How much does function alignment actually matter on modern processors?

Question

When I compile C code with a recent compiler on an amd64 or x86 system, functions are aligned to a multiple of 16 bytes. How much does this alignment actually matter on mode

Answer 1

TL;DR: Cache alignment matters. You don't want bytes that you won't execute.

You would, at least, want to avoid fetching instructions before the first one you will execute. Since this is a micro-benchmark, you most likely don't see any difference, but imagine on a full program, if you have an extra cache-miss on a bunch of functions because the first byte wasn't aligned to a cache-line and you eventually had to fetch a new cache line for the last N bytes of the function (where N <= the number of bytes before the function that you cached but didn't use).

Intel's optimization manual says this:

3.4.1.5 Code Alignment

Careful arrangement of code can enhance cache and memory locality. Likely sequences of basic blocks should be laid out contiguously in memory. This may involve removing unlikely code, such as code to handle error conditions, from the sequence. See Section 3.7, “Prefetching,” on optimizing the instruction prefetcher.

3-8 Assembly/Compiler Coding Rule 12. (M impact, H generality) All branch targets should be 16- byte aligned.

Assembly/Compiler Coding Rule 13. (M impact, H generality) If the body of a conditional is not likely to be executed, it should be placed in another part of the program. If it is highly unlikely to be executed and code locality is an issue, it should be placed on a different code page.

It also helps in explaining why you don't notice any difference in your program. All the code gets cached once and never leaves the cache (modulo context-switches, of course).