[LLVMdev] Keeping values in memory

[Evan Cheng]

On Sep 5, 2008, at 7:06 AM, Nicolas Capens wrote:

> Hi all,
>
> It looks like LLVM is quite eager to load values into registers when  
> they have multiple uses. Unfortunately, this increases register  
> pressure, specifically on x86. In my experience modern x86  
> processors are very capable of using memory operands as source. In  
> fact the only cases where a register is preferred over repeatedly  
> using the same memory operand is when multiple instructions could  
> use the value in the same clock cycle, or when it significantly  
> reduces code size.
>
> I have found this to be mostly a concern with SIMD code (MMX and  
> SSE). So if anyone could tell me where to find the code that  
> determines whether to load a value into a register or leave it in  
> memory, I could perform some tests to see if a better heuristic  
> exists.

This is done in instruction selection. In general using registers over  
memory is preferred. I can see certain situations where keeping values  
in registers can end up hurting performance due to added spilling.  
However, a heuristic is never going to be satisfactory solution  
because it's guaranteed to miss important cases.

The right approach to combat this is implement generalized re- 
materialization in the register allocator. That'll come sometime  
(hopefully) next year when I rewrite the register allocator.

Evan


>
> Thanks,
>
> Nicolas Capens
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