Q);
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KnownZero = KZF(KnownZero, ShiftAmt);
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KnownOne = KOF(KnownOne, ShiftAmt);
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// If there is conflict between KnownZero and KnownOne, this must be an
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// overflowing left shift, so the shift result is undefined. Clear KnownZero
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// and KnownOne bits so that other code could propagate this undef.
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if ((KnownZero & KnownOne) != 0) {
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KnownZero.clearAllBits();
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KnownOne.clearAllBits();
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}
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return;
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}
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@ -1065,13 +1073,23 @@ static void computeKnownBitsFromOperator(const Operator *I, APInt &KnownZero,
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}
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case Instruction::Shl: {
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// (shl X, C1) & C2 == 0 iff (X & C2 >>u C1) == 0
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auto KZF = [BitWidth](const APInt &KnownZero, unsigned ShiftAmt) {
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return (KnownZero << ShiftAmt) |
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APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
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bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
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auto KZF = [BitWidth, NSW](const APInt &KnownZero, unsigned ShiftAmt) {
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APInt KZResult =
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(KnownZero << ShiftAmt) |
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APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
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// If this shift has "nsw" keyword, then the result is either a poison
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// value or has the same sign bit as the first operand.
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if (NSW && KnownZero.isNegative())
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KZResult.setBit(BitWidth - 1);
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return KZResult;
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};
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auto KOF = [BitWidth](const APInt &KnownOne, unsigned ShiftAmt) {
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return KnownOne << ShiftAmt;
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auto KOF = [BitWidth, NSW](const APInt &KnownOne, unsigned ShiftAmt) {
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APInt KOResult = KnownOne << ShiftAmt;
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if (NSW && KnownOne.isNegative())
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KOResult.setBit(BitWidth - 1);
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return KOResult;
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};
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computeKnownBitsFromShiftOperator(I, KnownZero, KnownOne,
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55
test/Analysis/ValueTracking/known-signbit-shift.ll
Normal file
55
test/Analysis/ValueTracking/known-signbit-shift.ll
Normal file
@ -0,0 +1,55 @@
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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
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; RUN: opt < %s -instcombine -S | FileCheck %s
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; Result of left shifting a non-negative integer
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; with nsw flag should also be non-negative
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define i1 @test_shift_nonnegative(i32 %a) {
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; CHECK-LABEL: @test_shift_nonnegative(
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; CHECK-NEXT: ret i1 true
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;
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%b = lshr i32 %a, 2
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%shift = shl nsw i32 %b, 3
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%cmp = icmp sge i32 %shift, 0
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ret i1 %cmp
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}
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; Result of left shifting a negative integer with
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; nsw flag should also be negative
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define i1 @test_shift_negative(i32 %a, i32 %b) {
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; CHECK-LABEL: @test_shift_negative(
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; CHECK-NEXT: ret i1 true
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;
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%c = or i32 %a, -2147483648
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%d = and i32 %b, 7
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%shift = shl nsw i32 %c, %d
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%cmp = icmp slt i32 %shift, 0
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ret i1 %cmp
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}
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; If sign bit is a known zero, it cannot be a known one.
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; This test should not crash opt.
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define i32 @test_no_sign_bit_conflict1(i1 %b) {
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; CHECK-LABEL: @test_no_sign_bit_conflict1(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[SEL:%.*]] = select i1 %b, i32 -2147221504, i32 -2147483648
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; CHECK-NEXT: ret i32 [[SEL]]
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;
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entry:
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%sel = select i1 %b, i32 8193, i32 8192
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%mul = shl nsw i32 %sel, 18
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ret i32 %mul
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}
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; If sign bit is a known one, it cannot be a known zero.
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; This test should not crash opt.
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define i32 @test_no_sign_bit_conflict2(i1 %b) {
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; CHECK-LABEL: @test_no_sign_bit_conflict2(
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; CHECK-NEXT: entry:
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; CHECK-NEXT: [[SEL:%.*]] = select i1 %b, i32 2147221504, i32 2146959360
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; CHECK-NEXT: ret i32 [[SEL]]
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;
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entry:
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%sel = select i1 %b, i32 -8193, i32 -8194
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%mul = shl nsw i32 %sel, 18
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ret i32 %mul
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}
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