[llvm-dev] AARCH64 Code Size regression between 6/7

[Sjoerd Meijer via llvm-dev]

Bit of a drive-by comment as I haven't looked at the test case,
but could the tiny code model be helpful here (or is it perhaps related to that)?
Option -mcmodel=tiny was added not that long ago, possibly around that time.

Cheers,
Sjoerd.

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Rob via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 15 May 2019 18:57
To: Florian Hahn
Cc: llvm-dev
Subject: Re: [llvm-dev] AARCH64 Code Size regression between 6/7

I did a bit more poking, using llvm/clang versions 6-8.  The IR in all cases appears fundamentally identical.  I ran the IR generated by version 6 through llc on all three versions.  llc-7/8 produced the extra ADRPs, llc-6 did not.  So (to my untrained eyes), the IR is generated the same, it is in the IR->AARCH64 asm pass that the extra instructions are being generated.

On Wed, May 15, 2019 at 11:39 AM Florian Hahn <florian_hahn at apple.com<mailto:florian_hahn at apple.com>> wrote:
Hi,

> On May 15, 2019, at 16:27, Rob via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
>
> I am developing in C for an extremely memory constrained AARCH64 embedded environment.  Sometime between llvm 6 and 7, I'm seeing a code size regression when I make multiple accesses into a global struct.  Specifically, I have functions that perform several reads/writes into this global struct.
>
> In older versions (5/6)
> - a single ADRP/ADD combo is issued at the beginning of a function to get my structure address into a register
> - that register is preserved throughout the function
> - subsequent accesses into this structure are done as LDR/STR with offset from the preserved register
>
> In later versions (7/8)
> - the ADRP/ADD combo is performed every time I try to access something inside the struct.
>
> The net result is slightly larger code that has the potential to cause me issues. There are plenty of unused registers that could be used for the purpose of not constantly re-loading the address of my struct.  My current suspicion is that later versions are presuming fewer registers are not being preserved by other function calls, and therefore can't be relied upon to hold the address of my struct.  Assuming this is right, is there some way to encourage the behavior of the older versions?


Is the IR that gets fed into the backend equivalent between 5/6 and 7/8? This sounds like something could go wrong earlier, e.g. failing to eliminate congruent address computations in GVN.

In any case, to get to the bottom of this, a reproducer would be helpful.

Cheers,
Florian
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