Re: [PATCH] fwprop: Fix single_use_p calculation

[Richard Sandiford <richard.sandiford@arm.com>]

Ilya Leoshkevich <iii@linux.ibm.com> writes:
> On Mon, 2021-03-22 at 18:23 +0000, Richard Sandiford wrote:
>> Ilya Leoshkevich <iii@linux.ibm.com> writes:
>
> [...]
>
>> > Do you still want me to add single_nondebug_use() for completeness
>> > in
>> > this patch, or would it be better to add it later when it's
>> > actually
>> > needed?
>> 
>> I was thinking that the fwprop.c code would use
>> def->single_nondebug_use () instead of
>> def->single_nondebug_insn_use () && !def->has_phi_uses ().
>
> But these two are not equivalent, are they?  single_nondebug_use()
> that you proposed explicitly allows phis:
>
>   // If there is exactly one nondebug use of the set's result,
>   // return that use, otherwise return null.  The use might be in
>   // instruction or a phi node.
>   use_info *single_nondebug_use () const;
>
> but I don't think we want to propagate into phis here.
> Or should the check be a bit bigger, like the following?

But we're in the process of substituting the definition into an
insn use.  So we know that an insn use exists.  I think the
question we're trying to answer is: is this insn use the only
nondebug use?  I'd rather test that with a single accessor rather
than break it down into individual data structure tests.

Thanks,
Richard