[Bug target/56726] New: i386: MALLOC_ABI_ALIGNMENT is too small (usually)

["chip at pobox dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726

             Bug #: 56726
           Summary: i386: MALLOC_ABI_ALIGNMENT is too small (usually)
    Classification: Unclassified
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chip@pobox.com


Observed malloc alignment for the i386 ABI is double POINTER_SIZE. 
BITS_PER_WORD, the current default, is usually too small.  (It's right only on
X32.)

Proposed patch:

--- gcc/config/i386/i386.h      (revision 197055)
+++ gcc/config/i386/i386.h      (working copy)
@@ -815,6 +815,14 @@
    x86_field_alignment (FIELD, COMPUTED)
 #endif

+/* The maximum alignment 'malloc' honors.
+
+   This value is taken from glibc documentation for memalign().  It may
+   be up to double the very conservative GCC default.  This should be safe,
+   since even the GCC 4.8 default of BIGGEST_ALIGNMENT usually worked.  */
+
+#define MALLOC_ABI_ALIGNMENT (POINTER_SIZE * 2)
+
 /* If defined, a C expression to compute the alignment given to a
    constant that is being placed in memory.  EXP is the constant
    and ALIGN is the alignment that the object would ordinarily have