http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56434
Bug #: 56434
Summary: document that __attribute__((__malloc__)) assumes
returned pointer has BIGGEST_ALIGNMENT
Classification: Unclassified
Product: gcc
Version: 4.7.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: c...@pobox.com
The docs say that __attribute__((__malloc__)) only has one effect: informing
the compiler that returned pointers do not alias other pointers. But reading
the compiler output, and then reading gcc source code, proves that it also has
a second effect: informing the compiler that returned pointers are aligned to
BIGGEST_ALIGNMENT. To quote expand_call:
/* The return value from a malloc-like function is a pointer. */
if (TREE_CODE (rettype) == POINTER_TYPE)
mark_reg_pointer (temp, BIGGEST_ALIGNMENT);
This should be added to the documentation.
As a side issue, BIGGEST_ALIGNMENT changes on the i386 target depending on
whether -mavx is specified (128 vs. 256). Is it really a good idea for gcc to
assume different things about the behavior of malloc() depending on -mavx? It
seems that perhaps an alignment of 128 should always be conferred on malloc on
the i386 platform, regardless of -mavx?
What would the new target macro be? SMALLEST_BIGGEST_ALIGNMENT? :)
