[LLVMdev] disabling combining load/stores in optimizer.

[Chris Lattner]

On Aug 3, 2009, at 5:16 PM, Eli Friedman wrote:

> On Mon, Aug 3, 2009 at 4:34 PM, David Greene<dag at cray.com> wrote:
>> On Monday 03 August 2009 18:01, Chris Lattner wrote:
>>> On Aug 3, 2009, at 3:09 PM, Dan Gohman wrote:
>>>>> We are currently doing this, however I think disabling such
>>>>> optimizations is a much better solution.
>>>>
>>>> An LLVM design goal is that backends should be able to outsmart
>>>> instcombine when necessary, rather than having instcombine be able
>>>> to disable parts of itself in order to avoid foiling the backends.
>>>> Practicality sometimes steers elsewhere of course. Please explain
>>>> why you think suppressing this particular optimization is better;
>>>> it isn't obvious how it would look different in the end.
>>>
>>> Yeah, I agree.  LegalizeTypes should be able to trivially lower  
>>> this.
>>
>> "Should be" is the key clause here.  I agree with you and Dan in  
>> general.
>> However, we have at least one testcase where the overhead of  
>> legalize is
>> so huge it takes *hours* to finish.  This is with LLVM 2.4 and I'm  
>> waiting
>> on an LLVM 2.5 upgrade to re-test so I can file a bug if necessary.
>
> That doesn't seem relevant; we shouldn't mess with transformation
> passes to hide performance issues in CodeGen.

What Eli said. :)

It sounds like a bug in legalize if so, a bug we should fix!  That  
said, if there are well understood reasons why the code generator is  
not doing something, and if it isn't likely to change in the near  
future, we can consider making the optimizer aware of legal integer  
types.

-Chris