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three problems are addressed:
- use of pc arithmetic, which was difficult if not impossible to make
correct in thumb mode on all models, so that relative rather than
absolute pointers to the backends could be used. this was designed
back when there was no coherent model for the early stages of the
dynamic linker before relocations, and is no longer necessary.
- assumption that data (the relative pointers to the backends) can be
accessed at a constant displacement from the code. this will not be
possible on future fdpic subarchs (for cortex-m), so move
responsibility for loading the backend code address to the caller.
- hard-coded arm opcodes using the .word directive. instead, use the
.arch directive to work around the assembler's refusal to assemble
instructions not available (or in some cases, available but just
considered deprecated) in the target isa level. the obscure v6t2
arch is used for v6 code so as to (1) allow generation of thumb2
output if -mthumb is active, and (2) avoid warnings/errors for mcr
barriers that clang would produce if we just set arch to v7-a.
in addition, the __aeabi_read_tp function is moved out of the inner
workings and implemented as an asm wrapper around a C function, so
that asm code does not need to read global data. the asm wrapper
serves to satisfy the ABI calling convention requirements for this
function.
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