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### 4.3 `ld` and the ARM family

For the ARM, `ld` will generate code stubs to allow functions calls betweem ARM and Thumb code. These stubs only work with code that has been compiled and assembled with the ‘`-mthumb-interwork`’ command line option. If it is necessary to link with old ARM object files or libraries, which have not been compiled with the -mthumb-interwork option then the ‘`--support-old-code`’ command line switch should be given to the linker. This will make it generate larger stub functions which will work with non-interworking aware ARM code. Note, however, the linker does not support generating stubs for function calls to non-interworking aware Thumb code.

The ‘`--thumb-entry`’ switch is a duplicate of the generic ‘`--entry`’ switch, in that it sets the program's starting address. But it also sets the bottom bit of the address, so that it can be branched to using a BX instruction, and the program will start executing in Thumb mode straight away.

The ‘`--be8`’ switch instructs `ld` to generate BE8 format executables. This option is only valid when linking big-endian objects. The resulting image will contain big-endian data and little-endian code.

The ‘`R_ARM_TARGET1`’ relocation is typically used for entries in the ‘`.init_array`’ section. It is interpreted as either ‘`R_ARM_REL32`’ or ‘`R_ARM_ABS32`’, depending on the target. The ‘`--target1-rel`’ and ‘`--target1-abs`’ switches override the default.

The ‘`--target2=type`’ switch overrides the default definition of the ‘`R_ARM_TARGET2`’ relocation. Valid values for ‘`type`’, their meanings, and target defaults are as follows:

‘`rel`’
  ‘`R_ARM_REL32`’ (arm*-*-elf, arm*-*-eabi)
‘`abs`’
  ‘`R_ARM_ABS32`’ (arm*-*-symbianelf)
‘`got-rel`’
  ‘`R_ARM_GOT_PREL`’ (arm*-*-linux, arm*-*-*bsd)

The ‘`R_ARM_V4BX`’ relocation (defined by the ARM AAELF specification) enables objects compiled for the ARMv4 architecture to be interworking-safe when linked with other objects compiled for ARMv4t, but also allows pure ARMv4 binaries to be built from the same ARMv4 objects.

In the latter case, the switch `--fix-v4bx` must be passed to the linker, which causes v4t `BX rM` instructions to be rewritten as `MOV PC,rM`, since v4 processors do not have a `BX` instruction.

In the former case, the switch should not be used, and ‘`R_ARM_V4BX`’ relocations are ignored.

The ‘`--use-blx`’ switch enables the linker to use ARM/Thumb BLX instructions (available on ARMv5t and above) in various situations. Currently it is used to perform calls via the PLT from Thumb code using BLX rather than using BX and a mode-switching stub before each PLT entry. This should lead to such calls executing slightly faster.

This option is enabled implicitly for SymbianOS, so there is no need to specify it if you are using that target.