| .. title:: clang-tidy - altera-struct-pack-align |
| |
| altera-struct-pack-align |
| ======================== |
| |
| Finds structs that are inefficiently packed or aligned, and recommends |
| packing and/or aligning of said structs as needed. |
| |
| Structs that are not packed take up more space than they should, and accessing |
| structs that are not well aligned is inefficient. |
| |
| Fix-its are provided to fix both of these issues by inserting and/or amending |
| relevant struct attributes. |
| |
| Based on the `Altera SDK for OpenCL: Best Practices Guide |
| <https://www.altera.com/en_US/pdfs/literature/hb/opencl-sdk/aocl_optimization_guide.pdf>`_. |
| |
| .. code-block:: c++ |
| |
| // The following struct is originally aligned to 4 bytes, and thus takes up |
| // 12 bytes of memory instead of 10. Packing the struct will make it use |
| // only 10 bytes of memory, and aligning it to 16 bytes will make it |
| // efficient to access. |
| struct example { |
| char a; // 1 byte |
| double b; // 8 bytes |
| char c; // 1 byte |
| }; |
| |
| // The following struct is arranged in such a way that packing is not needed. |
| // However, it is aligned to 4 bytes instead of 8, and thus needs to be |
| // explicitly aligned. |
| struct implicitly_packed_example { |
| char a; // 1 byte |
| char b; // 1 byte |
| char c; // 1 byte |
| char d; // 1 byte |
| int e; // 4 bytes |
| }; |
| |
| // The following struct is explicitly aligned and packed. |
| struct good_example { |
| char a; // 1 byte |
| double b; // 8 bytes |
| char c; // 1 byte |
| } __attribute__((packed)) __attribute__((aligned(16)); |
| |
| // Explicitly aligning a struct to the wrong value will result in a warning. |
| // The following example should be aligned to 16 bytes, not 32. |
| struct badly_aligned_example { |
| char a; // 1 byte |
| double b; // 8 bytes |
| char c; // 1 byte |
| } __attribute__((packed)) __attribute__((aligned(32))); |