[llvm-dev] [RFC] Remarks-based code size analysis tool

[Sjoerd Meijer via llvm-dev]

Hi Jessica,

We often have exactly the same problem:

> I frequently find that I want to quickly find out which functions have changed the most in size when I compile a program with one version of the compiler and another version of the compiler.

Your tool looks very interesting and useful. I think we are very interested. In fact, I am looking at code-size problems at the moment, and will give it a try. I think we can help with the review too.

FWIW, when I investigate code-size problems, I usually start looking at the final linked image (for context, that's mostly baremetal images). Unused sections and functions will be removed by the linker, which e.g. avoids unnecessary analysing them which may happen when you only look at object files. I use a downstream tool for binaries that list the code-size per function. When I generate such a report for different images, I can easily diff them and see where codesize difference are the biggest. I then isolate a function, compile it with debug and print-after-all, so that I can do a diff again where changes are introduced. And when image sizes are small, and library inclusion is thus relatively important, the linker in verbose mode telling me why and which objects and functions are included is extremely useful too.

Cheers,
Sjoerd.



________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Jessica Paquette via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 14 June 2019 00:30
To: Jessica Paquette via llvm-dev
Subject: [llvm-dev] [RFC] Remarks-based code size analysis tool

Hi everyone,

I frequently find that I want to quickly find out which functions have changed the most in size when I compile a program with one version of the compiler and another version of the compiler.

Optimization remarks provide deeper insight into what the compiler does to individual functions during the compilation process. A tool based off of remarks would give us the means to say *exactly* what a pass did to a function during the compilation process. We also have remarks from the asm-printer which tell us the number of instructions in a function at any given time.

It’s now possible to access remarks through object files. So, I thought it would be an interesting experiment to write a C++ tool using the new remarks API to write a simple size tool. A size tool using remarks would be able to tell us about the following things:

1) Which functions grew the most in size between one compilation and another compilation
2) Which functions were/were not inlined between compilations (and similar insights into other passes)
3) Which compiler passes caused the largest increases in each function

I have a prototype for a size tool which accomplishes (1) using asm-printer remarks. (2) and (3) can be accomplished by searching for remarks corresponding to interesting passes and by searching for size-info remarks respectively. For now, I just decided to implement the minimum set of features for a useful tool. The patch is available here:

https://reviews.llvm.org/D63306

The tool in the patch works like this:

$ ./sizediff old_version_of_program new_version_of_program

And it produces output like this:

function_name old_instruction_count new_instruction_count delta

This output can tell me the following things:

- Which functions were not present in old_version_of_program (they have a size of 0)
- Which functions were not present in new_version_of_program (same idea)
- Which functions grew the most
- Which functions shrunk the most

For a concrete example of the output, here’s some output I got by compiling the stepanov_container benchmark for AArch64 with -Os and -O3 respectively:

https://hastebin.com/hucexocuxi.md

If anyone is interested, feel free to try it out/review/etc. I think that a tool like this would be quite useful. :)

- Jessica
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