[PATCH] D64854: [ELF] Delete redundant pageAlign of the first section after PT_GNU_RELRO after D58892

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added subscribers: sfertile, atanasyan.
MaskRay added a comment.

Currently we align p_vaddr to the next multiple of max-page-size, instead of `ALIGN(CONSTANT (MAXPAGESIZE)) + (. & (CONSTANT (MAXPAGESIZE) - 1))` as ld.bfd does in its `-z noseparate-code` mode and some cases in its `-z separate-code` mode. See the comment below for a ld.lld -z max-page-size=0x200000 case:

    [11] .rodata           PROGBITS        0000000000000558 000558 000004 04  AM  0   0  4                        
    [12] .eh_frame_hdr     PROGBITS        000000000000055c 00055c 00002c 00   A  0   0  4                        
    [13] .eh_frame         PROGBITS        0000000000000588 000588 0000cc 00   A  0   0  8             
  ////// gap due to separated R-- and R-X   
  ///// This gap can be saved in --no-rosegment mode.   
    [14] .text             PROGBITS        0000000000200000 200000 000161 00  AX  0   0 16                       
    [15] .init             PROGBITS        0000000000200164 200164 000017 00  AX  0   0  4                       
    [16] .fini             PROGBITS        000000000020017c 20017c 000009 00  AX  0   0  4                       
    [17] .plt              PROGBITS        0000000000200190 200190 000020 00  AX  0   0 16 
                        
    [18] .fini_array       FINI_ARRAY      0000000000400000 400000 000008 08  WA  0   0  8                       
    [19] .init_array       INIT_ARRAY      0000000000400008 400008 000008 08  WA  0   0  8                       
    [20] .dynamic          DYNAMIC         0000000000400010 400010 0001a0 10  WA  8   0  8                       
    [21] .got              PROGBITS        00000000004001b0 4001b0 000028 00  WA  0   0  8                       
    [22] .bss.rel.ro       NOBITS          00000000004001d8 4001d8 000000 00  WA  0   0  1         
  /// Gap due to PT_GNU_RELRO. It wasts almost 0x200000 bytes.
  /// If we change p_vaddr of the RW PT_LOAD from 0x600000 to 0x6001d8, its p_offset doesn't need to be aligned, and we can save nearly 0x200000 bytes in the file.
    [23] .data             PROGBITS        0000000000600000 600000 000010 00  WA  0   0  8                       
    [24] .tm_clone_table   PROGBITS        0000000000600010 600010 000000 00  WA  0   0  8                       
    [25] .got.plt          PROGBITS        0000000000600010 600010 000020 00  WA  0   0  8                       
    [26] .bss              NOBITS          0000000000600030 600030 000001 00  WA  0   0  1                       

We perform several max-page-size alignments for this file and each costs nearly 0x200000 bytes. If we do the optimization as described in the comment, we can save a lot of disk space. This is particularly relevant to targets with a large `defaultMaxPageSize` (AArch64, MIPS (@atanasyan), and PPC (@sfertile): 65536).

What do you think of this trick?


Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D64854/new/

https://reviews.llvm.org/D64854