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

[Paul Burton]

Hi Daniel,

On Friday, 16 June 2017 15:48:06 PDT Daniel Schwierzeck wrote:
> 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

This was due to an R_MIPS_26 relocation applied to a j instruction in 
show_board_info, which overflowed into the opcode field & changed the j into a 
jal.

v3 should fix it.

Thanks,
    Paul
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