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

[Daniel Schwierzeck]

Am 16.06.2017 um 02:05 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
> ---
> 
>  arch/mips/Makefile.postlink    |  23 +++
>  arch/mips/config.mk            |  19 +-
>  arch/mips/cpu/start.S          | 130 -------------
>  arch/mips/cpu/u-boot.lds       |  41 +---
>  arch/mips/include/asm/relocs.h |  24 +++
>  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 +++++++++++++++++++++++++++++++++++++++++
>  11 files changed, 656 insertions(+), 177 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
> 

there is a regression on qemu_mips when started with Qemu. The code
execution hangs in an endless loop and doesn't reach the console prompt.

I could debug it to following location:

int bootm_find_images(int flag, int argc, char * const argv[])
{
	int ret;

	/* find ramdisk */
	ret = boot_get_ramdisk(argc, argv, &images, IH_INITRD_ARCH,
			       &images.rd_start, &images.rd_end);
	if (ret) {
		puts("Ramdisk image is corrupt or invalid\n");
		return 1;
	}
    ...
}

The code flow goes into the "if (ret)" branch. At the "return 1" the $ra
register contains the address of bootm_find_images(). Thus the code is
executed in an endless loop. I don't know yet if that's a miscalculated
relocation or a stack overflow (maybe due to the changed 64KiB alignment
of the U-Boot relocation address) because $ra in bootm_find_images() is
loaded from stack:

bfc08f1c <bootm_find_images>:
bfc08f1c:	3c02bfc3 	lui	v0,0xbfc3
bfc08f20:	27bdffe0 	addiu	sp,sp,-32
...
bfc08f6c:	8fbf001c 	lw	ra,28(sp)
bfc08f70:	00601025 	move	v0,v1
bfc08f74:	03e00008 	jr	ra
bfc08f78:	27bd0020 	addiu	sp,sp,32


This regression leads to broken pytest for qemu_mips and therefore to
failing Travis CI [1]. I used the kernel.org MIPS toolchain with gcc-4.9
(same as in Travis CI). Could you please have a look?


[1] https://travis-ci.org/danielschwierzeck/u-boot/jobs/243667863

-- 
- Daniel

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