tree 88a3994fe4e3e2f36fd4d4546c56ca9e87e0c9bf
parent 9c014ac94c0bfdb05da414ca6a16c882dfd9ce11
author Fangrui Song <i@maskray.me> 1708466389 -0800
committer Copybara-Service <copybara-worker@google.com> 1708466546 -0800

[ELF] Support placing .lbss/.lrodata/.ldata after .bss

https://reviews.llvm.org/D150510 places .lrodata before .rodata to
minimize the number of permission transitions in the memory image.
However, this layout is less ideal for -fno-pic code (which is still
important).

Small code model -fno-pic code has R_X86_64_32S relocations with a range
of `[0,2**31)` (if we ignore the negative area). Placing `.lrodata`
earlier exerts relocation pressure on such code. Non-x86 64-bit
architectures generally have a similar `[0,2**31)` limitation if they
don't use PC-relative relocations.

If we place .lrodata later, we will need one extra PT_LOAD. Two layouts
are appealing:

* .bss/.lbss/.lrodata/.ldata (GNU ld)
* .bss/.ldata/.lbss/.lrodata

The GNU ld layout has the nice property that there is only one BSS
(except .tbss/.relro_padding). Add -z lrodata-after-bss to support
this layout.

Since a read-only PT_LOAD segment (for large data sections) may appear
after RW PT_LOAD segments. The placement of `_etext` has to be adjusted.

Pull Request: https://github.com/llvm/llvm-project/pull/81224

GitOrigin-RevId: 78762357d449cfcd11426c8e152302a27f2e7d4d