RPCS3/llvm-mirror
1
0
Fork 0
mirror of https://github.com/RPCS3/llvm-mirror.git synced 2024-11-25 20:23:11 +01:00
Code Issues Projects Releases Wiki Activity

Revert rGd65ddca83ff85c7345fe9a0f5a15750f01e38420 - "[ValueTracking] ComputeKnownBits - minimum leading/trailing zero bits in LSHR/SHL (PR44526)"

Browse Source 
This is causing sanitizer test failures that I haven't been able to fix yet.
This commit is contained in:
Simon Pilgrim 2021-02-24 18:03:05 +00:00
parent 2cf4f5f7af
commit bb3d1c71a7
3 changed files with 27 additions and 33 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
lib/Analysis/ValueTracking.cpp
View File

@ -994,26 +994,30 @@ static void computeKnownBitsFromShiftOperator(
bool ShiftAmtIsConstant = Known.isConstant();
bool MaxShiftAmtIsOutOfRange = Known.getMaxValue().uge(BitWidth);
// Use the KF callback to get an initial knownbits approximation.
Known = KF(Known2, Known);
if (ShiftAmtIsConstant) {
Known = KF(Known2, Known);
// If the known bits conflict, this must be an overflowing left shift, so
// the shift result is poison. We can return anything we want. Choose 0 for
// the best folding opportunity.
if (Known.hasConflict())
Known.setAllZero();
// If the known bits conflict, this must be an overflowing left shift, so
// the shift result is poison.
if (Known.hasConflict()) {
Known.resetAll();
return;
}
// Constant shift amount - we're not going to improve on this.
if (ShiftAmtIsConstant)
return;
// If the shift amount could be greater than or equal to the bit-width of the
// LHS, the value could be poison, but bail out because the check below is
// expensive.
// TODO: Should we just carry on?
if (MaxShiftAmtIsOutOfRange)
if (MaxShiftAmtIsOutOfRange) {
Known.resetAll();
return;
}
// It would be more-clearly correct to use the two temporaries for this
// calculation. Reusing the APInts here to prevent unnecessary allocations.
Known.resetAll();
// If we know the shifter operand is nonzero, we can sometimes infer more
// known bits. However this is expensive to compute, so be lazy about it and
@ -1053,9 +1057,10 @@ static void computeKnownBitsFromShiftOperator(
Known, KF(Known2, KnownBits::makeConstant(APInt(32, ShiftAmt))));
}
// If the known bits conflict, the result is poison.
// If the known bits conflict, the result is poison. Return a 0 and hope the
// caller can further optimize that.
if (Known.hasConflict())
Known.resetAll();
Known.setAllZero();
}
static void computeKnownBitsFromOperator(const Operator *I,
@ -1227,6 +1232,10 @@ static void computeKnownBitsFromOperator(const Operator *I,
};
computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
KF);
// Trailing zeros of a right-shifted constant never decrease.
const APInt *C;
if (match(I->getOperand(0), m_APInt(C)))
Known.Zero.setLowBits(C->countTrailingZeros());
break;
}
case Instruction::LShr: {
@ -1235,6 +1244,10 @@ static void computeKnownBitsFromOperator(const Operator *I,
};
computeKnownBitsFromShiftOperator(I, DemandedElts, Known, Known2, Depth, Q,
KF);
// Leading zeros of a left-shifted constant never decrease.
const APInt *C;
if (match(I->getOperand(0), m_APInt(C)))
Known.Zero.setHighBits(C->countLeadingZeros());
break;
}
case Instruction::AShr: {

19
lib/Support/KnownBits.cpp
View File

@ -187,25 +187,6 @@ KnownBits KnownBits::shl(const KnownBits &LHS, const KnownBits &RHS) {
MinTrailingZeros = std::min(MinTrailingZeros, BitWidth);
}
// If the maximum shift is in range, then find the common bits from all
// possible shifts.
APInt MaxShiftAmount = RHS.getMaxValue();
if (MaxShiftAmount.ult(BitWidth)) {
assert(MinShiftAmount.ult(MaxShiftAmount) && "Illegal shift range");
Known.Zero.setAllBits();
Known.One.setAllBits();
for (uint64_t ShiftAmt = MinShiftAmount.getZExtValue(),
MaxShiftAmt = MaxShiftAmount.getZExtValue();
ShiftAmt <= MaxShiftAmt; ++ShiftAmt) {
KnownBits SpecificShift;
SpecificShift.Zero = LHS.Zero << ShiftAmt;
SpecificShift.One = LHS.One << ShiftAmt;
Known = KnownBits::commonBits(Known, SpecificShift);
if (Known.isUnknown())
break;
}
}
Known.Zero.setLowBits(MinTrailingZeros);
return Known;
}

2
test/Transforms/InstSimplify/icmp-constant.ll
View File

@ -799,7 +799,7 @@ define i1 @eq_shl_by_constant_produces_poison(i8 %x) {
define i1 @eq_shl_by_variable_produces_poison(i8 %x) {
; CHECK-LABEL: @eq_shl_by_variable_produces_poison(
; CHECK-NEXT: ret i1 poison
; CHECK-NEXT: ret i1 false
;
%clear_high_bit = and i8 %x, 127 ; 0x7f
%set_next_high_bits = or i8 %clear_high_bit, 112 ; 0x70