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binutils commit bada43421274615d0d5f629a61a60b7daa71bc15 tightened
immediate fixup handling in gas in such a way that the final .arch of
an object file must be compatible with the fixups used when the
instruction was assembled; this in turn broke assembling of atomics.s,
at least in thumb mode.
it's not clear whether this should be considered a bug in gas, but
.object_arch is preferable anyway for our purpose here of controlling
the ISA level tag on the object file being produced, and it's the
intended directive for use in object files with runtime code
selection. research by Szabolcs Nagy confirmed that .object_arch is
supported in all relevant versions of binutils and clang's integrated
assembler.
patch by Reiner Herrmann.
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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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this file's .data section was not aligned, and just happened to get
the correct alignment with past builds. it's likely that the move of
atomic.s from arch/arm/src to src/thread/arm caused the change in
alignment, which broke the atomic and thread-pointer access fragments
on actual armv5 hardware.
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this is possible with the new build system that allows src/*/$(ARCH)/*
files which do not shadow a file in the parent directory, and yields a
more logical organization. eventually it will be possible to remove
arch/*/src from the build system.
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