[PATCH] D140636: [Support] Fix what I think is an off by 1 bug in UnsignedDivisionByConstantInfo.

Sat Dec 24 19:25:05 PST 2022

craig.topper added a comment.

In D140636#4016133 <https://reviews.llvm.org/D140636#4016133>, @barannikov88 wrote:

> In D140636#4016127 <https://reviews.llvm.org/D140636#4016127>, @lebedev.ri wrote:
>
>> Let me add some tests here.
>
> In case that helps, I've tried a few bitwidth variants.
> With LeadingZeros = 0, I get different magics iff the divisor has LSB and MSB ones and all other bits zeros. (I.e. 0b1000...0001 for any bitwidth.)
> Example: APInt(8, 0x81).
>
> With LeadingZeros != 0, the old algorithm asserts iff the divisor is a certain power of two. Examples:
>  D = APInt(8, 0x80) and LeadingZeros = 1
>  D = APInt(8, 0x40) and LeadingZeros = 2.
> The fixed version does not crash and returns what seems to be correct values (can that be enough for a unittest?)

I'm not sure I expect the algorithm to work if the divisor does not have at least as many leading zeros as the dividend. The AllOnes value the algorithm creates should be >= D. Norrmally when LeadingZeros is used both the dividend and the divisor have been shifted right by that many bits.

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https://reviews.llvm.org/D140636