https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037
--- Comment #15 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Jakub Jelinek from comment #14)
> (In reply to Uroš Bizjak from comment #9)
> > Created attachment 39845 [details]
> > Patch that introduces new builtins with UNSPEC_LZCNT and UNSPEC_TZCNT RTX
> > patterns
>
> Instead of UNSPEC, wouldn't it be better to model exactly what it does in
> RTL, i.e. op ? C[TL]Z (op) : <const> ? That way RTL optimizers could fold
> it.
> Of course folding it in gimple_fold target hook would be useful too.
The problem is that we have no way to macroize (const_int {16,32,64}) in the
pattern, and it would cause pattern explosion by writing out all define_insn
RTXes. So, I'd rather stay with UNSPEC and do constant folding in
ix86_fold_builtin.
