Hi! On Tue, Jun 03, 2025 at 11:01:11AM +0100, Tim Froggatt via Gcc-help wrote: > I'm compiling C software for an embedded ARM system with -mcpu=cortex-a8 > Currently my build system uses GCC version 12.2.1. > > I've been investigating the effects of -falign-functions=n:m > "Align the start of functions to the next power-of-two greater than or equal > to n, skipping up to m-1 bytes." > > I've set n=64 and I'm trying different values for m... > -falign-functions=64:5 > -falign-functions=64:6 > -falign-functions=64:7 > -falign-functions=64:8 > etc. > > m=5,6,7,8 all give similar output. The same for m=9,10,11,12. This is what > I'd > expect since all ARM32 instructions are 4 bytes long, so would be aligned > at a > minimum on 4-byte boundaries. > > However I'm confused by m=1,2,3,4. m=1 gives a very different result to > 2,3,4 > (and in my case performance is considerably worse for m=1). What is > -falign-functions=64:1 doing here? "Skipping up to m-1 bytes" means "do not skip anything at all", for m=1. I have no idea what you are seeing, and what about that confuses you, because you told us neither. > Also, what does it even mean when m=2, 3 or 4? If instructions are 4 bytes > long, > then I thought this wouldn't actually be doing any alignment. But it gives a > different result to specifying -fno-align-functions, so presumably there is > something I'm misunderstanding? Perhaps it rounds it up to a multiple of four here? I don't know what you are actually seeing (you didn't tell us), so that's about all I can say :-( Segher
