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From: "gcc at dixie dot net.nz" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/63435] New: Bad code with weak vs localalias on AIX
Date: Wed, 01 Oct 2014 23:13:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63435

            Bug ID: 63435
           Summary: Bad code with weak vs localalias on AIX
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at dixie dot net.nz
              Host: powerpc-ibm-aix7.1.0.0
            Target: powerpc-ibm-aix7.1.0.0
             Build: powerpc-ibm-aix7.1.0.0

Created attachment 33638
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33638&action=edit
testcase

Consider attached testcase.

The recvfrom function is generated as a weak symbol,
symtab_node::noninterposable_alias creates a localalias.
        .weak   recvfrom[DS]
        .weak   .recvfrom
        .csect recvfrom[DS]
recvfrom:
        .long .recvfrom, TOC[tc0], 0
        .csect .text[PR]
.recvfrom:

-------

gcc-4.9.1 generates local alias to the weak symbol:
        .lglobl .recvfrom.localalias.0
        .lglobl recvfrom.localalias.0
        .set    .recvfrom.localalias.0,.recvfrom
        .set recvfrom.localalias.0,recvfrom

-------

This local alias is called with local calling convention (missing nop):
        bl .recvfrom.localalias.0
        mr 9,3

-------

The linker discards the weak local recvfrom symbol, and chooses recvfrom from
libc.
The recvfrom in libc is remote and needs a nop instruction.

The linker correctly issues a diagnostic about this:
ld: 0711-768 WARNING: Object myrecvfrom.o, section 1, function .recvfrom:
        The branch at address 0xc8 is not followed by a recognized no-op
        or TOC-reload instruction. The unrecognized instruction is 0x7C691B78.

At runtime this results in corrupt TOC pointer (possibly requires larger
testcase to observe problems).