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

[Peter Collingbourne via llvm-commits]

On Thu, Feb 11, 2016 at 04:36:08PM -0800, Hans Wennborg wrote:
> On Thu, Feb 11, 2016 at 4:22 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> > On Thu, Feb 11, 2016 at 04:17:49PM -0800, Hans Wennborg wrote:
> >> 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?
> >
> > On account of the behaviour change in the last paragraph? I had to revert
> > that aspect of the change in r251342.
> 
> Sorry, I meant to quote the whole change description, of which I
> mostly read "Better codegen for global variable addresses", which
> sounded like a good thing to mention :-)

Ah. Sounds reasonable to me then, I'll try to get something written up soon.

Thanks,
-- 
Peter