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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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

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.