On Sun, Oct 26, 2014 at 10:24 PM, Chandler Carruth <[email protected]> wrote:
> 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. > I agree with Chandler on all points here. __builtin_clz and __builtin_ctz is clearly and unambiguously defined by GCC's specification for them, and we should not be providing additional definedness guarantees. If we want a defined-for-zero-input form of these intrinsics, they should have a different name (or an additional argument, or be written as "x == 0 ? 32 : __builtin_clz(x)" or similar). It is a major feature of Clang that we try not to lock people in by providing incompatible extensions.
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