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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 --use-nul-prefixed-import-tables switch is specifying, that the import tables idata4 and idata5 have to be generated with a zero elememt prefix for import libraries. This is the old style to generate import tables. By default this option is turned off.

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.

Replace BX rM instructions identified by R_ARM_V4BX relocations with a branch to the following veneer:

     TST rM, #1
     MOVEQ PC, rM
     BX Rn

This allows generation of libraries/applications that work on ARMv4 cores and are still interworking safe. Note that the above veneer clobbers the condition flags, so may cause incorrect progrm behavior in rare cases.

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.

The --no-wchar-size-warning switch prevents the linker from warning when linking object files that specify incompatible EABI wchar_t size attributes. For example, with this switch enabled, linking of an object file using 32-bit wchar_t values with another using 16-bit wchar_t values will not be diagnosed.

The --pic-veneer switch makes the linker use PIC sequences for ARM/Thumb interworking veneers, even if the rest of the binary is not PIC. This avoids problems on uClinux targets where --emit-relocs is used to generate relocatable binaries.

The linker will automatically generate and insert small sequences of code into a linked ARM ELF executable whenever an attempt is made to perform a function call to a symbol that is too far away. The placement of these sequences of instructions - called stubs - is controlled by the command line option --stub-group-size=N. The placement is important because a poor choice can create a need for duplicate stubs, increasing the code sizw. The linker will try to group stubs together in order to reduce interruptions to the flow of code, but it needs guidance as to how big these groups should be and where they should be placed.

The value of N, the parameter to the --stub-group-size= option controls where the stub groups are placed. If it is negative then all stubs are placed after the first branch that needs them. If it is positive then the stubs can be placed either before or after the branches that need them. If the value of N is 1 (either +1 or -1) then the linker will choose exactly where to place groups of stubs, using its built in heuristics. A value of N greater than 1 (or smaller than -1) tells the linker that a single group of stubs can service at most N bytes from the input sections.

The default, if --stub-group-size= is not specified, is N = +1.

Farcalls stubs insertion is fully supported for the ARM-EABI target only, because it relies on object files properties not present otherwise.