u-boot/arch/mips/config.mk
Paul Burton 703ec9ddf9 MIPS: Stop building position independent code
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
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Tested-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
2017-07-25 20:44:00 +02:00

69 lines
1.9 KiB
Makefile

#
# (C) Copyright 2003
# Wolfgang Denk, DENX Software Engineering, wd@denx.de.
#
# SPDX-License-Identifier: GPL-2.0+
#
ifdef CONFIG_SYS_BIG_ENDIAN
32bit-emul := elf32btsmip
64bit-emul := elf64btsmip
32bit-bfd := elf32-tradbigmips
64bit-bfd := elf64-tradbigmips
PLATFORM_CPPFLAGS += -EB
PLATFORM_LDFLAGS += -EB
endif
ifdef CONFIG_SYS_LITTLE_ENDIAN
32bit-emul := elf32ltsmip
64bit-emul := elf64ltsmip
32bit-bfd := elf32-tradlittlemips
64bit-bfd := elf64-tradlittlemips
PLATFORM_CPPFLAGS += -EL
PLATFORM_LDFLAGS += -EL
endif
ifdef CONFIG_32BIT
PLATFORM_CPPFLAGS += -mabi=32
PLATFORM_LDFLAGS += -m $(32bit-emul)
OBJCOPYFLAGS += -O $(32bit-bfd)
endif
ifdef CONFIG_64BIT
PLATFORM_CPPFLAGS += -mabi=64
PLATFORM_LDFLAGS += -m$(64bit-emul)
OBJCOPYFLAGS += -O $(64bit-bfd)
endif
PLATFORM_CPPFLAGS += -D__MIPS__
PLATFORM_ELFENTRY = "__start"
PLATFORM_ELFFLAGS += -B mips $(OBJCOPYFLAGS)
#
# From Linux arch/mips/Makefile
#
# GCC uses -G 0 -mabicalls -fpic as default. We don't want PIC in the kernel
# code since it only slows down the whole thing. At some point we might make
# use of global pointer optimizations but their use of $28 conflicts with
# the current pointer optimization.
#
# The DECStation requires an ECOFF kernel for remote booting, other MIPS
# machines may also. Since BFD is incredibly buggy with respect to
# crossformat linking we rely on the elf2ecoff tool for format conversion.
#
# cflags-y += -G 0 -mno-abicalls -fno-pic -pipe
# cflags-y += -msoft-float
# LDFLAGS_vmlinux += -G 0 -static -n -nostdlib
# MODFLAGS += -mlong-calls
#
ifndef CONFIG_SPL_BUILD
OBJCOPYFLAGS += -j .got -j .rel -j .padding -j .dtb.init.rodata
LDFLAGS_FINAL += --emit-relocs
endif
PLATFORM_CPPFLAGS += -G 0 -mno-abicalls -fno-pic
PLATFORM_CPPFLAGS += -msoft-float
PLATFORM_LDFLAGS += -G 0 -static -n -nostdlib
PLATFORM_RELFLAGS += -ffunction-sections -fdata-sections
LDFLAGS_FINAL += --gc-sections
OBJCOPYFLAGS += -j .text -j .rodata -j .data -j .u_boot_list