[llvm-dev] [RFC][AArch64] Homogeneous Prolog and Epilog for Size Optimization

[Vedant Kumar via llvm-dev]

This looks really interesting. In the slides, it’s mentioned that the combination of tuning the MachineOutliner for ThinLTO and of optimizing function prolog/epilogs improved measured run-time performance.

What kind of performance impact do you see from simply homogenizing prolog/epilogs? (If, say across LNT/aarch64/-Oz the performance impact is not large, it may make sense to have homogenization enabled by default.)

best,
vedant

> On Mar 23, 2020, at 11:32 PM, Kyungwoo Lee via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
> 
> I'd like to upstream our work over the time which the community would benefit from.
> This is a part of effort toward minimizing code size presented in here <https://llvm.org/devmtg/2020-02-23/slides/Kyungwoo-GlobalMachineOutlinerForThinLTO.pdf>. In particular, this RFC is about optimizing prolog and epilog for size.
> 
> Homogeneous Prolog and Epilog for Size Optimization, D76570 <https://reviews.llvm.org/D76570>:
> 
> Prolog and epilog to handle callee-save registers tend to be irregular with different immediate offsets, which are not often being outlined (by machine outliner) when optimizing for size. From D18619, combining stack operations stretched irregularity further.
> This patch tries to emit homogeneous stores and loads with the same offset for prolog and epilog respectively.  We have observed that this homogeneous prolog and epilog significantly increased the chance of outlining, resulting in a code size reduction. However, there were still a great deal of outlining opportunities left because the current outliner had to conservatively handle instructions with the return register, x30.
> Rather, this patch also forms a custom-outlined helper function on demand for prolog and epilog when lowering the frame code.
> 
> - Injects HOM_Prolog and HOM_Epilog pseudo instructions in Prolog and Epilog Injection Pass
> - Lower and optimize them in AArchLowerHomogneousPrologEpilog Pass
> - Outlined helpers are created on demand. Identical helpers are merged by the linker.
> - An opt-in flag is introduced to enable this feature. Another threshold flag is also introduced to control the aggressiveness of outlining for application's need.
> 
> This reduced an average of 4% of code size for LLVM-TestSuite/CTMark targeting arm64/-Oz. In a large mobile application, the size benefit was even larger reducing the page-faults as well.
>  
> Design Alternatives:
> 
> 1. Expand helpers eagerly by permuting all cases in an earlier module pass. Even though this is rather simple and less invasive, it creates many redundant helpers which need to be elided by the linker.
> 2. Turn Prolog-Epilog-Injection into a module pass. Need to plumb the module through the pass and architecture specific frame-lowering. Not sure about other architecture interaction with this module pass.
> 3. Runtime/compiler-rt for all helpers. The combinations of helpers are a lot and certainly this approach is not flexible.
> 
> Regards,
> Kyungwoo
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