[PATCH] D103788: [InstCombine] Eliminate casts to optimize ctlz operation

[Datta Nagraj via Phabricator via llvm-commits]

datta.nagraj added a comment.

In D103788#2813238 <https://reviews.llvm.org/D103788#2813238>, @spatel wrote:

> In D103788#2812375 <https://reviews.llvm.org/D103788#2812375>, @datta.nagraj wrote:
>
>> Removed the hard cut off check. Please review Sir.
>
> The hard check was definitely not right, but have you proven that a log2-of-bitwidth check is not necessary? The original Alive allowed for that kind of proof based on value type widths.
> If we can't prove it, then I would include that check in this patch to be safe. 
> I don't know what the motivating source code looks like. It seems unlikely that we would ever truncate to a type smaller than log2 of the wide type in real code, but we can't rule it out, so it still deserves a regression test.

Hi @spatel Sir, doesn't https://alive2.llvm.org/ce/z/89_wJb prove that this works for even type smaller than log2 of the wide type ? I can add the same test to the unit test, but I am in doubt whether to add it as a check, since the example here proves that it works for difference of bitwidth of more than log 2 as well. I tested with 63, 2 sizes as well, and that works too. Please suggest if we require to add the log2 check, since these are working fine from the above examples.


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