Re: [Qemu-devel] [PATCH for-2.5 04/10] tcg/optimize: allow constant to have copies

[Alex Bennée]

Aurelien Jarno <address@hidden> writes:

> On 2015-07-29 17:12, Alex Bennée wrote:
>> 
>> Aurelien Jarno <address@hidden> writes:
>> 
>> > Now that copies and constants are tracked separately, we can allow
>> > constant to have copies, deferring the choice to use a register or a
>> > constant to the register allocation pass. This prevent this kind of
>> > regular constant reloading:
>> >
>> > -OUT: [size=338]
>> > +OUT: [size=298]
>> >    mov    -0x4(%r14),%ebp
>> >    test   %ebp,%ebp
>> >    jne    0x7ffbe9cb0ed6
>> >    mov    $0x40002219f8,%rbp
>> >    mov    %rbp,(%r14)
>> > -  mov    $0x40002219f8,%rbp
>> >    mov    $0x4000221a20,%rbx
>> >    mov    %rbp,(%rbx)
>> >    mov    $0x4000000000,%rbp
>> >    mov    %rbp,(%r14)
<snip>
>> > --- a/tcg/optimize.c
>> > +++ b/tcg/optimize.c
>> > @@ -237,11 +237,6 @@ static void tcg_opt_gen_mov(TCGContext *s, TCGOp *op, 
>> > TCGArg *args,
>> >          return;
>> >      }
>> >  
>> > -    if (temp_is_const(src)) {
>> > -        tcg_opt_gen_movi(s, op, args, dst, temps[src].val);
>> > -        return;
>> > -    }
>> > -
>> 
>> That looks suspicious, surely we only want to drop the move if we
>> already have the const somewhere else?
>
> That's actually the while point of this patchset, to avoid converting
> mov into moving for constant values and thus loosing the link between
> temps.

I get that, I guess I didn't follow the previous loading of the constant
value. Maybe showing the ops in the commit message will make it clearer.

>
> At this moment point of the code, the only way to know that the source
> temp is a constant is when it has been set using a movi before.

OK.


-- 
Alex Bennée