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4.4 ld and the ARM family

For the ARM, ld will generate code stubs to allow functions calls between 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:

`R_ARM_REL32' (arm*-*-elf, arm*-*-eabi)
`R_ARM_ABS32' (arm*-*-symbianelf)
`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.

The `--vfp11-denorm-fix' switch enables a link-time workaround for a bug in certain VFP11 coprocessor hardware, which sometimes allows instructions with denorm operands (which must be handled by support code) to have those operands overwritten by subsequent instructions before the support code can read the intended values.

The bug may be avoided in scalar mode if you allow at least one intervening instruction between a VFP11 instruction which uses a register and another instruction which writes to the same register, or at least two intervening instructions if vector mode is in use. The bug only affects full-compliance floating-point mode: you do not need this workaround if you are using "runfast" mode. Please contact ARM for further details.

If you know you are using buggy VFP11 hardware, you can enable this workaround by specifying the linker option `--vfp-denorm-fix=scalar' if you are using the VFP11 scalar mode only, or `--vfp-denorm-fix=vector' if you are using vector mode (the latter also works for scalar code). The default is `--vfp-denorm-fix=none'.

If the workaround is enabled, instructions are scanned for potentially-troublesome sequences, and a veneer is created for each such sequence which may trigger the erratum. The veneer consists of the first instruction of the sequence and a branch back to the subsequent instruction. The original instruction is then replaced with a branch to the veneer. The extra cycles required to call and return from the veneer are sufficient to avoid the erratum in both the scalar and vector cases.

The `--no-enum-size-warning' switch prevents the linker from warning when linking object files that specify incompatible EABI enumeration size attributes. For example, with this switch enabled, linking of an object file using 32-bit enumeration values with another using enumeration values fitted into the smallest possible space will not be diagnosed.