mozilla::SignalTrampoline is designed to work around a bug in older ARM
kernels; it constructs a trampoline function with a NOP slide and then
calls a specified function. This feat is accomplished using inline
assembly and naked functions, which is a GCC extension where you get to
write the entire body of your function using GCC inline assembly.
Unfortunately, the particular implementation that it uses requires the
specified function's address to be loaded into a register. GCC permits
this and we use input arguments to the assembly statement to ensure that
GCC knows it shouldn't clobber the incoming argument registers when
trying to load the function's address.
clang, however, complains about the use of input parameters in naked
functions. So we need to find something that will work on both GCC and
clang.
The trick is to realize that we're a) tail-calling the specified
function and b) we don't have to worry about calling a fully-general
function. We just have to worry about calling a function inside libxul,
and we can therefore "assume" that the offset between the branch and the
called function fits into the immediate field of a Thumb (or ARM) branch
instruction. (This assumption is not strictly true; the branch range is
+/-16MB or so and libxul is actually quite a bit bigger than that. But
it works in practice, and the linker will insert branch stubs if
necessary to make things work out OK.)
The upshot is that we can use a "b" instruction instead of a "bx"
instruction, and this makes clang much happier. As a small bonus, the
stub gets ever-so-much-more efficient, which is probably the
least-significant micro-optimization ever.
Nathan Froyd
Jim Chen