Globally required fields are held in the global data structure. A pointer to the structure is available as symbol gd. The symbol is made available by the macro %DECLARE_GLOBAL_DATA_PTR.
On most architectures the global data pointer is stored in a register.
| ARC | r25 |
| ARM 32bit | r9 |
| ARM 64bit | x18 |
| M68000 | d7 |
| MicroBlaze | r31 |
| Nios II | gp |
| PowerPC | r2 |
| RISC-V | gp (x3) |
| SuperH | r13 |
| x86 32bit | fs |
The sandbox, x86_64, and Xtensa are notable exceptions.
Current implementation uses a register for the GD pointer because this results in smaller code. However, using plain global data for the GD pointer would be possible too (and simpler, as it does not require the reservation of a specific register for it), but the resulting code is bigger.
Clang for ARM does not support assigning a global register. When using Clang gd is defined as an inline function using assembly code. This adds a few bytes to the code size.
Binaries called by U-Boot are not aware of the register usage and will not conserve gd. UEFI binaries call the API provided by U-Boot and may return to U-Boot. The value of gd has to be saved every time U-Boot is left and restored whenever U-Boot is reentered. This is also relevant for the implementation of function tracing. For setting the value of gd function set_gd() can be used.
include/asm-generic/global_data.h