[Bug target/45296] register long double ICE at -O2, -Os, -O3

["jakub at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- 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.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45296