[Bug c/34803] New: wrong code for dereferencing type-punned pointer

["gin at mo dot msk dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

No simpler C code is (yet) known to hit this.  On request will
describe actual package where similar code occurs.

In the attached preprocessed code, incorrect arg 3, `%rdx', is passed
to `handle_out_external'.  Code expects this value, `extlen', to be
the same as value that is returned from `__builtin_alloca', stored in
`c', passed to `memcpy' as arg 1.  It is not.  Instead, the following
assembler code is output.

        movq    %rbx, %rdi
        call    memcpy
        movq    -72(%rbp), %rcx
        movq    -40(%rbp), %rdx
        movq    %rbx, -40(%rbp)

It passes in `%rbx' uninitialized value that was in `-40(%rbp)' and
only then writes into `-40(%rbp)' the correct value to pass.

Observing when compiling not only `-O3', but even
`-fno-strict-aliasing -O3 -fno-strict-aliasing'.  In these 2
invocations assembler output is the same.  That is,
`-fno-strict-aliasing' no longer disables optimizations that require
strict aliasing rules in code.  It, however, disables warnings output
when `-Wstrict-aliasing=2' is specified.


-- 
           Summary: wrong code for dereferencing type-punned pointer
           Product: gcc
           Version: 4.2.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gin at mo dot msk dot ru
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34803