Re: struct alignment in src/lib/libc/arch/m68k/softfloat

[Nathanial Sloss <nathanialsloss%yahoo.com.au@localhost>]

On Thu, 18 Sep 2025 02:39:48 Roland Illig wrote:
> Am 17.09.2025 um 13:32 schrieb Nathanial Sloss:
> > Module Name:	src
> > Committed By:	nat
> > Date:		Wed Sep 17 11:32:06 UTC 2025
> > 
> > Modified Files:
> > 	src/lib/libc/arch/m68k/softfloat: softfloat.h
> > 
> > Log Message:
> > Store 80 bit floats properly on m68k.
> > 
> > The libc/softfloat would store the high word as a short, where as gcc
> > will store it as a long with the sign and 15 bit exponent in the high
> > bytes.
> > 
> > I had to use the __packed attribute as they were stored with an 8-byte
> > alignment....Why????
> 
> struct floatx80 {
> 	uint64_t lo;
> 	uint16_t hi;
> };
> 
> When you have an array of this type, each access to arr[i].lo should be
> aligned at an 8-byte boundary. Therefore, there's 6 bytes of padding at
> the end of the struct.
> 
> On some architectures such as sparc, this padding is necessary to
> prevent SIGBUS, on other architectures such as x86_64, accessing an
> unaligned object results in bad performance, due to overlapping reads.
> 
> C23 6.7.3.2p16 doesn't directly impose any alignment requirements on a
> struct or its members, other that each member has to be aligned
> "appropriate to its type". On the other hand, C23 6.7.3.2p19 allows
> unnamed padding at the end of a struct, so that sizeof(arr) can be the
> same as sizeof(arr[0]) * array_length(arr).
> 
> Roland


Thank for the background information,  I was just worried that I was loosing 
memory throughout due to padding bytes.

Best regards,

Nat