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

[Jamie Hanlon via llvm-dev]

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?

 

Thanks in advance for any help on this.

 

Jamie Hanlon (from Graphcore, www.graphcore.ai)

[Hal Finkel via llvm-dev]

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 appreciated.

 -Hal

 

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

[Jamie Hanlon via llvm-dev]

Thanks for the clarification Hal, that’s really useful. We will be sure to upstream any generic work we do on this.

 

Jamie