#include <stdint.h>

register long double F80 __asm__("st");

typedef void (*inst_t)(uint32_t *ip);

void exec_inst(uint32_t *ip) {
  ((inst_t) (uint64_t) ip[0])(ip);
}

int main() {
  return 0;
}

At -O2 and above gcc generates:

register_long_double_ICE.c:3:1: warning: call-clobbered register used for
global register variable
register_long_double_ICE.c: In function ‘exec_inst’:
register_long_double_ICE.c:9:1: internal compiler error: in compensate_edge, at
reg-stack.c:2789
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-4.5/README.Bugs> for instructions.


-- 
           Summary: register long double ICE at -O2, -Os, -O3
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: adam at consulting dot net dot nz


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