[LLVMdev] A question about everyone's favorite constructs: NSW and NUW

Quentin Colombet qcolombet at apple.com
Tue Jan 7 12:09:30 PST 2014

On Jan 7, 2014, at 11:40 AM, Chandler Carruth <chandlerc at google.com> wrote:

> On Tue, Jan 7, 2014 at 2:24 PM, Quentin Colombet <qcolombet at apple.com> wrote:
> I agree with Andy and Jim.
> Actually, I have prototyped a compiler that does exactly this kind of promotion in CodeGenPrepare.
> Basically, I have updated the addressing mode matcher so that it moves a sext that is in a way of an addressing mode (i.e., it promotes the operand of the sext, let us call this operand def, if it is legal to do so, and sign extends the operands of def). When the matcher does not manage to absorb more computation after promoting def, it can revert the promotion.
> I am currently benchmarking this solution and I’ll update this thread with the results.
> Very cool. Could you share the patch? I can also run some benchmarks.
Sure, but be aware that this is a prototype (not structured, not commented) :).
In particular, it is missing the rollback if the promotion does not help

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