On 2018-09-08 11:58 +1000, Rob <sisyphus1@xxxxxxxxxxxxxxx> wrote: > -----Original Message----- > From: Xi Ruoyao > Sent: Saturday, September 08, 2018 1:43 AM > To: sisyphus1@xxxxxxxxxxxxxxx > Cc: gcc-help@xxxxxxxxxxx ; Jonathan Wakely > Subject: Re: A g++ bug ... or not ? > > [snip] > > > And libstdc++ is doing right thing (casting the `int` parameter to > > `double`). But C++03 is not so clear and does not define what should > > libstdc++ do. Libstdc++ chooses to call __builtin_powi. Unfortunately it > > is not so precise. > > Based on that, I expected that another solution to the problem (ie instead > of providing the explicit cast to double) would be to compile with > "-fno-builtin". > So I tried building the demo with "g++ -fno-builtin -std=c++03 -o try > try.c", but the explicit cast to double was still required. No. That won't work because libstdc++ is *explicitly* calling `__builtin_powi`. `-fno-builtin` can only stop GCC to convert, for e.g. `printf(...)` to `__builtin_printf(...)`. On Fri, 7 Sep 2018 23:47:48 +0100, Jonathan Wakely wrote: > Maybe. I think that might resolve > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78851 too We both try to make C++11 and C++03 behavior consistent but Vadim intend to use __builtin_pow for both of them... I believe he is wrong because Rob has shown that __builtin_pow is inprecise and evil. And, C++11 *defines* `double pow(double x, int y)` to behave *exactly* whatv `pow(x, (double)y)` does. Like Marc, I can't reproduce Vadim's result and think Vadim's libm is buggy. I'll try to make C++03 `pow(float, int)`, `pow(double, int)` and `pow(long double, int)` to call libm functions instead of builtin functions, then do a regression test. -- Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
