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

[Quentin Colombet]

> On Oct 1, 2014, at 11:07 AM, Serge Pavlov <sepavloff at gmail.com> wrote:
> 
> Hi Quentin,
> 
> 2014-10-02 0:21 GMT+07:00 Quentin Colombet <qcolombet at apple.com <mailto: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.

You mean by its encoding size or am I missing something?

>  
> 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 <http://llvm.org/bugs/show_bug.cgi?id=5124>.  

Thanks for the context.
This seems to confirm that, at least at first, we should do that just for Os and Oz functions (i.e., look for function attribute: OptimizeForSize and MinSize).

At this point, I’ll wait for Juergen's feedbacks on constant hoisting before doing anything.
Based on his answer, we will see how to move forward.

Thanks again,
-Quentin

> 
> 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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