Re: useless cast blocking some optimization in gcc 4.7.3

[Laurent Alfonsi <laurent dot alfonsi at st dot com>]

I understand, Thanks for your answer.
Looking at the standard, I was thinking the example you pointed was 
undefined.
I have created a bugzilla 
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56894) to track this big 
performance regression.

My investigations pointed on the scev_const_prop optimization.
The scalar evolution of "result" variable in the loop cannot be computed 
because of the cast :

    type <integer_type 0xf7de32a0 char sizes-gimplified public string-flag
type_6 QI[...]>
    arg 0 <polynomial_chrec 0xf7ecd180
        type <integer_type 0xf7de33c0 int sizes-gimplified public type_6
SI[...]>
        arg 0 <integer_cst 0xf7dcbe70 constant 2>
        arg 1 <integer_cst 0xf7dcba80 constant 0>
        arg 2 <integer_cst 0xf7dcba9c constant 1>>>

I don't know how to fix the case. But I believe it should be looked at, 
as it is a quite big perf regression in some testcases.
please advice.

Also, I didn't find the bugzilla that led to this change of the += 
operation. I would be interested to have a look at it, if you can find it.

Thanks
Laurent

On 09.04.2013 10:50, Richard Biener wrote:
> On Mon, Apr 8, 2013 at 9:13 PM, Laurent Alfonsi <laurent.alfonsi@st.com> wrote:
> >      Hello,
> >
> > I have identified a big performance regression between 4.6 and 4.7. (I have
> > enclosed a pathological test).
> >
> > After investigation, it is because of the += statement applied on 2 signed
> > chars.
> >    - It is now type-promoted to "int" when it is written "result += foo()".
> > (since 4.7)
> >    - it is type promoted to "unsigned char" when it is written "result =
> > result + foo()".
> >
> > The "char->int->char" cast is blocking some optimizations in later phases.
> > Anyway, this doesn't look wrong, so I extended fold optimization in order to
> > catch this case. (patch enclosed)
> > The patch basically transforms :
> >      (TypeA)  ( (TypeB) a1 + (TypeB) a2 )                /* with a1 and a2 of
> > the signed type TypeA */
> > into :
> >      a1 + a2
> >
> > I believe this is legal for any licit a1/a2 input values (no overflow on
> > signed char).
> > No new failure on the two tested targets : sh-superh-elf and
> > x86_64-unknown-linux-gnu.
> > Should I enter a bugzilla to track this ? Is it ok for trunk ?
> Please open a bugzilla.  No, the patch is not ok, as said by Marc.
> It's possible to shorten the operation by performing it using
> unsigned arithmetic, that is (signed)((unsigned)a1 + (unsigned)a2).
> But if really "casts" cause the issue you are seeing that will not
> help you.
>
> Richard.
>
> > 2013-04-08  Laurent Alfonsi  <laurent.alfonsi@st.com>
> >
> >         * fold-const.c (fold_unary_loc): Suppress useless type promotion.
> >
> >
> > Thanks,
> > Laurent
> .