Fix PR64876, regressions in powerpc64 Go testsuite

[David Edelsohn]

On Tue, Feb 3, 2015 at 8:57 AM, Alan Modra <amodra@gmail.com> wrote:
> This fixes a large number of Go testsuite failures on powerpc64 ELFv1,
> caused by loading r11 from a function descriptor and thus trashing the
> value set up from CALL_EXPR_STATIC_CHAIN.  So don't load r11 if it
> already contains a useful value.  Whether r11 has been set is found
> directly by examining rtl.  Conveniently, looking at the previous
> sequence on the rtl sequence stack lets us skip over anything already
> emitted for GEN_CALL, and the static chain assignment, if present,
> happens to be the last insn of that sequence (calls.c emit_call_1
> stuff).
>
> Alternative approaches considered:
> 1) Turn off TARGET_POINTERS_TO_NESTED_FUNCTIONS for Go in
>    rs6000_option_override_internal, similar to the hack posted in the
>    PR.  That fixes Go, but leaves __builtin_call_with_static_chain
>    broken.
> 2) Turn off TARGET_POINTERS_TO_NESTED_FUNCTIONS everywhere.  This
>    means rewriting rs6000_trampoline_init to not put the static chain
>    value into the trampoline function descriptor, and possibly other
>    code.  Might also affect user code.
> 3) Arrange to have a new flag set in the third arg of rs6000_call_aix.
>    This isn't simple due to none of INIT_CUMULATIVE_ARGS or various
>    targetm.calls hooks having access to the call expression.  We don't
>    have a function decl either, since this is an indirect call.
>
> Bootstrapped and regression tested powerpc64-linux.  OK to apply?
>
>         PR target/64876
>         * config/rs6000/rs6000.c (chain_already_loaded): New function.
>         (rs6000_call_aix): Use it.

Okay with Jakub's suggested change.

Thanks, David