[llvm-dev] llvm and clang are getting slower

[Xinliang David Li via llvm-dev]

On Tue, Mar 8, 2016 at 10:55 AM, mats petersson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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.
>
>
The number of BBs -- Kosyia can point you to the compile time bug that is
exposed by asan .

David



> 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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>
>
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