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

Rob via llvm-dev
Wed, 15 May 2019 15:27:22 -0000

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?

Thanks,
Robert M
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