[LLVMdev] Missing optimizations (again -> pass ordering)

[Carl-Philip Hänsch]

Hi,

I'm here again with an other issue with pass ordering:

Take a look at the following code.
#include <stdio.h>
#include <functional>

int main() {
        std::function<void(const char*)> hello = [](const char
*name){printf("Hello %s\n", name);};
        hello("World");
        return 0;
}

When I compile it with -O3 the optimizer does not recognize all
optimizations. Instead I get 143 lines of complicated landing pad code,
allocas etc..

When I put this code into an other opt -O3 process, I get much better code:
23 lines of code where the main function simply consists of a single printf
call. (1 line) -> optimal code that I would write in C.

So, when using sophisticated stl data structures like std::function,
std::vector, std::string, they are not molten down to their C equivalents
until the optimizer runs twice, there is something wrong with pass
ordering. Existing passes are okay, but they are not called often enough to
remove all overhead. I hope there will be further work on rerunning passes.

Cheers
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