Cache Line Alignment (Need clarification on article)

Question

I've recently encountered what I think is a false-sharing problem in my application, and I've looked up Sutter's article on how to align my data to cache lines. He suggests the following C++ code: ``` // C++ (using C++0x alignment syntax) template<typename T> struct cache_line_storage { [[ align(CACHE_LINE_SIZE) ]] T data; char pad[ CACHE_LINE_SIZE > sizeof(T) ? CACHE_LINE_SIZE - sizeof(T) : 1 ]; }; ``` I can see how this would work when `CACHE_LINE_SIZE > sizeof(T)` is true -- the struct `cache_line_storage` just ends up taking up one full cache line of memory. However, when the `sizeof(T)` is larger than a single cache line, I would think that we should pad the data by `CACHE_LINE_SIZE - T % CACHE_LINE_SIZE` bytes, so that the resulting struct has a size that is an integral multiple of the cache line size. What is wrong with my understanding? Why does padding with 1 byte suffice?

Accepted Answer

You can't have arrays of size 0, so 1 is required to make it compile. However, the current draft version of the spec says that such padding is unecessary; the compiler must pad up to the struct's alignment.

Note also that this code is ill-formed if `CACHE_LINE_SIZE` is smaller than `alignof(T)`. To fix this, you should probably use `[[align(CACHE_LINE_SIZE), align(T)]]`, which will ensure that a smaller alignment is never picked.

Cache Line Alignment (Need clarification on article)

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