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this depends on commit 9f5eb77992b42d484d69e879d24ef86466f20f21, which
made it possible to use a .c file for arch-specific replacements, and on
commit 2f853dd6b9a95d5b13ee8f9df762125e0588df5d, the out-of-tree build
support, which made it so that src/*/$(ARCH)/* 'replacement' files get
used even if they don't match the base name of a .c file in the parent
directory.
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the idea of the three-instruction sequence being removed was to be
able to return to thumb code when used on armv4t+ from a thumb caller,
but also to be able to run on armv4 without the bx instruction
available (in which case the low bit of lr would always be 0).
however, without compiler support for generating such a sequence from
C code, which does not exist and which there is unlikely to be
interest in implementing, there is little point in having it in the
asm, and it would likely be easier to add pre-armv4t support via
enhanced linker handling of R_ARM_V4BX than at the compiler level.
removing this code simplifies adding support for building libc in
thumb2-only form (for cortex-m).
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contrary to commit 9367fe926196f407705bb07cd29c6e40eb1774dd, all
relevant gas versions actually do support .syntax unified.
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this file had been a mess that went unnoticed ever since it was
imported. some lines used spaces for indention while others used tabs,
and tabs were used for alignment.
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commit 27828f7e9adb6b4f93ca56f6f98ef4c44bb5ed4e fixed compatibility
with clang's internal assembler, but broke compatibility with gas and
the traditional arm asm syntax by switching to the arm "unified
assembler language" (UAL). recent versions of gas also support UAL,
but require the .syntax directive to be used to switch to it. clang on
the other hand defaults to UAL. and old versions of gas (still
relevant) don't support UAL at all.
for the conditional ldm/stm instructions, "ia" is default and can just
be omitted, resulting in a mnemonic that's compatible with both
traditional and UAL syntax. but for byte/halfword loads and stores,
there seems to be no mnemonic compatible with both, and thus .word is
used to produce the desired opcode explicitly. the .inst directive is
not used because it is not compatible with older assemblers.
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the approach of this implementation was heavily investigated prior to
adopting it. attempts to obtain similar performance with pure C code
were capping out at about 75% of the performance of the asm, with
considerably larger code size, and were fragile in that the compiler
would sometimes compile part of memcpy into a call to itself.
therefore, just using the asm seems to be the best option.
this commit is the first to make use of the new subarch-specific asm
framework. the new armel directory is the location for arm asm that
should not be used for all arm subarchs, only the default one. armhf
is the name of the little-endian hardfloat-ABI subarch, which can use
the exact same asm. in both cases, the build system finds the asm by
following a memcpy.sub file.
the other two subarchs, armeb and armebhf, would need a big-endian
variant of this code. it would not be hard to adapt the code to big
endian, but I will hold off on doing so until there is demand for it.
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