Sorry for the late replies, but as the last person to work on the CLZ / CTZ stuff, I have some small opinions here. =]
On Sun, Oct 26, 2014 at 9:52 PM, David Majnemer <[email protected]> wrote: > I think Paul's patch is fine. > I firmly disagree. > > Having `__builtin_ctz` asking the target if `isCLZForZeroUndef` seems > busted and this patch seems to fix that just fine. > Yes, that is busted and we should fix that aspect regardless. But changing the behavior on x86 seems actively harmful to portability. So, let's look at the history. I went and taught LLVM to produce optimal code in cases where undef-at-zero was acceptable and only instructions with undef-at-zero were available. In order to generate reasonable code for these scenarios, Clang and LLVM had to respect the contract specified for the GCC __builtin_clz and __builtin_ctz. That spec[1] *very* clearly states that if the input is zero the result is undefined. 1: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html However, after this Bob changed ARM to not follow this spec citing a radar only in r149086. I objected at the time, and continue to feel this is the *wrong* approach for ARM. We should not be deviating from the clear, documented, and widely followed spec here. However, I had no real basis for arguing about the behavior of ARM at the time, and I doubt that I have much say in the behavior there even now. =] That said, on x86, I am firmly opposed to deviating from the clear and unambiguous spec. If folks want a builtin to access unambiguously defined-at-zero CLZ or CTZ behavior, we should define *new* builtins which have a clear spec here. However, I'm unpersuaded that this is better or more portable. The alternative code seems quite palatable: (x == 0 ? 32 : __builtin_clz(x)) If LLVM fails to turn this into the maximally efficient code on some architecture, then much as others have suggested, we should enhance LLVM. -Chandler
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