MIPS: relocation: add relocation support

this patch i a port of following patch from u-boot with some additional
integration changes and fixes of original code:
| Subject: [PATCH] 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>

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 24 insertions, 3 deletions

diff --git a/arch/mips/lib/barebox.lds.S b/arch/mips/lib/barebox.lds.S
index 8ddf954517..58b0c5919d 100644
--- a/arch/mips/lib/barebox.lds.S
+++ b/arch/mips/lib/barebox.lds.S
@@ -13,6 +13,8 @@ SECTIONS
 	. = TEXT_BASE;
 
 	. = ALIGN(4);
+
+	.image_start : { *(.__image_start) }
 	.text      :
 	{
 		_stext = .;
@@ -79,9 +81,28 @@ SECTIONS
 	.dtb : { BAREBOX_DTB() }
 
 	_edata = .;
+	.image_end : { *(.__image_end) }
+
 	. = ALIGN(4);
-	__bss_start = .;
-	.bss : { *(.bss*) }
-	__bss_stop = .;
+	/*
+	 * .rel must come last so that the mips-relocs tool can shrink
+	 * the section size & the PT_LOAD program header filesz.
+	 */
+	.data.reloc : {
+		__rel_start = .;
+		BYTE(0x0)
+		. += (32 * 1024) - 1;
+	}
+
 	_end = .;
+
+	.bss __rel_start (OVERLAY) : {
+		__bss_start = .;
+		*(.sbss.*)
+		*(.bss.*)
+		*(COMMON)
+		. = ALIGN(4);
+		__bss_stop = .;
+	}
+
 }