[RFC] Do not consider volatile asms as optimization barriers #1

[Eric Botcazou]

> ...it's so loosely defined.  If non-local labels are the specific problem,
> I think it'd be better to limit the flush to that.

No, there was "e.g." written so non-local labels are not the only problem.

> I'm back to throwing examples around, sorry, but take the MIPS testcase:
> 
>     volatile int x = 1;
> 
>     void foo (void)
>     {
>       x = 1;
>       __builtin_mips_set_fcsr (0);
>       x = 2;
>     }
> 
> where __builtin_mips_set_fcsr is a handy way of getting unspec_volatile.
> (I'm not interested in what the function does here.)  Even at -O2,
> the cse.c code successfully prevents %hi(x) from being shared,
> as you'd expect:
> 
>         li      $3,1                    # 0x1
>         lui     $2,%hi(x)
>         sw      $3,%lo(x)($2)
>         move    $2,$0
>         ctc1    $2,$31
>         li      $3,2                    # 0x2
>         lui     $2,%hi(x)
>         sw      $3,%lo(x)($2)
>         j       $31
>         nop
> 
> But put it in a loop:
> 
>     void frob (void)
>     {
>       for (;;)
>         {
>           x = 1;
>           __builtin_mips_set_fcsr (0);
>           x = 2;
>         }
>     }
> 
> and we get the rather bizarre code:
> 
>         lui     $2,%hi(x)
>         li      $6,1                    # 0x1
>         move    $5,$0
>         move    $4,$2
>         li      $3,2                    # 0x2
>         .align  3
> .L3:
>         sw      $6,%lo(x)($2)
>         ctc1    $5,$31
>         sw      $3,%lo(x)($4)
>         j       .L3
>         lui     $2,%hi(x)
> 
> Here the _second_ %hi(x), the 1 and the 2 have been hoisted but the first
> %hi(x) is reloaded each time.  So what's the correct behaviour here?
> Should the hoisting of the second %hi(x) have been disabled because the
> loop contains an unspec_volatile?  What about the 1 (from the first store)
> and the 2?

Well, I personally wouldn't spend much time on the code generated in a loop 
containing an UNSPEC_VOLATILE.  If an instruction or a builtin is supposed to 
be performance-sensitive, then don't use an UNSPEC_VOLATILE by all means and 
properly model it instead!

-- 
Eric Botcazou