[llvm] r251322 - ARM/ELF: Better codegen for global variable addresses.

[Hans Wennborg via llvm-commits]

On Mon, Oct 26, 2015 at 11:23 AM, Peter Collingbourne via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: pcc
> Date: Mon Oct 26 13:23:16 2015
> New Revision: 251322
>
> URL: http://llvm.org/viewvc/llvm-project?rev=251322&view=rev
> Log:
> ARM/ELF: Better codegen for global variable addresses.
>
> In PIC mode we were previously computing global variable addresses (or GOT
> entry addresses) by adding the PC, the PC-relative GOT displacement and
> the GOT-relative symbol/GOT entry displacement. Because the latter two
> displacements are fixed, we ended up performing one more addition than
> necessary.
>
> This change causes us to compute addresses using a single PC-relative
> displacement, resulting in a shorter code sequence. This reduces code size
> by about 4% in a recent build of Chromium for Android.
>
> As a result of this change we no longer need to compute the GOT base address
> in the ARM backend, which allows us to remove the Global Base Reg pass and
> SDAG lowering for the GOT.
>
> We also now no longer use the GOT when addressing a symbol which is known
> to be defined in the same linkage unit. Specifically, the symbol must have
> either hidden visibility or a strong definition in the current module in
> order to not use the the GOT.
>
> This is a change from the previous behaviour where we would use the GOT to
> address externally visible symbols defined in the same module. I think the
> only cases where this could matter are cases involving symbol interposition,
> but we don't really support that well anyway.
>
> Differential Revision: http://reviews.llvm.org/D13650

Might be worth mentioning in the ARM section of the release notes?

 - Hans