[Bug target/57438] bootstrap fails on x86_64 darwin in stage2 linking cc1

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57438

--- Comment #30 from Iain Sandoe <iains at gcc dot gnu.org> ---
Author: iains
Date: Sun Nov 27 14:50:58 2016
New Revision: 242897

URL: https://gcc.gnu.org/viewcvs?rev=242897&root=gcc&view=rev
Log:
[Darwin] Fix PR57438 by avoiding empty function bodies and trailing labels.

A.
Empty function bodies causes two problems for Darwin's linker (i) zero-length
FDEs and (ii) coincident label addresses that might point to items of
differing weakness.

B.
Trailing local labels can be problematic when they end a function because
similarly they might apparently point to a following weak function, leading
to the linker concluding that there's a pointer-diff to a weak symbol
(which is not allowed).

Both conditions arise from __builtin_unreachable() lowering to a barrier.

The solution for both is to emit some finite amount of code; in the case of A
a trap is emitted, in the case of B a nop.

gcc/

2016-11-27  Iain Sandoe  <iain@codesourcery.com>

        PR target/57438
        * config/i386/i386.c (ix86_code_end): Note that we emitted code
        where the function might otherwise appear empty for picbase thunks.
        (ix86_output_function_epilogue): If we find a zero-sized function
        assume that reaching it is UB and trap.  If we find a trailing label
        append a nop.
        * config/rs6000/rs6000.c (rs6000_output_function_epilogue): If we
        find a zero-sized function assume that reaching it is UB and trap.
        If we find a trailing label, append a nop.

gcc/testsuite/

2016-11-27  Iain Sandoe  <iain@codesourcery.com>

        PR target/57438
        * gcc.dg/pr57438-1.c: New Test.
        * gcc.dg/pr57438-2.c: New Test.


Added:
    trunk/gcc/testsuite/gcc.dg/pr57438-1.c
    trunk/gcc/testsuite/gcc.dg/pr57438-2.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/i386/i386.c
    trunk/gcc/config/rs6000/rs6000.c
    trunk/gcc/testsuite/ChangeLog