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Fix APFloat::convert so that it handles narrowing conversions correctly; it

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was returning incorrect values in rare cases, and incorrectly marking
exact conversions as inexact in some more common cases. Fixes PR11406, and a
missed optimization in test/CodeGen/X86/fp-stack-O0.ll.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145141 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eli Friedman 2011-11-26 03:38:02 +00:00
parent 7c5025bbee
commit 4455142a95
3 changed files with 61 additions and 50 deletions
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85
lib/Support/APFloat.cpp
View File

@ -1854,20 +1854,33 @@ APFloat::convert(const fltSemantics &toSemantics,
lostFraction lostFraction;
unsigned int newPartCount, oldPartCount;
opStatus fs;
int shift;
const fltSemantics &fromSemantics = *semantics;
assertArithmeticOK(*semantics);
assertArithmeticOK(fromSemantics);
assertArithmeticOK(toSemantics);
lostFraction = lfExactlyZero;
newPartCount = partCountForBits(toSemantics.precision + 1);
oldPartCount = partCount();
shift = toSemantics.precision - fromSemantics.precision;
/* Handle storage complications. If our new form is wider,
re-allocate our bit pattern into wider storage. If it is
narrower, we ignore the excess parts, but if narrowing to a
single part we need to free the old storage.
Be careful not to reference significandParts for zeroes
and infinities, since it aborts. */
bool X86SpecialNan = false;
if (&fromSemantics == &APFloat::x87DoubleExtended &&
&toSemantics != &APFloat::x87DoubleExtended && category == fcNaN &&
(!(*significandParts() & 0x8000000000000000ULL) ||
!(*significandParts() & 0x4000000000000000ULL))) {
// x86 has some unusual NaNs which cannot be represented in any other
// format; note them here.
X86SpecialNan = true;
}
// If this is a truncation, perform the shift before we narrow the storage.
if (shift < 0 && (category==fcNormal || category==fcNaN))
lostFraction = shiftRight(significandParts(), oldPartCount, -shift);
// Fix the storage so it can hold to new value.
if (newPartCount > oldPartCount) {
// The new type requires more storage; make it available.
integerPart *newParts;
newParts = new integerPart[newPartCount];
APInt::tcSet(newParts, 0, newPartCount);
@ -1875,60 +1888,34 @@ APFloat::convert(const fltSemantics &toSemantics,
APInt::tcAssign(newParts, significandParts(), oldPartCount);
freeSignificand();
significand.parts = newParts;
} else if (newPartCount < oldPartCount) {
/* Capture any lost fraction through truncation of parts so we get
correct rounding whilst normalizing. */
if (category==fcNormal)
lostFraction = lostFractionThroughTruncation
(significandParts(), oldPartCount, toSemantics.precision);
if (newPartCount == 1) {
integerPart newPart = 0;
if (category==fcNormal || category==fcNaN)
newPart = significandParts()[0];
freeSignificand();
significand.part = newPart;
}
} else if (newPartCount == 1 && oldPartCount != 1) {
// Switch to built-in storage for a single part.
integerPart newPart = 0;
if (category==fcNormal || category==fcNaN)
newPart = significandParts()[0];
freeSignificand();
significand.part = newPart;
}
// Now that we have the right storage, switch the semantics.
semantics = &toSemantics;
// If this is an extension, perform the shift now that the storage is
// available.
if (shift > 0 && (category==fcNormal || category==fcNaN))
APInt::tcShiftLeft(significandParts(), newPartCount, shift);
if (category == fcNormal) {
/* Re-interpret our bit-pattern. */
exponent += toSemantics.precision - semantics->precision;
semantics = &toSemantics;
fs = normalize(rounding_mode, lostFraction);
*losesInfo = (fs != opOK);
} else if (category == fcNaN) {
int shift = toSemantics.precision - semantics->precision;
// Do this now so significandParts gets the right answer
const fltSemantics *oldSemantics = semantics;
semantics = &toSemantics;
*losesInfo = false;
// No normalization here, just truncate
if (shift>0)
APInt::tcShiftLeft(significandParts(), newPartCount, shift);
else if (shift < 0) {
unsigned ushift = -shift;
// Figure out if we are losing information. This happens
// if are shifting out something other than 0s, or if the x87 long
// double input did not have its integer bit set (pseudo-NaN), or if the
// x87 long double input did not have its QNan bit set (because the x87
// hardware sets this bit when converting a lower-precision NaN to
// x87 long double).
if (APInt::tcLSB(significandParts(), newPartCount) < ushift)
*losesInfo = true;
if (oldSemantics == &APFloat::x87DoubleExtended &&
(!(*significandParts() & 0x8000000000000000ULL) ||
!(*significandParts() & 0x4000000000000000ULL)))
*losesInfo = true;
APInt::tcShiftRight(significandParts(), newPartCount, ushift);
}
*losesInfo = lostFraction != lfExactlyZero || X86SpecialNan;
// gcc forces the Quiet bit on, which means (float)(double)(float_sNan)
// does not give you back the same bits. This is dubious, and we
// don't currently do it. You're really supposed to get
// an invalid operation signal at runtime, but nobody does that.
fs = opOK;
} else {
semantics = &toSemantics;
fs = opOK;
*losesInfo = false;
}

2
test/CodeGen/X86/fp-stack-O0.ll
View File

@ -10,7 +10,7 @@ declare i32 @x2(x86_fp80, x86_fp80) nounwind
; Pass arguments on the stack.
; CHECK-NEXT: movq %rsp, [[RCX:%r..]]
; Copy constant-pool value.
; CHECK-NEXT: fldt LCPI
; CHECK-NEXT: fldl LCPI
; CHECK-NEXT: fstpt 16([[RCX]])
; Copy x1 return value.
; CHECK-NEXT: fstpt ([[RCX]])

24
unittests/ADT/APFloatTest.cpp
View File

@ -653,4 +653,28 @@ TEST(APFloatTest, getLargest) {
EXPECT_EQ(1.7976931348623158e+308, APFloat::getLargest(APFloat::IEEEdouble).convertToDouble());
}
TEST(APFloatTest, convert) {
bool losesInfo;
APFloat test(APFloat::IEEEdouble, "1.0");
test.convert(APFloat::IEEEsingle, APFloat::rmNearestTiesToEven, &losesInfo);
EXPECT_EQ(1.0f, test.convertToFloat());
EXPECT_FALSE(losesInfo);
test = APFloat(APFloat::x87DoubleExtended, "0x1p-53");
test.add(APFloat(APFloat::x87DoubleExtended, "1.0"), APFloat::rmNearestTiesToEven);
test.convert(APFloat::IEEEdouble, APFloat::rmNearestTiesToEven, &losesInfo);
EXPECT_EQ(1.0, test.convertToDouble());
EXPECT_TRUE(losesInfo);
test = APFloat(APFloat::IEEEquad, "0x1p-53");
test.add(APFloat(APFloat::IEEEquad, "1.0"), APFloat::rmNearestTiesToEven);
test.convert(APFloat::IEEEdouble, APFloat::rmNearestTiesToEven, &losesInfo);
EXPECT_EQ(1.0, test.convertToDouble());
EXPECT_TRUE(losesInfo);
test = APFloat(APFloat::x87DoubleExtended, "0xf.fffffffp+28");
test.convert(APFloat::IEEEdouble, APFloat::rmNearestTiesToEven, &losesInfo);
EXPECT_EQ(4294967295.0, test.convertToDouble());
EXPECT_FALSE(losesInfo);
}
}