[Bug target/47120] New: -malign-double does not align doubles in function calls

[pip88nl at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47120

           Summary: -malign-double does not align doubles in function
                    calls
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: pip88nl@gmail.com


According to the gcc manual, -malign-double creates object code with an ABI
that is incompatible to code that was not compiled with that switch. With this
switch, structs containing doubles are aligned correctly (according to double's
alignment requirements) but doubles passed to functions are not. E.g.:

  void foo (int i, double d);
  int main () {
    foo (1, 2);
    return 0;
  }

On x86, this will emit code storing the double value of 2.0 to a word-aligned
stack location, not a double-word aligned location, as the manual implies.

I suppose the reason for not aligning doubles on a two word boundary in this
instance is that it would, in addition to the struct ABI, break the x86 calling
convention. However, not aligning the double correctly will make code fail that
requires aligned access.