[llvm] [KnownBits] Improve minimum denominator bounds for sdiv (PR #227264)

[via llvm-commits]

anandkvinu wrote:

@ZERICO2005 this is the example i did in unit test:

TEST(KnownBitsTest, NonZeroDenomSDiv) {
KnownBits Num = KnownBits::makeConstant(APInt(4, -8, true));
KnownBits Denom(4);

// Denom = 0b0?00.
Denom.Zero.setBit(3);
//Denom.Zero.setBit(2);
Denom.Zero.setBit(1);
Denom.Zero.setBit(0);

KnownBits Result = KnownBits::sdiv(Num, Denom);
KnownBits Answer = KnownBits::makeConstant(APInt(4, -2, true));

EXPECT_EQ(Result, Answer);
}

and this was the output of the test:

/home/user/Documents/anand/llvm23/llvm-project/llvm/unittests/Support/KnownBitsTest.cpp:710: Failure
Expected equality of these values:
Result
Which is: 111?
Answer
Which is: 1110

[ FAILED ] KnownBitsTest.NonZeroDenomSDiv (0 ms)
[----------] 1 test from KnownBitsTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[ PASSED ] 0 tests.
[ FAILED ] 1 test, listed below:
[ FAILED ] KnownBitsTest.NonZeroDenomSDiv

1 FAILED TEST

as you can see here the value stored in result is 111? and not 1110 . if i test it with the Answer.One.setHighBits(3);
instead of Answer = KnownBits::makeConstant(APInt(4, -2, true));
the test passes!! its checking the setbits.

is this what you meant when you said "where you would check that the known bits of the return value are 0b111??? or etc for example"???
let me know? thanks.

https://github.com/llvm/llvm-project/pull/227264