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Systems demand that certain primitives be aligned to certain points within the memory (ints to bytes that are multiples of 4, shorts to bytes that are multiples of 2, etc.). Of course, these can be optimized to waste the least space in padding.
My question is why doesn't GCC do this automatically? Is the more obvious heuristic (order variables from biggest size requirement to smallest) lacking in some way? Is some code dependent on the physical ordering of its structs (is that a good idea)?
I'm only asking because GCC is super optimized in a lot of ways but not in this one, and I'm thinking there must be some relatively cool explanation (to which I am oblivious).
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No. GCC passes your assembly source through the preprocessor and then to the assembler. At no time are any optimisations performed.
GCC has a range of optimization levels, plus individual options to enable or disable particular optimizations. The overall compiler optimization level is controlled by the command line option -On, where n is the required optimization level, as follows: -O0 . (default).
The compiler optimizes to reduce the size of the binary instead of execution speed. If you do not specify an optimization option, gcc attempts to reduce the compilation time and to make debugging always yield the result expected from reading the source code.
Pragmas are implementation specific but, in this case (gcc), it sets the optimisation level to 3 (high), similar in effect to using -O3 on the command line. Details on optimisation levels for gcc , and the individual flags that get set in response, can be found here.
gcc does not reorder the elements of a struct, because that would violate the C standard. Section 6.7.2.1 of the C99 standard states:
Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared.
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Structs are frequently used as representations of the packing order of binary file formats and network protocols. This would break if that were done. In addition, different compilers would optimize things differently and linking code together from both would be impossible. This simply isn't feasible.
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