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.

// 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)));