If you build FF/TB with Clang in i386, than it stops with the below error. This is an integrated assembler bug, but for llvm with very low priority. So a workaround would be nice, until llvm will support it. As you can read in Comment 2: > A work around would be to use a "wait" instruction before the "fnstcw" > instruction, which is the same as the "fstcw". http://llvm.org/bugs/show_bug.cgi?id=7557#c2 The error is: /Volumes/Developer/temp/src/mozilla/js/src/jscntxt.h:2213:14: warning: unknown pragma ignored [-Wunknown-pragmas] # pragma GCC visibility pop ^ /Volumes/Developer/temp/src/mozilla/js/src/jsnum.cpp:673:9: error: unrecognized instruction asm("fstcw %0" : "=m" (control) : ); ^ <inline asm>:1:2: note: instantated into assembly here fstcw 26(%esp) ^ /Volumes/Developer/temp/src/mozilla/js/src/jsnum.cpp:673:9: error: unrecognized instruction asm("fstcw %0" : "=m" (control) : ); ^ <inline asm>:1:2: note: instantated into assembly here fstcw 42(%esp) ^ 5 warnings and 2 errors generated. make[4]: *** [jsnum.o] Error 1 make[4]: *** Waiting for unfinished jobs.... 4 warnings generated. 2 warnings generated. 2 warnings generated. 5 warnings generated. 2 warnings generated. 2 warnings generated. 2 warnings generated. make[3]: *** [libs_tier_js] Error 2 make[2]: *** [tier_js] Error 2 make[1]: *** [default] Error 2 make: *** [build] Error 2

This should now be fixed in clang since r115015 (didn't test it): http://llvm.org/bugs/show_bug.cgi?id=7557#c5 So we don't need a workaround anymore.