> On Jul 4, 2016, at 11:09 AM, Dmitry Markman <[email protected]> wrote: > > clang can’t optimize malloc here, because result of the malloc is used.
It’s only used by free(), whose behavior is also known to the compiler, not by anything in your code. Thus the compiler saw that the malloc and free together formed a no-op, and optimized them both away. > also if I use > std::cout << "data != NULL" << data << std::endl; > malloc wasn’t optimized Right, because then the program actually uses the pointer, so the compiler can’t optimize it away. > we have much more complex use case where > malloc was optimized out, Did this code actually use the pointer returned by malloc? If so, that would seem to be a bug. But the example you showed here isn’t. > On Jul 4, 2016, at 11:55 AM, Quincey Morris > <[email protected]> wrote: > > I don’t know the answer, but it seems to me at least possible that it didn’t > fail. It’s at least possible that it gave you an unmapped virtual allocation > of the size you asked for. No, it does fail — I just tried it on OS X 10.11.5, and with no optimizations (-O0) the malloc call isn’t optimized away, and logs: a.out(9897,0x7fff75a05000) malloc: *** mach_vm_map(size=281474976710656) failed (error code=3) *** error: can't allocate region *** set a breakpoint in malloc_error_break to debug —Jens
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