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@imgtec.com Cc: Daniel Schwierzeck daniel.schwierzeck@gmail.com Cc: u-boot@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