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add busybox_ldscript.README.txt

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
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Denys Vlasenko 2022-01-23 15:46:05 +01:00
parent 39369ff460
commit 33a9f34df5
1 changed files with 47 additions and 0 deletions
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/* Add SORT_BY_ALIGNMENT to linker script (found in busybox_unstripped.out):
## .rodata : { *(.rodata SORT_BY_ALIGNMENT(.rodata.*) .gnu.linkonce.r.*) }
## .data : { *(.data SORT_BY_ALIGNMENT(.data.*) .gnu.linkonce.d.*) }
## .bss : { *(.bss SORT_BY_ALIGNMENT(.bss.*) .gnu.linkonce.b.*) }
## This will eliminate most of the padding (~3kb).
## Hmm, "ld --sort-section alignment" should do it too.
##
## There is a ld hack which is meant to decrease disk usage
## at the cost of more RAM usage (??!!) in standard ld script:
## . = ALIGN (0x1000) - ((0x1000 - .) & (0x1000 - 1)); . = DATA_SEGMENT_ALIGN (0x1000, 0x1000);
## Replace it with:
## . = ALIGN (0x1000); . = DATA_SEGMENT_ALIGN (0x1000, 0x1000);
## to unconditionally align .data to the next page boundary,
## instead of "next page, plus current offset in this page"
*/
/* To reduce the number of VMAs each bbox process has,
## move *(.bss SORT_BY_ALIGNMENT(.bss.*) ...)
## part from .bss : {...} block to .data : { ... } block.
## (This usually increases .data section by only one page).
## Result:
##
## text data bss dec hex filename
## 1050792 560 7580 1058932 102874 busybox.bss
## 1050792 8149 0 1058941 10287d busybox.nobss
##
## $ exec busybox.bss pmap $$
## 0000000008048000 1028K r-xp /path/to/busybox.bss
## 0000000008149000 8K rw-p /path/to/busybox.bss
## 000000000814b000 4K rw-p [ anon ] <---- this VMA is eliminated
## 00000000085f5000 4K ---p [heap]
## 00000000085f6000 4K rw-p [heap]
## 00000000f7778000 8K rw-p [ anon ]
## 00000000f777a000 12K r--p [vvar]
## 00000000f777d000 8K r-xp [vdso]
## 00000000ff7e9000 132K rw-p [stack]
##
## $ exec busybox.nobss pmap $$
## 0000000008048000 1028K r-xp /path/to/busybox.nobss
## 0000000008149000 12K rw-p /path/to/busybox.nobss
## 00000000086f0000 4K ---p [heap]
## 00000000086f1000 4K rw-p [heap]
## 00000000f7783000 8K rw-p [ anon ]
## 00000000f7785000 12K r--p [vvar]
## 00000000f7788000 8K r-xp [vdso]
## 00000000ffac0000 132K rw-p [stack]
*/