[llvm-dev] Pre/post-increment addressing mode in LSR

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 23 06:09:45 PST 2017

On 01/23/2017 05:52 AM, Jamie Hanlon via llvm-dev wrote:
> Dear all,
> From what I can gather, there is currently no way for loop strength 
> reduction to target pre- and post-increment addressing modes. This is 
> because the target hook `isLegalAddressingMode` in 
> TargetTransformInfo.h doesn’t allow for pre- and post-increment. There 
> is in fact a comment to that effect on the function prototype: “TODO: 
> handle pre/postinc as well” (see 
> http://llvm.org/docs/doxygen/html/TargetTransformInfo_8h_source.html 
> line 310).
> So I was wondering: is there a way to work around this limitation? And 
> are there any plans to add support for pre- and post-increment 
> addressing modes?

It is, unfortunately, a long-standing deficiency. There are some 
work-arounds in some targets (e.g. 
lib/Target/PowerPC/PPCLoopPreIncPrep.cpp), but work on this would be 


> Thanks in advance for any help on this.
> Jamie Hanlon (from Graphcore, www.graphcore.ai)
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Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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