[PATCH v2] Add uaddv_optab, usubv4_optab

[Segher Boessenkool]

Hi Richard,

On Sun, Nov 22, 2015 at 11:38:31AM +0100, Richard Henderson wrote:
> One of which I believe I've worked around in the i386 backend, but I 
> believe to be a latent problem within combine.
> 
> With the following patch, disable the add<mode>3_*_overflow_2 patterns.  
> Then compile c-c++-common/torture/builtin-arith-overflow-4.c with -O2 and 
> you'll see
> 
>  t151_2add:
>        testb   %dil, %dil
>        leal    -1(%rdi), %eax
>        jne     .L644
> 
> which is incorrect.  Combine has looked through two comparisons, seen the 
> NE in the second comparison, and then converted a CCCmode compare to a 
> CCZmode compare.

It sees the *first* comparison, and its use, and has simplified that.
As far as I see, anyway.  (It will never look outside a basic block,
combine isn't *that* scary!)

0xff + x < 0xff  (everything as unsigned char) is the same as  x != 0 .


Segher