Hi,
when looking at the m68k I realized the following, which is
a general problem...
If the alignment of the structure is less than sizeof(field), the
strict volatile bitfields code may read beyond the end of the
structure!
Consider this example:
struct s
{
char x : 8;
volatile unsigned int y : 31;
volatile unsigned int z : 1;
} __attribute__((packed));
struct s global;
Here we have sizeof(struct s) = 5, alignment(global) == 1,
However when we access global.z we read a 32-bit word
at offset 4, which touches 3 bytes that are not safe to use.
Something like that does never happen with -fno-strict-volatile-bitfields,
because IIRC, with the only exception of the simple_mem_bitfield_p code path,
there is never an access mode used which is larger than MEM_ALIGN(x).
In this example, if I want to use the packed attribute,
I also have to use the aligned(4) attribute, this satisfies the
check "MEM_ALIGN (op0) < modesize", which is IMO always necessary
for strict volatile bitfields, not only on STRICT_ALIGNMENT targets.
On a target, that has BIGGEST_ALIGNMENT < BITS_PER_WORD,
to use the strict volatile bitfields, you have to add the
__attribute__((aligned(4)))
to the structure.
I had to do that on the pr23623.c test case, to have it passed on m68k for
instance.
I have attached the updated patch. As explained before, the check
MEM_ALIGN (op0) < modesize should always be done in strict_volatile_bitfield_p.
For the targets, that usually enable -fstrict-volatile-bitfields, nothing
changes,
Except when we use "packed" on the structure, we need to add also an aligned(4)
attribute. For m68k where the natural alignment of any structure is <=2 we need
to
force aligned(4) if we want to ensure the access is in SImode.
Boot-strapped and reg-tested on x86_64-linux-gnu.
OK for trunk?
Thanks
Bernd.
gcc:
2015-03-15 Bernd Edlinger <bernd.edlin...@hotmail.de>
* expmed.c (strict_volatile_bitfield_p): Check that MEM_ALIGN allows
a MODESIZE access.
(store_bit_field, extract_bit_field): For !STRICT_ALIGNMENT explicitly
generate an unaligned access if the field crosses a word boundary.
testsuite:
2015-03-15 Bernd Edlinger <bernd.edlin...@hotmail.de>
* gcc.dg/pr23623.c: Added aligned attribute.
* gcc.dg/20141029-1.c: Likewise.
* gcc.dg/20150306-1.c: New test.
patch-volatile-bitfields-1.diff
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