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[ValueTracking] Improve ComputeNumSignBits for SRem. 

The result will have the same sign as the dividend unless the
result is 0. The magnitude of the result will always be less
than or equal to the dividend. So the result will have at least
as many sign bits as the dividend.

Previously we would do this if the divisor was a positive constant,
but that isn't required.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D97170
This commit is contained in:
Craig Topper 2021-02-22 14:34:06 -08:00
parent 7a0da88943
commit 89440df64a
2 changed files with 20 additions and 27 deletions
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41
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -2784,6 +2784,8 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
}
case Instruction::SRem: {
Tmp = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
const APInt *Denominator;
// srem X, C -> we know that the result is within [-C+1,C) when C is a
// positive constant. This let us put a lower bound on the number of sign
@ -2791,30 +2793,25 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
if (match(U->getOperand(1), m_APInt(Denominator))) {
// Ignore non-positive denominator.
if (!Denominator->isStrictlyPositive())
break;
if (Denominator->isStrictlyPositive()) {
// Calculate the leading sign bit constraints by examining the
// denominator. Given that the denominator is positive, there are two
// cases:
//
// 1. The numerator is positive. The result range is [0,C) and
// [0,C) u< (1 << ceilLogBase2(C)).
//
// 2. The numerator is negative. Then the result range is (-C,0] and
// integers in (-C,0] are either 0 or >u (-1 << ceilLogBase2(C)).
//
// Thus a lower bound on the number of sign bits is `TyBits -
// ceilLogBase2(C)`.
// Calculate the incoming numerator bits. SRem by a positive constant
// can't lower the number of sign bits.
unsigned NumrBits = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
// Calculate the leading sign bit constraints by examining the
// denominator. Given that the denominator is positive, there are two
// cases:
//
// 1. the numerator is positive. The result range is [0,C) and [0,C) u<
// (1 << ceilLogBase2(C)).
//
// 2. the numerator is negative. Then the result range is (-C,0] and
// integers in (-C,0] are either 0 or >u (-1 << ceilLogBase2(C)).
//
// Thus a lower bound on the number of sign bits is `TyBits -
// ceilLogBase2(C)`.
unsigned ResBits = TyBits - Denominator->ceilLogBase2();
return std::max(NumrBits, ResBits);
unsigned ResBits = TyBits - Denominator->ceilLogBase2();
Tmp = std::max(Tmp, ResBits);
}
}
break;
return Tmp;
}
case Instruction::AShr: {

6
llvm/test/Transforms/InstCombine/with_overflow.ll
View File

@ -332,11 +332,7 @@ define i1 @overflow_mod_overflow_mul(i32 %v1, i32 %v2) nounwind {
define i1 @overflow_mod_mul2(i16 %v1, i32 %v2) nounwind {
; CHECK-LABEL: @overflow_mod_mul2(
; CHECK-NEXT: [[A:%.*]] = sext i16 [[V1:%.*]] to i32
; CHECK-NEXT: [[REM:%.*]] = srem i32 [[A]], [[V2:%.*]]
; CHECK-NEXT: [[T:%.*]] = call { i32, i1 } @llvm.smul.with.overflow.i32(i32 [[REM]], i32 [[REM]])
; CHECK-NEXT: [[OBIT:%.*]] = extractvalue { i32, i1 } [[T]], 1
; CHECK-NEXT: ret i1 [[OBIT]]
; CHECK-NEXT: ret i1 false
;
%a = sext i16 %v1 to i32
%rem = srem i32 %a, %v2