On Fri, Oct 6, 2023 at 6:49 PM Jakub Svojgr via Gcc <gcc@gcc.gnu.org> wrote:
> Hello,
> this is an extremely specific scenario - however while compiling
> computer-generated code for a fibonnacci function which looks like this:
>
> I32 GF_fib_I_I(I32 L_0_0)
> {
> I32 L_2_0; I32 L_5_0; I32 L_6_0;
> bb0:;
> bool L_0_1 = ((I32)L_0_0) == ((I32)0);
> if (L_0_1)
> {
> goto bb1;
> }
> L_2_0 = L_0_0;
> goto bb2;
> bb1:;
> return ((I32)1);
> bb2:;
> bool L_2_1 = ((I32)L_2_0) == ((I32)(0));
> if (L_2_1)
> {
> goto bb4;
> }
> L_5_0 = L_2_0;
> goto bb5;
> bb4:;
> return ((I32)1);
> bb5:;
> I32 L_5_1 = ((I32)L_5_0) - ((I32)1);
> I32 L_5_2 = GF_fib_I_I(L_5_1);
> I32 L_5_3 = ((I32)L_5_0) - ((I32)2);
> I32 L_5_4 = GF_fib_I_I(L_5_3);
> I32 L_5_5 = ((I32)L_5_2) + ((I32)L_5_4);
> return L_5_5;
> }
>
> Now, when compiling this with x86-64 gcc version 13.2 on GodBolt (
> https://godbolt.org/z/5sj7hPchs)
> using the O0 option generates not-so-bad, slightly naive code with 52
> lines of assembly
> using O1 generates good code with 20 lines of assembly
> however when O2 is turned on the generated assembly blows up to 230 lines
> for this function!
> I am not sure if this is classified as a bug because the code does behave
> correctly however it is strange.
>
Have you measured the performance?
GCC -O2 does not target minimum code size and one should not assume that
smaller code is faster code given the details of processor
microarchitectures.
If you are reporting that -O2 is producing worse performance, then that is
a regression. Large code size with -O2 is not a regression and may
actually be correct and beneficial because the option is intended to
improve performance, not reduce code size.
Thanks, David
