[PING] [PATCH] Fix another wrong-code bug with -fstrict-volatile-bitfields

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

Hi,

I'd like to ping for this patch, which I hope can still go in the gcc-5 release:
See https://gcc.gnu.org/ml/gcc-patches/2015-03/msg00817.html for the
patch files.


Thanks,
Bernd.


> Date: Mon, 16 Mar 2015 11:53:00 +0100
>
>
> 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.
>