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

[Xi Ruoyao <xry111@xry111.site>]

On Mon, 2023-12-18 at 08:39 -0700, Jeff Law wrote:
> 
> 
> On 12/18/23 06:42, Xi Ruoyao wrote:
> > With simplify_gen_unary we end up with a not fully expanded RTX like
> > 
> >      (set (reg:SI 90) (and:SI (neg:SI (reg:SI 80)) (const_int 63)))
> > 
> > Then it will cause an ICE with unrecognizable insn.
> > 
> > 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".

To me, technically the following operation

other_amount
  = simplify_gen_binary (AND, GET_MODE (op1), other_amount,
                         gen_int_mode (mask, GET_MODE (op1)));

should also be something negate_rtx too or we may still end up with an
ICE with backends incapable to match

(set (reg:SI XX) (and (reg:SI 81) (const_int 31)))

But fortunately most backends has an immediate and operation so it's
unlikely to blow up...

> If so, this patch is fine for the trunk.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University