[Bug tree-optimization/37242] missed FRE opportunity because of signedness of addition

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #9 from rguenther at suse dot de  2008-08-27 19:12 -------
Subject: Re:  missed FRE opportunity because of
 signedness of addition

On Wed, 27 Aug 2008, bonzini at gnu dot org wrote:

> ------- Comment #8 from bonzini at gnu dot org  2008-08-27 17:50 -------
> Subject: Re:  missed FRE opportunity because
>  of signedness of addition
> 
> Maybe we can lookup the non-GIMPLE operands in simplify_unary_expression
> and replace them with existing SSA_NAMES if they have been value numbered.

So when we see

pretmp.36_86 = (unsigned int) maxIdx_24;

we do not even simplify it to (unsigned int)maxIdx_59 + 1.

So much for the fold theory...  instead we come from

#9  0x080b75dd in pointer_int_sum (resultcode=PLUS_EXPR, ptrop=0xb7c9d57c, 
    intop=0xb7ca37a0) at /home/richard/src/trunk/gcc/c-common.c:3361
3361      ret = fold_build2 (POINTER_PLUS_EXPR, result_type, ptrop, 
intop);
(gdb) call debug_generic_expr (ptrop)
y + 4
(gdb) call debug_generic_expr (intop)
(unsigned int) ((unsigned int) x * 4)

go through y p+ (4 + (unsigned int) ((unsigned int) x * 4)) which we
fold by fold_plusminus_mult_expr to ((unsigned int) x + 1) * 4.

I saw your patch that adds this folding.  And indeed we should
be able to lookup (unsigned int) x + 1 in two steps.

I may look into this at some point.

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37242