mirror of git://git.musl-libc.org/musl
e3c682ab52
in thumb mode, r7 is the ABI frame pointer register, and unless frame pointer is disabled, gcc insists on treating it as a fixed register, refusing to spill it to satisfy constraints. unfortunately, r7 is also used in the syscall ABI for passing the syscall number. up til now we just treated this as a requirement to disable frame pointer when generating code as thumb, but it turns out gcc forcibly enables frame pointer, and the fixed register constraint that goes with it, for functions which contain VLAs. this produces an unacceptable arch-specific constraint that (non-arm-specific) source files making syscalls cannot use VLAs. as a workaround, avoid r7 register constraints when producing thumb code and instead save/restore r7 in a temp register as part of the asm block. at some point we may want/need to support armv6-m/thumb1, so the asm has been tweaked to be thumb1-compatible while also near-optimal for thumb2: it allows the temp and/or syscall number to be in high registers (necessary since r0-r5 may all be used for syscalll args) and in thumb2 mode allows the syscall number to be an 8-bit immediate. |
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README
musl libc
musl, pronounced like the word "mussel", is an MIT-licensed
implementation of the standard C library targetting the Linux syscall
API, suitable for use in a wide range of deployment environments. musl
offers efficient static and dynamic linking support, lightweight code
and low runtime overhead, strong fail-safe guarantees under correct
usage, and correctness in the sense of standards conformance and
safety. musl is built on the principle that these goals are best
achieved through simple code that is easy to understand and maintain.
The 1.1 release series for musl features coverage for all interfaces
defined in ISO C99 and POSIX 2008 base, along with a number of
non-standardized interfaces for compatibility with Linux, BSD, and
glibc functionality.
For basic installation instructions, see the included INSTALL file.
Information on full musl-targeted compiler toolchains, system
bootstrapping, and Linux distributions built on musl can be found on
the project website:
http://www.musl-libc.org/