[LLVMdev] Area for improvement

[Chris Lattner]

On Tue, 22 Feb 2005, Jeff Cohen wrote:
> Sounds reasonable to me.  It seems to me it would be best if LSR was part of 
> the low-level optimization framework, otherwise it would be forced to 
> duplicate too much work on its own.
>
> There is still probably a need for a mid-level LSR that handles the explicit 
> multiplications, divisions, etc...

You're right: strength reduction is the #1 gray area between the LLVM 
level and the code generator level.  Doing it in the backend has several 
nice advantages (e.g. handling explicit multiplies and gep instructions is 
same analysis, you have a better understanding of register pressure, etc), 
but doing it at the llvm level also has advantages (higher level 
information is available, the xform is faster and cheaper to do, etc). 
Currently we also have the infrastructure available to do the 
transformation on the LLVM level, but don't really have it available in 
the code generator.

> Also, some of what LSR needs to decide is architecture dependent.  For 
> example, it may not want to strength reduce a multiplication which multiplies 
> by a small power of two, as this is handled by addressing modes on some 
> architectures.

You're right.  However, we can choose to expose information about target 
parameters through the Target* interfaces that llvm->llvm passes can use 
as well, so at least this aspect is not a killer issue.

-Chris

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