[llvm-dev] GEP transformation by InstCombiner

[Demikhovsky, Elena via llvm-dev]

I tried to retrieve anything from DataLayout. It contains pointer size, but how can I conclude that the GEP index can't be widened?

-           Elena

From: Hal Finkel [mailto:hfinkel at anl.gov]
Sent: Monday, January 15, 2018 20:34
To: Demikhovsky, Elena <elena.demikhovsky at intel.com>; llvm-dev at lists.llvm.org; Sanjay Patel (spatel at rotateright.com) <spatel at rotateright.com>; Chandler Carruth (chandlerc at gmail.com) <chandlerc at gmail.com>; Quentin Colombet (qcolombet at apple.com) <qcolombet at apple.com>; Craig Topper (craig.topper at gmail.com) <craig.topper at gmail.com>
Cc: Breger, Igor <igor.breger at intel.com>
Subject: Re: GEP transformation by InstCombiner


On 01/15/2018 12:21 PM, Demikhovsky, Elena wrote:
Hi all,

I'm working on an out-of-tree target and encountered the following problem:

InstCombiner "normalizes" GEPs and extends Index operand to the Pointer width.
It works fine if you can convert pointer to integer for address calculation and I assume that all registered targets do this.

The target I'm working on has very restricted ISA for the pointer calculation:
ptr + int,   ptr - int,    ptr - ptr    and   ptr-compare

I have full arithmetic set for 32-bit integers, but the Ptr is wider. Extending index to the Ptr width requires full arithmetic support for pointers.
But, actually, it does not come from C-sources (casting Ptr to int means truncation).

I'd like to add TTI (TargetTransformInfo) to InstCombiner in order to configure the width of GEP indices.
The current default behavior will be preserved.
What do you think?

Given that this affects the canonical form of the IR, based on our current practice, it should go in DataLayout (not in TTI). InstCombine should probably know how to do the right thing for the IR even if the particular target is not compiled in.

 -Hal



Thanks.

*         Elena




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