Struct layout in apcs-gnu ABI

Question

For this code:

struct S {     unsigned char ch[2]; };

int main(void)
{
    _Static_assert( sizeof(struct S) == 2, "size was not 2");
}

using GCC (various versions) for ARM with the ABI apcs-gnu (aka. OABI, or EABI version 0), I get the assertion fails. It turns out the size of the struct is 4.

I can work around this by using __attribute__((packed)); but my questions are:

• What is the rationale for making this struct size 4?
• Is there any documentation specifying the layout of structs in this ABI?

On the ARM website I found documentation for aapcs (EABI version 5) which does specify this struct as having a size of 2; but I could not find anything about apcs-gnu.

Answer 1

This is a GCC-specific decision to trade-off size for performance. It can be overridden with -mstructure-size-boundary=8.

An excerpt from source code:

/* Setting STRUCTURE_SIZE_BOUNDARY to 32 produces more efficient code, but the
value set in previous versions of this toolchain was 8, which produces more
compact structures.  The command line option -mstructure_size_boundary=<n>
can be used to change this value.  For compatibility with the ARM SDK
however the value should be left at 32.  ARM SDT Reference Manual (ARM DUI
0020D) page 2-20 says "Structures are aligned on word boundaries".
The AAPCS specifies a value of 8.  */
#define STRUCTURE_SIZE_BOUNDARY arm_structure_size_boundary