On 5 October 2012 08:12, Russell King - ARM Linux <li...@arm.linux.org.uk> wrote: > On Fri, Oct 05, 2012 at 03:25:16AM +0100, Mans Rullgard wrote: >> On 5 October 2012 02:56, Rob Herring <robherri...@gmail.com> wrote: >> > This struct is the IP header, so a struct ptr is just set to the >> > beginning of the received data. Since ethernet headers are 14 bytes, >> > often the IP header is not aligned unless the NIC can place the frame at >> > a 2 byte offset (which is something I need to investigate). So this >> > function cannot make any assumptions about the alignment. Does the ABI >> > define structs have some minimum alignment? Does the struct need to be >> > declared as packed or something? >> >> The ABI defines the alignment of structs as the maximum alignment of its >> members. Since this struct contains 32-bit members, the alignment for the >> whole struct becomes 32 bits as well. Declaring it as packed tells gcc it >> might be unaligned (in addition to removing any holes within). > > This has come up before in the past. > > The Linux network folk will _not_ allow - in any shape or form - for > this struct to be marked packed (it's the struct which needs to be > marked packed) because by doing so, it causes GCC to issue byte loads/ > stores on architectures where there isn't a problem, and that decreases > the performance of the Linux IP stack unnecessarily.
Which architectures? I have never seen anything like that. -- Mans Rullgard / mru _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
