[llvm-commits] [llvm] r55085 - in /llvm/trunk: include/llvm/CodeGen/FastISel.h lib/CodeGen/SelectionDAG/FastISel.cpp lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

[Evan Cheng]

On Aug 20, 2008, at 7:20 PM, Chris Lattner wrote:

>
> On Aug 20, 2008, at 6:21 PM, Evan Cheng wrote:
>
>>>
>>> How would converting a multiply into a shift benefit compile time?
>>
>> It doesn't. This would be a code quality optimization. We want to be
>> fast but it doesn't mean generating poor code. :-) We can do some dag
>> combiner type optimizations if they are cheap.
>
> For now, it probably isn't worth it to do this sort of thing.

We'll see. People want fast and reasonable code. On some targets  
multiplication is really slow so it might be worthwhile to do cheap  
optimizations. We're certainly not at a point where we should worry  
about this.

>
>
> in practice, what we'll want to do for GEPs is try to fold them into
> loads and stores where possible.  This will produce smaller code and
> is probably going to be more significant than mul -> shift for fast
> compiles.

Sure. We'll look at optimizations that require examining multiple  
instructions at a time. It's probably a good idea to keep these ideas  
as FIXME's and / or in README files. We're obviously not ready to  
tackle this either.

Evan

>
>
> -Chris
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