[cfe-dev] computed goto/labels as values in interpreters

[Carsten Mattner]

On Fri, Aug 17, 2012 at 7:40 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Fri, Aug 17, 2012 at 8:14 AM, Carsten Mattner
> <carstenmattner at gmail.com> wrote:
>> After searching list archives and bug reports I wasn't able to conclude
>> whether computed goto/labels as values as used in Erlang and other (Guile)
>> interpreters is supported in clang 3.1. With some BSDs working to make
>> clang/clang++ the base cc/c++ this is going to be even more important.
>>
>> Is clang 3.1 supposed to compile beam_emu.c correctly and with the
>> same or comparable optimization result?
>>
>> https://github.com/erlang/otp/blob/master/erts/emulator/beam/beam_emu.c
>
> Yes, it's supposed to.

Yes, seems to work but with some interesting differences compared to
gcc. While gcc-4.7 takes 6s to recompile Erlang's emulator if I
'touch beam_emu.c', clang takes 2m11s and consumes around 141MB RES vs
40MB gcc. Not sure why but clang also seemed to result in a longer
time to run ar. Tests were conducted on i386 linux 3.5.1 with
glibc-2.15 against erlang's maint branch. Next, a process which is
known to be slow is building some of the generated C++ files in lib/wx
(wxWidgets Erlang bindings). Now, when linking erl_gl.so with clang
it took forever to finish (didn't time). This is similar to the ar
archive step in erts/emulator. Is there anything inherently slow with
gcc-4.7, glibc-2.15, and binutils-2.22? ar and ld from binutils at
fault? While linking erl_gl.so RES of clang was at 265MB. It seemed
to be an infinite loop and I stopped it after more than 5 minutes.

Are these expected performance results?