[PATCH] Extending Vector GetElementPtr - please review

[Demikhovsky, Elena]

I really need this change to optimize masked gather and scatter code. I have a mail thread in LLVM dev, where I justify this change and people agreed with me.

Please see the start of discussion: http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-March/082921.html
And the end agreement: http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-March/082989.html

> I'd have thought that it should be pretty simple for any front-end or back-end to do the splat on their own.
The front-end can do the splat, but I want to allow front-end to leave a scalar value and let the backend to splat.
When the frontend splats the scalar, it generates a broadcast instruction before GEP. While looking at the GEP in backend, I can't find this broadcast (but I need it!), it is moved up to another basic block.

> To justify such change, I'm looking for some kind of transformation / optimisation / selection that cannot be done unless the base pointer is a scalar.
Initially I added an optimization for masked gather/scatter  to this patch, but Hal asked me to remove it and submit separately.

I plan to add masked gather and scatter to the loop vectorizer. I want to use the updated form of Vector GEP there.

-  Elena


-----Original Message-----
From: Renato Golin [mailto:renato.golin at linaro.org] 
Sent: Wednesday, July 01, 2015 11:13
To: Demikhovsky, Elena; nrotem at apple.com; aschwaighofer at apple.com; mzolotukhin at apple.com; dblaikie at gmail.com; hfinkel at anl.gov
Cc: llvm-commits at cs.uiuc.edu
Subject: Re: [PATCH] Extending Vector GetElementPtr - please review

Hi Elena,

Maybe I'm missing something, but can you give an example on where this will be clearly beneficial? From what I see, you're just moving the time when you go from i8* to <N x i8*>, a bit further down the pipeline, and complicating the IR in the process. I'd have thought that it should be pretty simple for any front-end or back-end to do the splat on their own.

To justify such change, I'm looking for some kind of transformation / optimisation / selection that cannot be done unless the base pointer is a scalar.

cheers,
--renato


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