[U-Boot] [PATCH v3 2/2] MIPS: Stop building position independent code

[Daniel Schwierzeck]

Am 19.06.2017 um 20:53 schrieb Paul Burton:
> U-Boot has up until now built with -fpic for the MIPS architecture,
> producing position independent code which uses indirection through a
> global offset table, making relocation fairly straightforward as it
> simply involves patching up GOT entries.
> 
> Using -fpic does however have some downsides. The biggest of these is
> that generated code is bloated in various ways. For example, function
> calls are indirected through the GOT & the t9 register:
> 
>   8f998064   lw     t9,-32668(gp)
>   0320f809   jalr   t9
> 
> Without -fpic the call is simply:
> 
>   0f803f01   jal    be00fc04 <puts>
> 
> This is more compact & faster (due to the lack of the load & the
> dependency the jump has on its result). It is also easier to read &
> debug because the disassembly shows what function is being called,
> rather than just an offset from gp which would then have to be looked up
> in the ELF to discover the target function.
> 
> Another disadvantage of -fpic is that each function begins with a
> sequence to calculate the value of the gp register, for example:
> 
>   3c1c0004   lui    gp,0x4
>   279c3384   addiu  gp,gp,13188
>   0399e021   addu   gp,gp,t9
> 
> Without using -fpic this sequence no longer appears at the start of each
> function, reducing code size considerably.
> 
> This patch switches U-Boot from building with -fpic to building with
> -fno-pic, in order to gain the benefits described above. The cost of
> this is an extra step during the build process to extract relocation
> data from the ELF & write it into a new .rel section in a compact
> format, plus the added complexity of dealing with multiple types of
> relocation rather than the single type that applied to the GOT. The
> benefit is smaller, cleaner, more debuggable code. The relocate_code()
> function is reimplemented in C to handle the new relocation scheme,
> which also makes it easier to read & debug.
> 
> Taking maltael_defconfig as an example the size of u-boot.bin built
> using the Codescape MIPS 2016.05-06 toolchain (gcc 4.9.2, binutils
> 2.24.90) shrinks from 254KiB to 224KiB.
> 
> Signed-off-by: Paul Burton <paul.burton at imgtec.com>
> Cc: Daniel Schwierzeck <daniel.schwierzeck at gmail.com>
> Cc: u-boot at lists.denx.de
> 
> ---
> 
> Changes in v3:
> - Fix R_MIPS_26 case that was breaking qemu_mips_defconfig.
> 
> Changes in v2:
> - Drop unused .$@.relocs from Makefile.postlink.
> - Remove PF_OBJCOPY & assign to OBJCOPYFLAGS directly in config.mk.
> - Move declaration of __rel_start to asm/sections.h.
> 
>  arch/mips/Makefile.postlink      |  23 +++
>  arch/mips/config.mk              |  21 +-
>  arch/mips/cpu/start.S            | 130 ------------
>  arch/mips/cpu/u-boot.lds         |  41 +---
>  arch/mips/include/asm/relocs.h   |  24 +++
>  arch/mips/include/asm/sections.h |   7 +
>  arch/mips/lib/Makefile           |   1 +
>  arch/mips/lib/reloc.c            | 164 +++++++++++++++
>  common/board_f.c                 |   2 +-
>  tools/.gitignore                 |   1 +
>  tools/Makefile                   |   2 +
>  tools/mips-relocs.c              | 426 +++++++++++++++++++++++++++++++++++++++
>  12 files changed, 663 insertions(+), 179 deletions(-)
>  create mode 100644 arch/mips/Makefile.postlink
>  create mode 100644 arch/mips/include/asm/relocs.h
>  create mode 100644 arch/mips/lib/reloc.c
>  create mode 100644 tools/mips-relocs.c
> 
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck at gmail.com>
Tested-by: Daniel Schwierzeck <daniel.schwierzeck at gmail.com>


applied to u-boot-mips/next, thanks.

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