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Re: useless cast blocking some optimization in gcc 4.7.3

I understand, Thanks for your answer.
Looking at the standard, I was thinking the example you pointed was undefined. I have created a bugzilla ( 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
        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.


On 09.04.2013 10:50, Richard Biener wrote:
On Mon, Apr 8, 2013 at 9:13 PM, Laurent Alfonsi <> wrote:

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
   - 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
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.


2013-04-08  Laurent Alfonsi  <>

        * fold-const.c (fold_unary_loc): Suppress useless type promotion.



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