tidy - altera-struct-pack-align — Extra Clang Tools 22.0.0git documentation (original) (raw)

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.

// 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)));