[LLVMdev] disabling combining load/stores in optimizer.

[Eli Friedman]

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.

-Eli