Re: [PATCH 1/2] BaseTools/GenFw AARCH64: convert ADRP to ADR if binary size allows it

[Leif Lindholm <leif.lindholm@linaro.org>]

Apologies, lost track of this one.

On Mon, Aug 01, 2016 at 01:53:09PM +0200, Ard Biesheuvel wrote:
> On 27 July 2016 at 13:26, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > The ADRP instruction in the AArch64 ISA requires the link time and load
> > time offsets of a binary to be equal modulo 4 KB. The reason is that this
> > instruction always produces a multiple of 4 KB, and relies on a subsequent
> > ADD or LDR instruction to set the offset into the page. The resulting
> > symbol reference only produces the correct value if the symbol in question
> > resides at that exact offset into the page, and so loading the binary at
> > arbitrary offsets is not possible.
> >
> > Due to the various levels of padding when packing FVs into FVs into FDs,
> > this alignment is very costly for XIP code, and so we would like to relax
> > this alignment requirement if possible.
> >
> > Given that symbols that are sufficiently close (within 1 MB) of the
> > reference can also be reached using an ADR instruction which does not
> > suffer from this alignment issue, let's replace ADRP instructions with ADR
> > after linking if the offset can be encoded in this instruction's immediate
> > field. Note that this only makes sense if the section alignment is < 4 KB.
> > Otherwise, replacing the ADRP has no benefit, considering that the
> > subsequent ADD or LDR instruction is retained, and that micro-architectures
> > are more likely to be optimized for ADRP/ADD pairs (i.e., via micro op
> > fusing) than for ADR/ADD pairs, which are non-typical.
> >
> > Contributed-under: TianoCore Contribution Agreement 1.0
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> 
> @Liming, @Leif:
> 
> are there any objections to these patches? I know it is unfortunate
> that we need to modify instructions as part of the ELF to PE/COFF
> conversion, but it is very effective

It's absolutely horrid, but extremely useful.
For the series:
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

> ArmVirtQemu-AARCH64 built with CLANG35:
> 
> Before:
> 
> FVMAIN_COMPACT [41%Full] 2093056 total, 868416 used, 1224640 free
> FVMAIN [99%Full] 4848064 total, 4848008 used, 56 free
> 
> After:
> 
> FVMAIN_COMPACT [36%Full] 2093056 total, 768064 used, 1324992 free
> FVMAIN [99%Full] 4848064 total, 4848008 used, 56 free
> 
> For comparision, GCC49
> 
> FVMAIN_COMPACT [35%Full] 2093056 total, 749960 used, 1343096 free
> FVMAIN [99%Full] 3929088 total, 3929032 used, 56 free
> 
> and GCC5 (with LTO)
> 
> FVMAIN_COMPACT [34%Full] 2093056 total, 732400 used, 1360656 free
> FVMAIN [99%Full] 3730240 total, 3730216 used, 24 free
> 
> In other words, it turns CLANG35 from a pathetic outlier into
> something usable :-)
> 
> Regards,
> Ard.