Hi. Thanks for your help. To give you some context, I'm compiling code that forwards packets on an embedded network device. The code accesses the hardware directly, there is no operating system. The speed of network traffic is limited by the CPU and I'm experimenting with different GCC optimisation options, and seeing how fast I can get the same code to forward network traffic. I don't have my actual results to hand now, but the following example shows the pattern that I see... (in real life, the differences are relatively not so drastic, but the specific values aren't important) -fno-align-functions 60 Mbps -falign-functions=64:1 40 Mbps -falign-functions=64:2 70 Mbps -falign-functions=64:3 70 Mbps -falign-functions=64:4 70 Mbps -falign-functions=64:5 90 Mbps -falign-functions=64:6 90 Mbps -falign-functions=64:7 90 Mbps -falign-functions=64:8 90 Mbps -falign-functions=64:9 100 Mbps -falign-functions=64:10 100 Mbps -falign-functions=64:11 100 Mbps -falign-functions=64:12 100 Mbps -falign-functions=64:13 50 Mbps -falign-functions=64:14 50 Mbps -falign-functions=64:15 50 Mbps -falign-functions=64:16 50 Mbps -falign-functions=64:17 40 Mbps -falign-functions=64:18 40 Mbps -falign-functions=64:19 40 Mbps -falign-functions=64:20 40 Mbps etc... On 03/06/2025 17:02, Segher Boessenkool wrote: > I have no idea what you are seeing, and what about that confuses you, > because you told us neither. I hope the example above shows you what I'm seeing. I think it makes sense that every 4 values of m produces the same code - because all ARM32 instructions are 4 bytes long, and would be aligned at a minimum on 4-byte boundaries. I am confused why m=1,2,3,4 doesn't fit this pattern even though all other values do. And I'm confused why neither m=1 nor m=2,3,4 produce the same code as -fno-align-functions.
"Skipping up to m-1 bytes" means "do not skip anything at all", for m=1.
Yes. So I don't understand why "do not skip anything at all" is different to -fno-align-functions. And also why m=2,3,4 also doesn't mean "do not skip anything at all". All instructions are 4 bytes long, so shouldn't they also mean don't skip anything? Kind regards, Tim
