"Dario Saccavino" <kathoum@xxxxxxxxx> writes: > I noticed that with the -pedantic option, the compiler was able to > transform the recursive call in a loop, but the same optimization > didn't happen without -pedantic. Look at the bottom of the post for > the full assembly listings. Interesting. It has to do with the handling of the const variable j. It calls the function decl_constant_value_for_broken_optimization which has this interesting comment. /* Return either DECL or its known constant value (if it has one), but return DECL if pedantic or DECL has mode BLKmode. This is for bug-compatibility with the old behavior of decl_constant_value (before GCC 3.0); every use of this function is a bug and it should be removed before GCC 3.1. It is not appropriate to use pedantic in a way that affects optimization, and BLKmode is probably not the right test for avoiding misoptimizations either. */ The effect is that a pedantic compilation sees an add of the variable where a non-pedantic compilation sees an add of the constant 1. This then leads to slightly different generated code, which causes the non-pedantic compilation to fail to see that it can generate a tail call. Interestingly, this particular test case will work in 4.1.3 (if there is a 4.1.3) because of the fix for bug 30364, which changes the association of the generated code. Clearly we need to actually remove decl_constant_value_for_broken_optimization as promised in the comment. Ian
