[PATCH] [X86] New pass that moves immediate operands to registers.

[Serge Pavlov]

Hi Quentin,

2014-10-02 0:21 GMT+07:00 Quentin Colombet <qcolombet at apple.com>:

> The constant hoisting pass does this kind of things. Should we try to
>> teach it to handle this kind of cases?
>>
>> That would be interesting. However this pass is x86 specific and can use
> processor features (subregister structure, loading 64-bit value with 32-bit
> move). Can theses features be used by constant hoisting?
>
>
> Maybe. This pass has a bunch of target hooks if I remember correctly.
> Juergen would know better :).
>

Ok, looking forward to advice:)


> Moreover, this may be beneficial for code size, but I guess it is
>> generally not beneficial for performances. Therefore, I believe this should
>> be done for functions with the Os or Oz attributes only.
>>
>> Just curious, why? Moves from register must be faster than move from
> memory.
>
>
> Yes, but those are moves from immediate, which does not require memory at
> all.
>

On the other hand it loads memory bus.


> My performance concerns are:
> - Register pressure, like Rafael mentioned.
> - Additional scheduling dependencies.
>
> Going back to your example:
> This yields two independent chain of computation that can be scheduled
> independently. Moreover, you need just one register to realize this
> sequence.
>   mov $0, 0x4(%esi)
>    mov $0, 0x8(%esi)
>
> The two sequences of computations have now to wait for the first mov
> immediate. Moreover, this sequence requires 2 registers.
>    mov $0, %eax
>    mov %eax, 0x4(%esi)
>    mov %eax, 0x8(%esi)
>
>
Looks reasonable, thank you for explanation.


>
> Both gcc and icc use moves from register when compiling with optimization.
>
>
>
> Sure. What I am saying is that generally speaking, trading an immediate to
> register copy against a register to register copy does not sound like
> beneficial to me.
>
> Except from code size improvements, what kind of improvements are you
> seeing?
>

The main goal was code size. In fact the main interest is optimization of
memset, the problem is described in
http://llvm.org/bugs/show_bug.cgi?id=5124.

Also, how big are those improvements?
>
>
Compilation of PHP distribution with and without this pass shows size
reduction about 0.3%.


 --Serge
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