[llvm-dev] llvm and clang are getting slower

[mats petersson via llvm-dev]

I have noticed that LLVM doesn't seem to "like" large functions, as a
general rule. Admittedly, my experience is similar with gcc, so I'm not
sure it's something that can be easily fixed. And I'm probably sounding
like a broken record, because I have said this before.

My experience is that the time it takes to compile something is growing
above linear with size of function.

Of course, the LLVM code is growing over time, both to support more
features and to support more architectures, new processor types and
instruction sets, at least of which will lead to larger functions in
general [and this is the function "after inlining", so splitting small
'called once' functions out doesn't really help much].

I will have a little play to see if I can identify more of a cuplrit [at
the very least if it's "large basic blocks" or "large functions" that is
the problem] - of course, this could be unrelated and irellevant to the
problem Daniel is pointing at, and it may or may not be easily resolved...

--
Mats

On 8 March 2016 at 18:42, Richard Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Tue, Mar 8, 2016 at 8:13 AM, Rafael Espíndola
> <llvm-dev at lists.llvm.org> wrote:
> > I have just benchmarked building trunk llvm and clang in Debug,
> > Release and LTO modes (see the attached scrip for the cmake lines).
> >
> > The compilers used were clang 3.5, 3.6, 3.7, 3.8 and trunk. In all
> > cases I used the system libgcc and libstdc++.
> >
> > For release builds there is a monotonic increase in each version. From
> > 163 minutes with 3.5 to 212 minutes with trunk. For comparison, gcc
> > 5.3.2 takes 205 minutes.
> >
> > Debug and LTO show an improvement in 3.7, but have regressed again in
> 3.8.
>
> I'm curious how these times divide across Clang and various parts of
> LLVM; rerunning with -ftime-report and summing the numbers across all
> compiles could be interesting.
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