maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

[APInt] Fix 1-bit edge case in smul_ov() 

The sdiv used to check for overflow can itself overflow if the
LHS is signed min and the RHS is -1. The code tried to account for
this by also checking the commuted version. However, for 1-bit
values, signed min and -1 are the same value, so both divisions
overflow. As such, the overflow for -1 * -1 was not detected
(which results in -1 rather than 1 for 1-bit values). Fix this by
explicitly checking for this case instead.

Noticed while adding exhaustive test coverage for smul_ov(),
which is also part of this commit.
This commit is contained in:
Nikita Popov 2021-10-16 20:25:18 +02:00
parent 2c941fa2f9
commit 492a4a428f
2 changed files with 21 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
llvm/lib/Support/APInt.cpp
View File

@ -1952,8 +1952,9 @@ APInt APInt::sdiv_ov(const APInt &RHS, bool &Overflow) const {
APInt APInt::smul_ov(const APInt &RHS, bool &Overflow) const {
APInt Res = *this * RHS;
if (*this != 0 && RHS != 0)
Overflow = Res.sdiv(RHS) != *this || Res.sdiv(*this) != RHS;
if (RHS != 0)
Overflow = Res.sdiv(RHS) != *this ||
(isMinSignedValue() && RHS.isAllOnes());
else
Overflow = false;
return Res;

21
llvm/unittests/ADT/APIntTest.cpp
View File

@ -2729,9 +2729,24 @@ TEST(APIntTest, umul_ov) {
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);
APInt N1 = APInt(Bits, A), N2 = APInt(Bits, B);
APInt Narrow = N1.umul_ov(N2, Overflow);
APInt Wide = N1.zext(2 * Bits) * N2.zext(2 * Bits);
EXPECT_EQ(Wide.trunc(Bits), Narrow);
EXPECT_EQ(Narrow.zext(2 * Bits) != Wide, Overflow);
}
}
TEST(APIntTest, smul_ov) {
for (unsigned Bits = 1; Bits <= 5; ++Bits)
for (unsigned A = 0; A != 1u << Bits; ++A)
for (unsigned B = 0; B != 1u << Bits; ++B) {
bool Overflow;
APInt N1 = APInt(Bits, A), N2 = APInt(Bits, B);
APInt Narrow = N1.smul_ov(N2, Overflow);
APInt Wide = N1.sext(2 * Bits) * N2.sext(2 * Bits);
EXPECT_EQ(Wide.trunc(Bits), Narrow);
EXPECT_EQ(Narrow.sext(2 * Bits) != Wide, Overflow);
}
}