Dear GCC experts,
I'm having trouble compiling code that uses the following macro from the
Apache Portable Runtime:
#define apr_atomic_cas(mem,with,cmp) \
({ apr_atomic_t prev; \
asm volatile ("lock; cmpxchgl %1, %2" \
: "=a" (prev) \
: "r" (with), "m" (*(mem)), "0"(cmp) \
: "memory"); \
prev;})
It seems that on some machines it tries to use the AMD64 register %dil,
which does not exist on a regular x86 machine, and gives an assembler error.
I've found bug 10153 and its duplicates which seem to be describing
essentially the same thing, but say that the input is invalid because it
uses "r" rather than "q". I don't know enough about x86 to understand
this; presumably only certain registers can be used with this
instruction, or something.
However, naively, I'd argue that it is never right for gcc to use a
register that doesn't exist on the target architecture. Can someone
justify its behaviour? If the input is illegal surely it should
generate an error.
Can you suggest how Apache should correct this code?
(I should explain that I have not seen this error first-hand myself.
I'm the author of a GPL program, Anyterm, and have had this error
reported by other users. I'm therefore "debugging at a distance". The
platform that causes the problems is Mandrake 10.1. I'm afraid I don't
even know what version of gcc it has.)
Thanks for any suggestions.
--Phil.
