[PATCH] D151867: [Clang][RISCV] Make generic clz/ctz builtins defined for zero on RISCV targets.

Wed Jun 7 20:01:36 PDT 2023

Yunzezhu abandoned this revision.
Yunzezhu added a comment.

In D151867#4401987 <https://reviews.llvm.org/D151867#4401987>, @craig.topper wrote:

> In D151867#4401952 <https://reviews.llvm.org/D151867#4401952>, @Yunzezhu wrote:
>
>> In D151867#4400255 <https://reviews.llvm.org/D151867#4400255>, @craig.topper wrote:
>>
>>> From the C language perspective with this change, __builtin_clz/ctz is still considered undefined for 0 and code that uses it is ill-formed. `isCLZForZeroUndef` is only intended to prevent the middle end from optimizing based on the undefinedness and creating surprises. See also https://discourse.llvm.org/t/should-ubsan-detect-0-input-to-builtin-clz-ctz-regardless-of-target/71060
>>
>> I see __builtin_clz/ctz returning an undefined value for 0 input matches gcc's document, but when I test __builtin_clz/ctz with 0 input on gcc, it returns a valid value rather than an undefined value. It looks gcc does not follow gcc's document. I'm not sure which one is better that match document to return undefined for 0, or match gcc's behavior to return defined value?
>
> From what I can see in the assembly here https://godbolt.org/z/s4qqz83EK, gcc's undefined behavior sanitizer does consider an input of 0 to be undefined.

Got it.

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https://reviews.llvm.org/D151867