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

Rob via llvm-dev
Wed, 15 May 2019 17:57:52 -0000

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>
wrote:

> Hi,
>
> > On May 15, 2019, at 16:27, Rob via llvm-dev <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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