[llvm-dev] Status of the function merging pass?

[Vedant Kumar via llvm-dev]

Yikes, Mail.app dropped a bunch of CC’s. I’ll try to re-add them now.


> On Jan 31, 2019, at 12:24 PM, Shoaib Meenai <smeenai at fb.com> wrote:
> 
> (Disclaimer: I don’t know anything about MergeFunctions; I’m just assuming functionality from the name)
>  
> How does MergeFunctions compare to performing identical code folding (ICF) in the linker?

IIUC there are a couple important differences:

1) Linker ICF tends to be fairly restricted. E.g. on Darwin, the linker only deduplicates if a) the compiler has marked the symbol “auto-hide”, b) the text of both symbols literally memcmp’s to 0, and c) they have exactly the same relocations. MergeFunctions results in substantial code size reductions even with linker deduplication enabled, probably because it can detect more kinds of equivalent functions.

2) MergeFunctions runs much earlier in the pipeline, so it can save compile-time (no need to optimize forwarding thunks)

vedant

>  
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Vedant Kumar via llvm-dev <llvm-dev at lists.llvm.org>
> Reply-To: Vedant Kumar <vsk at apple.com>
> Date: Thursday, January 31, 2019 at 11:52 AM
> To: llvm-dev <llvm-dev at lists.llvm.org>
> Cc: "nikic at php.net" <nikic at php.net>
> Subject: [llvm-dev] Status of the function merging pass?
>  
> Hi, 
>  
> I'm interested in finding ways to reduce code size. LLVM's MergeFunctions pass seems like a promising option, and I'm curious about its status in tree.
>  
> Enabling MergeFunctions gives a 1% code size reduction across the entire iOS shared cache (a collection of a few hundred system-critical DSO's). The numbers are even more compelling for Swift code. In fact, the swift compiler enables MergeFunctions by default when optimizing, along with an even more aggressive merging pass which handles equivalence-modulo-constant-uses (https://github.com/apple/swift/blob/master/lib/LLVMPasses/LLVMMergeFunctions.cpp <https://github.com/apple/swift/blob/master/lib/LLVMPasses/LLVMMergeFunctions.cpp>).
>  
> Is anyone actively working on enabling MergeFunctions in LLVM's default pipelines? Is there a roadmap for doing so?
>  
> ISTM that preventing miscompiles when merging functions is a serious, unsolved problem. I.e., it's hard for the MergeFunctions pass to be *really sure* that two functions are a) really identical and b) safe to merge.
>  
> Is there a systematic solution at the IR-level, given that the semantics of IR are subject to change? Is extensive testing the only solution? Or is this intractable, and the only safe approach is to perform merging post-regalloc (or, at some late point when equivalence is easier to determine)?
>  
> thanks,
> vedant

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