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.bur...@imgtec.com> > Cc: Daniel Schwierzeck <daniel.schwierz...@gmail.com> > Cc: u-boot@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.schwierz...@gmail.com> Tested-by: Daniel Schwierzeck <daniel.schwierz...@gmail.com>
applied to u-boot-mips/next, thanks.
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