On Sat, Oct 22, 2016 at 01:46:30PM +0200, Uros Bizjak wrote:
> On Fri, Oct 21, 2016 at 5:37 PM, Uros Bizjak <ubiz...@gmail.com> wrote:
> > On Fri, Oct 21, 2016 at 5:26 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> >
> >> This patch on top of the just posted patch adds folding for a couple more
> >> builtins (though, hundreds or thousands of other md builtins remain
> >> unfolded
> >> even though they actually could be folded for e.g. const arguments).
>
> Just a few words regarding other unfolded builtins. x86 intrinsics
> (and consequently builtins) are considered as a convenient way to emit
> assembly instructions. So, the same rules as when writting assembly,
> although slightly relaxed, should apply there. IMO, compiler
> optimizations with intrinsics should be an exception, not the rule. As
> an example, __builtin_ctz, __builtin_clz and functionaly similar
> target-builtins are rather messy w.r.t to "undefinedness", so I think
> this fact warrants some help from the compiler. But there is no need
> to handle every single builtin - only a competent person that knows
> the background of these intrinsics should use them.
Generally constant folding what we can is a good thing, usually people will
not use the intrinsics when they are passing constants directly, but
constants could appear there through inlining and other optimizations.
If we do constant fold the x86 intrinsics, we allow further constant folding
and optimizations down the road.
For various x86 intrinsics we do some constant folding, but only late
(during RTL optimizations), and only if the insn patterns don't contain
UNSPECs.
Besides the BMI/BMI2/TBM/LZCNT intrinsics that are already folded or I've
posted patch for, intrinsics that IMHO would be nice to be folded are e.g.
__builtin_ia32_bsr*, __builtin_ia32_ro[rl]*, maybe
__builtin_ia32_{,r}sqrtps*, __builtin_ia32_rcpps, etc.
For __builtin_ia32_addps and the like the question is why we have those
builtins at all, it would be better to just use normal vector arithmetics.
__builtin_ia32_cmp*p[sd], __builtin_ia32_{min,max}[ps][sd] etc. are also
nicely constant foldable, etc.
Jakub
