[LLVMdev] Register pressure mechanism in PRE or Smarter rematerialization/split/spiller/coalescing ?

[Daniel Berlin]

On Fri, Jul 17, 2015 at 10:07 AM, Philip Reames
<listmail at philipreames.com> wrote:
> On 07/16/2015 06:04 PM, Daniel Berlin wrote:
>>
>> That should be literally impossible, which makes me think something
>> was tested wrong
>>
>> The second patch i posted disables scalar pre (assuming you use
>> -disable-pre) but not load pre.
>>
>> Since the patch you reverted touched only scalar pre, disabling scalar
>> pre should *also* do the same thing.
>
> Unless that benchmark is relying on *some* scalar PRE happening, but not
> "too much".  i.e. it could be a really really fragile benchmark which seems
> to fit the description.

Sorry, yes, let me rephrase: It should not be having performance loss
due to the same root cause.
The failure mode described was GEP live ranges being extended due to
new phis that PRE was inserting.
That should not be happening, because it can't be doing that anymore :)