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[APInt] Optimize umul_ov 

Change two costly udiv() calls to lshr(1)*RHS + left-shift + plus

On one 64-bit umul_ov benchmark, I measured an obvious improvement: 12.8129s -> 3.6257s

Note, there may be some value to special case 64-bit (the most common
case) with __builtin_umulll_overflow().

Differential Revision: https://reviews.llvm.org/D60669

llvm-svn: 358730
This commit is contained in:
Fangrui Song 2019-04-19 02:06:06 +00:00
parent 9206335e9d
commit acc7641bcb
2 changed files with 48 additions and 5 deletions
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17
llvm/lib/Support/APInt.cpp
View File

@ -1914,12 +1914,19 @@ APInt APInt::smul_ov(const APInt &RHS, bool &Overflow) const {
}
APInt APInt::umul_ov(const APInt &RHS, bool &Overflow) const {
APInt Res = *this * RHS;
if (countLeadingZeros() + RHS.countLeadingZeros() + 2 <= BitWidth) {
Overflow = true;
return *this * RHS;
}
if (*this != 0 && RHS != 0)
Overflow = Res.udiv(RHS) != *this || Res.udiv(*this) != RHS;
else
Overflow = false;
APInt Res = lshr(1) * RHS;
Overflow = Res.isNegative();
Res <<= 1;
if ((*this)[0]) {
Res += RHS;
if (Res.ult(RHS))
Overflow = true;
}
return Res;
}

36
llvm/unittests/ADT/APIntTest.cpp
View File

@ -2381,6 +2381,42 @@ TEST(APIntTest, RoundingSDiv) {
}
}
TEST(APIntTest, umul_ov) {
const std::pair<uint64_t, uint64_t> Overflows[] = {
{0x8000000000000000, 2},
{0x5555555555555556, 3},
{4294967296, 4294967296},
{4294967295, 4294967298},
};
const std::pair<uint64_t, uint64_t> NonOverflows[] = {
{0x7fffffffffffffff, 2},
{0x5555555555555555, 3},
{4294967295, 4294967297},
};
bool Overflow;
for (auto &X : Overflows) {
APInt A(64, X.first);
APInt B(64, X.second);
(void)A.umul_ov(B, Overflow);
EXPECT_TRUE(Overflow);
}
for (auto &X : NonOverflows) {
APInt A(64, X.first);
APInt B(64, X.second);
(void)A.umul_ov(B, Overflow);
EXPECT_FALSE(Overflow);
}
for (unsigned Bits = 1; Bits <= 5; ++Bits)
for (unsigned A = 0; A != 1u << Bits; ++A)
for (unsigned B = 0; B != 1u << Bits; ++B) {
APInt C = APInt(Bits, A).umul_ov(APInt(Bits, B), Overflow);
APInt D = APInt(2 * Bits, A) * APInt(2 * Bits, B);
EXPECT_TRUE(D.getHiBits(Bits).isNullValue() != Overflow);
}
}
TEST(APIntTest, SolveQuadraticEquationWrap) {
// Verify that "Solution" is the first non-negative integer that solves
// Ax^2 + Bx + C = "0 or overflow", i.e. that it is a correct solution