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