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

[Carsten Mattner]

On Fri, Aug 17, 2012 at 8:50 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Fri, Aug 17, 2012 at 11:42 AM, Carsten Mattner
> <carstenmattner at gmail.com> wrote:
>> 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.
>
> IIRC, there's been some work in this area since 3.1.  If you're still
> seeing issues on trunk, please file a bug.

Thanks for the info. From what I can see beam_emu did get built and
I was able to run the emulator. I didn't do any performance test because
AFAIR previously it didn't result in a runnable executable at all. This
result with enough success for now.

>> 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.
>
> I'm not following this... linking shouldn't be spending any
> substantial amount of time in the "clang" process.  clang just invokes
> ld.

Me neither, it's totally unexpected and surprising.

> If you have a reproducible infinite loop, please file a bug.

I won't be able to conduct another set of tests including clang trunk
before the middle of next week, sorry.

If anyone wants to try in the meantime here are the steps:

$ git clone git://github.com/erlang/otp.git
$ cd otp
$ export ERL_TOP=$PWD
$ export PATH=$ERL_TOP/bin:$PATH
$ export CC=clang
$ export CXX=clang++
$ ./otp_build setup -a --prefix=$PWD/localinstall
$ make install
You can run and test the emulator without installing locally.
If you want to build the emulator only to see about beam_emu.c
$ ./otp_build autoconf && ./otp_build configure --prefix=$PWD/localinstall
$ cd erts && make
erl_gl.so is built in lib/wx
$ cd lib/wx && make
This naturally requires wxWidgets