------- Comment #8 from uweigand at gcc dot gnu dot org 2008-01-09 19:23 ------- This is a long-standing problem in gen_reload. This routine fundamentally assumes that every PLUS expression that describes a legitimate address can be reloaded into a register without requiring any additional scratch registers.
It first attempts to generate a simple (set (...) (plus ...)) pattern. If the target does not accept this pattern, it tries as a fall-back to generate a two-insn sequence to perform the load. This code looks fundamentally broken to me, in that it does not check whether the first of those two insns clobbers a register still required by the second. However, even if such a check were added, at this point we could not actually solve the problem, because it's far too late to allocate another register. I ran into the same problem in the s390 back-end a long time ago, and solved it by providing a back-end specific secondary reload rule (requiring a scratch register) to reload PLUS patterns that require multiple instructions. I think you'll have to do the same for rs6000. (As a separate issue, the broken code in gen_reload should be replaced by an abort -- at least back-ends where the secondary reload is missing would no longer generate wrong code in that case.) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34529
