Large data sections — lld 20.1.8 documentation

# Large data sections

When linking very large binaries, lld may report relocation overflows like

    relocation R_X86_64_PC32 out of range: 2158227201 is not in [-2147483648, 2147483647]

This happens when running into architectural limitations. For example, in x86-64 PIC code, a reference to a static global variable is typically done with a `R_X86_64_PC32` relocation, which is a 32-bit signed offset from the PC. That means if the global variable is laid out further than 2GB (2^31 bytes) from the instruction referencing it, we run into a relocation overflow.

lld normally lays out sections as follows:

The largest relocation pressure is usually from `.text` to the beginning of `.rodata` or `.text` to the end of `.bss`.

Some code models offer a tradeoff between relocation pressure and performance. For example, x86-64’s medium code model splits global variables into small and large globals depending on if their size is over a certain threshold. Large globals are placed further away from text and we use 64-bit references to refer to them.

Large globals are placed in separate sections from small globals, and those sections have a “large” section flag, e.g. `SHF_X86_64_LARGE` for x86-64. The linker places large sections on the outer edges of the binary, making sure they do not affect the distance of small globals to text. The large versions of `.rodata`, `.bss`, and `.data` are `.lrodata`, `.lbss`, and `.ldata`, and they are laid out as follows:

We try to keep the number of `PT_LOAD` segments to a minimum, so we place large sections next to the small sections with the same RWX permissions when possible.

`.lbss` is right after `.bss` so that they are merged together and we minimize the number of segments with `p_memsz > p_filesz`.

Note that the above applies to PIC code. For less common non-PIC code with absolute relocations instead of relative relocations, 32-bit relocations typically assume that symbols are in the lower 2GB of the address space. So for non-PIC code, large sections should be placed after all small sections to avoid `.lrodata` pushing small symbols out of the lower 2GB of the address space. `-z lrodata-after-bss` changes the layout to be: