Ping again... it's a wrong code bug.
Thanks Bernd. > Date: Mon, 23 Mar 2015 10:09:35 +0100 > > 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. >> >
