Following along from this question: how-do-i-check-if-gcc-is-performing-tail-recursion-optimization, I noticed that using gcc with -fPIC seems to destroy this optimization. I am creating a shared library, but I doesn t seem to need the -fPIC option.
Well, my question is, why does -fPIC change gcc optimizations ? Do I need to keep -fPIC for any reason ?
In absence of details such as target architecture and compiler version, a possible explanation is this:
In position-dependent code, the tail-recursion optimization is essentially to reuse the current stack frame, and to replace the considered call by a jump. The syntax may be call function replaced by jmp <small offset of function>.
In position-independent code, the call may be written call function@PLT if the instruction set allows it (this example is amd64). It could perfectly well be replaced by jmp <small offset of function>@PLT, but the two settings do interfere and perhaps gcc developers did not get around to implementing tail-call optimization in the latter mode.
In ia32 linux, use of fpic means that you don t have ebx available for general purpose use, which will surely impact optimization. Perhaps the compiler decided against the tail recursion optimization due to register scheduling pressure.
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