there's a lot I don't understand about optimizers, but is it unreasonable to expect a modern C++ compiler to turn foo() into an add instruction?
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"std::vector<int> v {a, b}" seems to make a huge difference with gcc trunk. But still not a simple add. godbolt.org/z/DDrOCO
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In general, I don't think either Clang or GCC is capable of getting rid of malloc/free or new/delete outside of a special case for completely dead stores. They aren't capable of doing escape analysis and lowering it to a stack allocation / virtual registers so it won't go away.
Last time I checked, the GCC dead store elimination also only works for malloc/free and only when the code is buggy and lacks an out-of-memory check. Clang is better at this, but it lacks a way to turn this into a dead store anyway. It can't work with malloc/new directly either.
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So yes, I'm sure it understands that it's just an add. It knows the memory returned from new/malloc doesn't alias anything else. It just has absolutely no way to get rid of it by lowering it to something else and also has no way to get rid of it in a single step. Not implemented.
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They're happy to get rid of them in cases where doing do is an invalid transformation, like p=malloc(-1); if(p) puts("bad"); free(p);
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In general, they make up their own semantics for C standard library functions. A good example is that Clang doesn't support floating point rounding modes or signalling NaN. It explicitly chooses to implement optimizations incompatible with an implementation of those features.
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