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: