[llvm] [APFloat] Fix `IEEEFloat::addOrSubtractSignificand` and `IEEEFloat::normalize` (PR #98721)
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Mon Nov 25 10:54:48 PST 2024
https://github.com/beetrees updated https://github.com/llvm/llvm-project/pull/98721
>From 0dec30d6a63f9b2eeb3939a71b6c475f31cf0db6 Mon Sep 17 00:00:00 2001
From: beetrees <b at beetr.ee>
Date: Sat, 13 Jul 2024 07:50:08 +0100
Subject: [PATCH] [APFloat] Fix `IEEEFloat::addOrSubtractSignificand` and
`IEEEFloat::normalize`
---
llvm/include/llvm/ADT/APFloat.h | 2 +
llvm/lib/Support/APFloat.cpp | 50 +++++---
llvm/unittests/ADT/APFloatTest.cpp | 179 +++++++++++++++++++++++++++++
3 files changed, 216 insertions(+), 15 deletions(-)
diff --git a/llvm/include/llvm/ADT/APFloat.h b/llvm/include/llvm/ADT/APFloat.h
index 4ca928bf4f49e3..a561164ed13be2 100644
--- a/llvm/include/llvm/ADT/APFloat.h
+++ b/llvm/include/llvm/ADT/APFloat.h
@@ -750,6 +750,8 @@ class IEEEFloat final {
/// Sign bit of the number.
unsigned int sign : 1;
+
+ friend class IEEEFloatUnitTestHelper;
};
hash_code hash_value(const IEEEFloat &Arg);
diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 81e297c3ab033e..494c9f1049cdca 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -1682,7 +1682,8 @@ APFloat::opStatus IEEEFloat::normalize(roundingMode rounding_mode,
/* Before rounding normalize the exponent of fcNormal numbers. */
omsb = significandMSB() + 1;
- if (omsb) {
+ // Only skip this `if` if the value is exactly zero.
+ if (omsb || lost_fraction != lfExactlyZero) {
/* OMSB is numbered from 1. We want to place it in the integer
bit numbered PRECISION if possible, with a compensating change in
the exponent. */
@@ -1864,7 +1865,7 @@ APFloat::opStatus IEEEFloat::addOrSubtractSpecials(const IEEEFloat &rhs,
/* Add or subtract two normal numbers. */
lostFraction IEEEFloat::addOrSubtractSignificand(const IEEEFloat &rhs,
bool subtract) {
- integerPart carry;
+ integerPart carry = 0;
lostFraction lost_fraction;
int bits;
@@ -1882,11 +1883,13 @@ lostFraction IEEEFloat::addOrSubtractSignificand(const IEEEFloat &rhs,
"This floating point format does not support signed values");
IEEEFloat temp_rhs(rhs);
+ bool lost_fraction_is_from_rhs = false;
if (bits == 0)
lost_fraction = lfExactlyZero;
else if (bits > 0) {
lost_fraction = temp_rhs.shiftSignificandRight(bits - 1);
+ lost_fraction_is_from_rhs = true;
shiftSignificandLeft(1);
} else {
lost_fraction = shiftSignificandRight(-bits - 1);
@@ -1894,23 +1897,40 @@ lostFraction IEEEFloat::addOrSubtractSignificand(const IEEEFloat &rhs,
}
// Should we reverse the subtraction.
- if (compareAbsoluteValue(temp_rhs) == cmpLessThan) {
- carry = temp_rhs.subtractSignificand
- (*this, lost_fraction != lfExactlyZero);
+ cmpResult cmp_result = compareAbsoluteValue(temp_rhs);
+ if (cmp_result == cmpLessThan) {
+ bool borrow =
+ lost_fraction != lfExactlyZero && !lost_fraction_is_from_rhs;
+ if (borrow) {
+ // The lost fraction is being subtracted, borrow from the significand
+ // and invert `lost_fraction`.
+ if (lost_fraction == lfLessThanHalf)
+ lost_fraction = lfMoreThanHalf;
+ else if (lost_fraction == lfMoreThanHalf)
+ lost_fraction = lfLessThanHalf;
+ }
+ carry = temp_rhs.subtractSignificand(*this, borrow);
copySignificand(temp_rhs);
sign = !sign;
- } else {
- carry = subtractSignificand
- (temp_rhs, lost_fraction != lfExactlyZero);
+ } else if (cmp_result == cmpGreaterThan) {
+ bool borrow = lost_fraction != lfExactlyZero && lost_fraction_is_from_rhs;
+ if (borrow) {
+ // The lost fraction is being subtracted, borrow from the significand
+ // and invert `lost_fraction`.
+ if (lost_fraction == lfLessThanHalf)
+ lost_fraction = lfMoreThanHalf;
+ else if (lost_fraction == lfMoreThanHalf)
+ lost_fraction = lfLessThanHalf;
+ }
+ carry = subtractSignificand(temp_rhs, borrow);
+ } else { // cmpEqual
+ zeroSignificand();
+ if (lost_fraction != lfExactlyZero && lost_fraction_is_from_rhs) {
+ // rhs is slightly larger due to the lost fraction, flip the sign.
+ sign = !sign;
+ }
}
- /* Invert the lost fraction - it was on the RHS and
- subtracted. */
- if (lost_fraction == lfLessThanHalf)
- lost_fraction = lfMoreThanHalf;
- else if (lost_fraction == lfMoreThanHalf)
- lost_fraction = lfLessThanHalf;
-
/* The code above is intended to ensure that no borrow is
necessary. */
assert(!carry);
diff --git a/llvm/unittests/ADT/APFloatTest.cpp b/llvm/unittests/ADT/APFloatTest.cpp
index f291c814886d35..0023b87ef82e1b 100644
--- a/llvm/unittests/ADT/APFloatTest.cpp
+++ b/llvm/unittests/ADT/APFloatTest.cpp
@@ -47,6 +47,37 @@ static std::string convertToString(double d, unsigned Prec, unsigned Pad,
return std::string(Buffer.data(), Buffer.size());
}
+namespace llvm {
+namespace detail {
+class IEEEFloatUnitTestHelper {
+public:
+ static void runTest(bool subtract, bool lhsSign,
+ APFloat::ExponentType lhsExponent,
+ APFloat::integerPart lhsSignificand, bool rhsSign,
+ APFloat::ExponentType rhsExponent,
+ APFloat::integerPart rhsSignificand, bool expectedSign,
+ APFloat::ExponentType expectedExponent,
+ APFloat::integerPart expectedSignificand,
+ lostFraction expectedLoss) {
+ // `addOrSubtractSignificand` only uses the sign, exponent and significand
+ IEEEFloat lhs(1.0);
+ lhs.sign = lhsSign;
+ lhs.exponent = lhsExponent;
+ lhs.significand.part = lhsSignificand;
+ IEEEFloat rhs(1.0);
+ rhs.sign = rhsSign;
+ rhs.exponent = rhsExponent;
+ rhs.significand.part = rhsSignificand;
+ lostFraction resultLoss = lhs.addOrSubtractSignificand(rhs, subtract);
+ EXPECT_EQ(resultLoss, expectedLoss);
+ EXPECT_EQ(lhs.sign, expectedSign);
+ EXPECT_EQ(lhs.exponent, expectedExponent);
+ EXPECT_EQ(lhs.significand.part, expectedSignificand);
+ }
+};
+} // namespace detail
+} // namespace llvm
+
namespace {
TEST(APFloatTest, isSignaling) {
@@ -560,6 +591,104 @@ TEST(APFloatTest, FMA) {
EXPECT_EQ(-8.85242279E-41f, f1.convertToFloat());
}
+ // The `addOrSubtractSignificand` can be considered to have 9 possible cases
+ // when subtracting: all combinations of {cmpLessThan, cmpGreaterThan,
+ // cmpEqual} and {no loss, loss from lhs, loss from rhs}. Test each reachable
+ // case here.
+
+ // Regression test for failing the `assert(!carry)` in
+ // `addOrSubtractSignificand` and normalizing the exponent even when the
+ // significand is zero if there is a lost fraction.
+ // This tests cmpEqual, loss from lhs
+ {
+ APFloat f1(-1.4728589E-38f);
+ APFloat f2(3.7105144E-6f);
+ APFloat f3(5.5E-44f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(-0.0f, f1.convertToFloat());
+ }
+
+ // Test cmpGreaterThan, no loss
+ {
+ APFloat f1(2.0f);
+ APFloat f2(2.0f);
+ APFloat f3(-3.5f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(0.5f, f1.convertToFloat());
+ }
+
+ // Test cmpLessThan, no loss
+ {
+ APFloat f1(2.0f);
+ APFloat f2(2.0f);
+ APFloat f3(-4.5f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(-0.5f, f1.convertToFloat());
+ }
+
+ // Test cmpEqual, no loss
+ {
+ APFloat f1(2.0f);
+ APFloat f2(2.0f);
+ APFloat f3(-4.0f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(0.0f, f1.convertToFloat());
+ }
+
+ // Test cmpLessThan, loss from lhs
+ {
+ APFloat f1(2.0000002f);
+ APFloat f2(2.0000002f);
+ APFloat f3(-32.0f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(-27.999998f, f1.convertToFloat());
+ }
+
+ // Test cmpGreaterThan, loss from rhs
+ {
+ APFloat f1(1e10f);
+ APFloat f2(1e10f);
+ APFloat f3(-2.0000002f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(1e20f, f1.convertToFloat());
+ }
+
+ // Test cmpGreaterThan, loss from lhs
+ {
+ APFloat f1(1e-36f);
+ APFloat f2(0.0019531252f);
+ APFloat f3(-1e-45f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(1.953124e-39f, f1.convertToFloat());
+ }
+
+ // {cmpEqual, cmpLessThan} with loss from rhs can't occur for the usage in
+ // `fusedMultiplyAdd` as `multiplySignificand` normalises the MSB of lhs to
+ // one bit below the top.
+
+ // Test cases from #104984
+ {
+ APFloat f1(0.24999998f);
+ APFloat f2(2.3509885e-38f);
+ APFloat f3(-1e-45f);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(5.87747e-39f, f1.convertToFloat());
+ }
+ {
+ APFloat f1(4.4501477170144023e-308);
+ APFloat f2(0.24999999999999997);
+ APFloat f3(-8.475904604373977e-309);
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(2.64946468816203e-309, f1.convertToDouble());
+ }
+ {
+ APFloat f1(APFloat::IEEEhalf(), APInt(16, 0x8fffu));
+ APFloat f2(APFloat::IEEEhalf(), APInt(16, 0x2bffu));
+ APFloat f3(APFloat::IEEEhalf(), APInt(16, 0x0172u));
+ f1.fusedMultiplyAdd(f2, f3, APFloat::rmNearestTiesToEven);
+ EXPECT_EQ(0x808eu, f1.bitcastToAPInt().getZExtValue());
+ }
+
// Test using only a single instance of APFloat.
{
APFloat F(1.5);
@@ -8089,4 +8218,54 @@ TEST(APFloatTest, Float4E2M1FNToFloat) {
EXPECT_TRUE(SmallestDenorm.isDenormal());
EXPECT_EQ(0x0.8p0, SmallestDenorm.convertToFloat());
}
+
+TEST(APFloatTest, AddOrSubtractSignificand) {
+ typedef detail::IEEEFloatUnitTestHelper Helper;
+ // Test cases are all combinations of:
+ // {equal exponents, LHS larger exponent, RHS larger exponent}
+ // {equal significands, LHS larger significand, RHS larger significand}
+ // {no loss, loss}
+
+ // Equal exponents (loss cannot occur as their is no shifting)
+ Helper::runTest(true, false, 1, 0x10, false, 1, 0x5, false, 1, 0xb,
+ lfExactlyZero);
+ Helper::runTest(false, false, -2, 0x20, true, -2, 0x20, false, -2, 0,
+ lfExactlyZero);
+ Helper::runTest(false, true, 3, 0x20, false, 3, 0x30, false, 3, 0x10,
+ lfExactlyZero);
+
+ // LHS larger exponent
+ // LHS significand greater after shitfing
+ Helper::runTest(true, false, 7, 0x100, false, 3, 0x100, false, 6, 0x1e0,
+ lfExactlyZero);
+ Helper::runTest(true, false, 7, 0x100, false, 3, 0x101, false, 6, 0x1df,
+ lfMoreThanHalf);
+ // Significands equal after shitfing
+ Helper::runTest(true, false, 7, 0x100, false, 3, 0x1000, false, 6, 0,
+ lfExactlyZero);
+ Helper::runTest(true, false, 7, 0x100, false, 3, 0x1001, true, 6, 0,
+ lfLessThanHalf);
+ // RHS significand greater after shitfing
+ Helper::runTest(true, false, 7, 0x100, false, 3, 0x10000, true, 6, 0x1e00,
+ lfExactlyZero);
+ Helper::runTest(true, false, 7, 0x100, false, 3, 0x10001, true, 6, 0x1e00,
+ lfLessThanHalf);
+
+ // RHS larger exponent
+ // RHS significand greater after shitfing
+ Helper::runTest(true, false, 3, 0x100, false, 7, 0x100, true, 6, 0x1e0,
+ lfExactlyZero);
+ Helper::runTest(true, false, 3, 0x101, false, 7, 0x100, true, 6, 0x1df,
+ lfMoreThanHalf);
+ // Significands equal after shitfing
+ Helper::runTest(true, false, 3, 0x1000, false, 7, 0x100, false, 6, 0,
+ lfExactlyZero);
+ Helper::runTest(true, false, 3, 0x1001, false, 7, 0x100, false, 6, 0,
+ lfLessThanHalf);
+ // LHS significand greater after shitfing
+ Helper::runTest(true, false, 3, 0x10000, false, 7, 0x100, false, 6, 0x1e00,
+ lfExactlyZero);
+ Helper::runTest(true, false, 3, 0x10001, false, 7, 0x100, false, 6, 0x1e00,
+ lfLessThanHalf);
+}
} // namespace
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