[En-Nut-Discussion] ARM GCC 4.4 Alignment Problems

[Harald Kipp]

Hi Bernd,

It's always a pleasure to see you jumping in when things become a bit
complicated.

On 01.04.2010 20:06, Bernd Walter wrote:
> I'm not so much a fan about missaligned data.

Who is? ;-)

> In almost every case it is avoidable without much trouble.
> One of the most annoying points with network data is the 6 bytes long
> ethernet header, which usually garanties missaligned IP headers, but
> usually this is avoided by setting RX buffers at a 2 byte offset or
> copying the data before parsing if HW require 32bit aligned DMA RX
> buffers - AT91SAM7X should be happy with 2 byte offset, AT91RM9200
> are not and require copying.

Using a 2 byte offset is indeed an option worth to be evaluated.
Copying, however, is something that really consumes CPU power.


> Packed on ARM has different reasons - older ARM CPUs required bytes
> and 16bit words to be on the same alignment because they had to
> mask bytes out from 32bit memory operations and therefor structs
> containing 3 bytes are 4 bytes long, so that an array of those structs
> always start the same members at the same 32bit offset.

Not a real problem, but of course additional instructions are required
compared to 32-bit aligned elements.


> Usually this isn't a problem and it is also Ok with C-standards, but
> parsing network data with structs can be a problem.
> Since we don't need ABI compatibility with older ARM systems it
> shouldn't be a problem to use -mstructure-size-boundary=8 (bits)
> and don't use packed.

This option will reduce the total size of a structure, but it will not
pack its members.

  typedef struct __attribute__((packed)) ether_header {
    uint8_t  ether_dhost[ETHER_ADDR_LEN];
    uint8_t  ether_shost[ETHER_ADDR_LEN];
    uint16_t ether_type;
  } ETHERHDR;

  struct __attribute__((packed)) frame {
    ETHERHDR hdr;
    uint32_t data[8];
  };

ETHERHDR is 14 bytes and struct frame is 8 * 4 + 14 = 46 bytes.

When removing packed and instead compiling with
-mstructure-size-boundary=8, then ETHERHDR is still 14 bytes only, but
struct frame will grow by 2 bytes, because data[] will become aligned.

The problem is not the size of structures, but the alignment of their
members.

Please correct me if I'm wrong, I'm just evaluating this stuff.

Harald