Re: volatile and R/M/W operations

[Robert Dewar <dewar at adacore dot com>]

Samuel Tardieu wrote:
> When looking at an Ada PR, I stumbled upon the equivalent of the
> following C code:
>
> unsigned char x;
> volatile unsigned char y;
>
> void f ()
> {
>         x |= 16;
>         y |= 32;
> }
>
> With trunk/i686, the following code is generated (-O3 -fomit-frame-pointer):
>
> f:
>         movzbl  y, %eax
>         orb     $16, x
>         orl     $32, %eax
>         movb    %al, y
>         ret
>
> I cannot see a reason not to use "orb $32,y" here instead of a three
> steps read/modify/write operation. Is this only a missed optimization?
> (in which case I will open a PR)

Are you sure it is an optimization, the timing on these things is
very subtle. What evidence do you have that there is a missed
optimization here?