------- Comment #4 from jakub at gcc dot gnu dot org 2010-08-17 06:12 ------- The GCC manual says that the register should be call-saved, that isn't the case in your testcase, so it isn't valid. But, more importantly, the i387 floating point stack is even more special, as the st* registers change meaning during compilation - before reg-stack pass they are just flat 8 registers and the reg-stack pass changes them into real stacked registers, adding various fixup instructions. So, what is "st(1)" before reg-stack can be at various depths in the i387 stack during the function. There is really no way to make this work. You can use inline-asm with "t" resp. "u" constraints, but that's the only way you can directly influence the operation of the i387 register stack.
-- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45296
