Re: [PATCH] middle-end: Call negate_rtx instead of simplify_gen_unary expanding rotate shift [PR113033]

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Dec 19, 2023 at 12:48:46AM +0800, Xi Ruoyao wrote:
> > > gcc/ChangeLog:
> > > 
> > > 	PR middle-end/113033
> > > 	* expmed.cc (expand_shift_1): When expanding rotate shift, call
> > > 	negate_rtx instead of simplify_gen_unary (NEG, ...).
> 
> > The key difference being that using negate_rtx will go through the 
> > expander which knows how to synthesize negation whereas 
> > simplify_gen_unary will just generate a (neg ...) and assume it matches 
> > something in the backend, right?
> 
> For PR113033 the key difference (to me) is negate_rtx emits an insn to
> set a new pseudo reg to -x.  So the result will be
> 
> (set (reg:SI 81) (neg:SI (reg:SI 80)))
> 
> then
> 
> (and (reg:SI 81) (const_int 31))
> 
> instead of a consolidated
> 
> (and:SI (neg:SI (reg:SI IN)) (const_int 63))
> 
> AFAIK no backends have an instruction doing "negate an operand then and
> bitwisely".

Can you explain why it doesn't work as is though?
I mean, expand_shift_1 with that (and (neg (reg ...)) (const_int ...))
should try to legitimize the operand (e.g. in maybe_legitimize_operand
-> force_operand and force_operand should be able to deal with that,
AND is binary op, so it recurses on the 2 operands and NEG is UNARY_P,
so the recursion should deal with that if it is not general_operand.

	Jakub