On Thu, Jan 09, 2020 at 02:26:10PM +0100, Richard Biener wrote:
> > >> + tree lhs = gimple_assign_lhs (stmt);
> > >> + bool zero_ok = CTZ_DEFINED_VALUE_AT_ZERO (TYPE_MODE (type), val);
> > >
> > > since we're using the optab entry shouldn't you check for == 2 here?
> >
> > Yes, that looks more correct (it's not clear what 1 means exactly).
1 is what e.g. x86 uses with -mbmi, and I think it is what most targets
actually define if they have any defined value there, exception is
aarch64/arm, powerpc, gcn, nvptx and mips.
Given:
int foo (int x) { return __builtin_ctz (x); }
int bar (int x) { return x ? __builtin_ctz (x) : 32; }
on x86 -mbmi we with CTZ_DEFINED_VALUE_AT_ZERO being 1 and value type_size
emit the same code, we'd need to find out if that is something done for all
targets or just a few, and if for all that have CTZ_DEFINED_VALUE_AT_ZERO 1,
we could perhaps just emit branchy code and wait for RTL to fix that up.
That said, perhaps we should also check if the argument isn't known to be
non-zero from VRP, in that case we wouldn't have to bother with the zero
value stuff at all.
Jakub
