On Fri, Oct 24, 2014 at 4:53 PM, Arthur O'Dwyer <[email protected]> wrote:
> On Fri, Oct 24, 2014 at 3:55 PM, Sean Silva <[email protected]> wrote: > > > > What is the harm of documenting it? By defining this behavior we are > > pledging to forever support it. > > Exactly. That's harm enough, isn't it? > It looks like clang already has precedent for doing this, and a corresponding infrastructure for doing so, so it shouldn't be too problematic. > > >> This allows us to optimize > >> unsigned foo(unsigned x) { return (x ? __builtin_clz(x) : 32) } > >> into a single LZCNT instruction on a BMI-capable X86, if we choose. > > I'm no expert, but isn't this kind of peephole optimization supposed > to be handled by the LLVM (backend) side of things, not the Clang > side? > It's not an optimization that is being talked about here. It's a change of semantic meaning to define a previously undefined situation. Presumably it's a win for the user that thinks that __builtin_clz means "the best clz-like instruction available for the target", rather than the independently defined meaning given in the docs. -- Sean Silva > > The behavior of __builtin_clz(0) is undefined, so no portable program > can use it. Sure, it might happen to work on your machine, but then > you switch to a different architecture, or even a slightly older x86, > or a year from now Intel introduces an *even faster* bit-count > instruction that yields the wrong value for 0... and suddenly your > program breaks, and you have to waste time tracking down the bug. > > Therefore, portable programs WILL use > > unsigned foo(unsigned x) { return (x ? __builtin_clz(x) : 32) } > > Therefore, it would be awesome if the LLVM backend for these > particular Intel x86 machines would generate the most-optimized LZCNT > instruction sequence for "foo". This seems relatively easy and > shouldn't involve any changes to the Clang front-end, and *certainly* > not to the documentation. > > Then you get the best of both worlds — old portable code continues to > work (but gets faster), and newly written code continues to be > portable. > > Andrea wrote: > >> > My concern is that your suggested approach would force people to > >> > always guard calls to __builtin_ctz/__builtin_clz against zero. > >> > From a customer point of view, the compiler knows exactly if ctz and > >> > clz is defined on zero. It is basically pushing the problem on the > >> > customer by forcing them to guard all the calls to ctz/clz against > >> > zero. We've already had a number of customer queries/complaints about > >> > this and I personally don't think it is unreasonable to have ctz/clz > >> > defined on zero on our target (and other x86 targets where the > >> > behavior on zero is clearly defined). > > Sounds like a good argument in favor of introducing a new builtin, > spelled something like "__builtin_lzcnt", that does what the customer > wants. It could be implemented internally as (x ? __builtin_clz(x) : > 32), and lowered to a single instruction if possible by the LLVM > backend for your target. > > my $.02, > –Arthur >
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