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.. 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)));
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