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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
This commit is contained in:
Evgeny Stupachenko 2016-08-24 23:01:33 +00:00
parent b6cd5121b7
commit d7f9c3564a
2 changed files with 78 additions and 5 deletions
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28
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -795,6 +795,14 @@ static void computeKnownBitsFromShiftOperator(
computeKnownBits(I->getOperand(0), KnownZero, KnownOne, Depth + 1, Q);
KnownZero = KZF(KnownZero, ShiftAmt);
KnownOne = KOF(KnownOne, ShiftAmt);
// If there is conflict between KnownZero and KnownOne, this must be an
// overflowing left shift, so the shift result is undefined. Clear KnownZero
// and KnownOne bits so that other code could propagate this undef.
if ((KnownZero & KnownOne) != 0) {
KnownZero.clearAllBits();
KnownOne.clearAllBits();
}
return;
}
@ -1065,13 +1073,23 @@ static void computeKnownBitsFromOperator(const Operator *I, APInt &KnownZero,
}
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) |
APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
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,

55
llvm/test/Analysis/ValueTracking/known-signbit-shift.ll Normal file
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@ -0,0 +1,55 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; 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-NEXT: 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-NEXT: 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
}
; If sign bit is a known zero, it cannot be a known one.
; This test should not crash opt.
define i32 @test_no_sign_bit_conflict1(i1 %b) {
; CHECK-LABEL: @test_no_sign_bit_conflict1(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[SEL:%.*]] = select i1 %b, i32 -2147221504, i32 -2147483648
; CHECK-NEXT: ret i32 [[SEL]]
;
entry:
%sel = select i1 %b, i32 8193, i32 8192
%mul = shl nsw i32 %sel, 18
ret i32 %mul
}
; If sign bit is a known one, it cannot be a known zero.
; This test should not crash opt.
define i32 @test_no_sign_bit_conflict2(i1 %b) {
; CHECK-LABEL: @test_no_sign_bit_conflict2(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[SEL:%.*]] = select i1 %b, i32 2147221504, i32 2146959360
; CHECK-NEXT: ret i32 [[SEL]]
;
entry:
%sel = select i1 %b, i32 -8193, i32 -8194
%mul = shl nsw i32 %sel, 18
ret i32 %mul
}