[U-Boot] ARM Relocation compiler and linker switches (was: AT91: problems master vs. next)

[Albert ARIBAUD]

Le 21/09/2010 16:00, Albert ARIBAUD a écrit :

> This is due to the fact that each routine has to recompute the PIC
> register. As a test, I tried adding -msingle-pic-base to -fPIC (this
> computes the PIC register once for the whole code) and the code size
> falls back to 123764 bytes, 'only' 5.5% more than the non relocating
> case. Using -fPIE -pie -msingle-pic-base lowers this again to 5.2%.
>
> Of course you cannot just turn -msingle-pic-base on; you've got to have
> the code in start.S that computes the register. Also, switching from PIC
> to PIE needs to be verified. I've got the code in my local tree, but
> it's not tested yet. I'll test it tonight and post it if it works.

I haven't fully tested yet, but I've given a look at -fPIC vs -fPIE vs 
-fPIE -pie, with and without -msingle-pic-base, and here are the 
results; note that the following numbers were measured on the edminiv2 
config (I'd previously used a not-yet-submitted net5big config).

Compiling u-boot with '#define CONFIG_SYS_ARM_WITHOUT_RELOC' yields a 
133700 byte u-boot.bin whereas '#undef CONFIG_SYS_ARM_WITHOUT_RELOC' 
yields 145588, an increase of +8.9% -- AIUI this increase is due only to 
relocation changes as all the rest is unchanged, including Wolfgang's 
environment patches. This increase is explained by the fact that all 
global object accesses now need an additional indirection through the GOT.

Replacing -fPIC with -fPIE reduces size down to 144856 (+8.3%), by 
replacing the GOT indirection for initialized data with sl-relative 
addressing (uninitialized data still go through the GOT). This has the 
added marginal benefit that the GOT shrinks from 520 down to 296 bytes. 
The condition for -fPIE to work is that the relative position of .data 
to .text must remain constant across relocations, which is the case for 
u-boot.

Unfortunately I haven't seen any gcc/ld option which would replace 
indirection by sl-relative addressing for .bss.

Adding linker option -pie to -fPIE has no noticeable effect on size, 
though it does move things around -- I'll have to dig deeper into this 
one; until I know its effects, I'll just not add it.

Finally, adding -msingle-pic-base to any of the previous obviously 
reduces code size by not recomputing the pic base register in every 
function; for -fPIE -msingle-pic-base, the u-boot size shrinks down to 
140992 (+5.5%). Since this recomputation is useful only if relocation 
scatters .text, which is not a requirement (that I know of) for u-boot, 
we can safely add -msingle-pic-base, provided start.S computes the pic 
base register.

As for implementing these changes: going from -fPIC to -fPIE can be done 
at ARM level because it only requires modifying PLATFORM_RELFLAGS in 
arch/arm/config.mk. Adding -msingle-pic-base, though, requires a change 
to start.S for computing the pic base into r10 and r9 (only one of these 
will be used, depending on whether stack checking is enabled or not); 
since start.S is cpu-specific, the change must be at cpu, not arch, 
level. I'll provide a tested patch for arm926ejs along with the -fPIE 
patch; for other arm cpus, I can provide a patch but not test it.

Amicalement,
-- 
Albert.