[llvm-dev] Scaling to many basic blocks

David Jones via llvm-dev llvm-dev at lists.llvm.org
Sat Aug 22 16:51:49 PDT 2015

A somewhat more general question:

What characteristics of generated IR contribute to poor optimization/code
generation times?

I recently found a pathological case in my own code of data tables that
caused poor performance in the IR verifier.

In general, I am not happy with the performance of LLVM on my code, so I
know I need to do better. But how? I have a few ideas, but trying any of
them could be time-consuming, with no guaranteed return at the end of it.

Currently, I don't emit TBAA metadata. I have a hunch that doing so might
speed up compilation, as alias analyses can be reduced in size, but I don't
know that for sure, or how much of a benefit it would be. Yes, I need to do
this in any case, but if my users complain of long compile times, I'd like
to know what to work on first.

On Sat, Aug 22, 2015 at 6:57 PM, Michael Zolotukhin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
> Several passes would have troubles with such code (namely, GVN and
> JumpThreading). Recently we exposed similar issues when new more aggressive
> unrolling heuristic was implemented - we unrolled big loops expecting that
> the linear code would be then optimized, and it indeed was optimized, but
> compile time regressed significantly. But probably some of these issues
> have been fixed already - I didn’t check.
> Thanks,
> Michael
> > On Aug 22, 2015, at 3:03 PM, Russell Wallace via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > How well does LLVM scale to many basic blocks? Let's say you have a
> single function consisting of a million basic blocks each with a few tens
> of instructions (and assuming the whole thing isn't trivially repetitive so
> the number of simultaneously live variables and whatever is large) and you
> feed that through the optimisers into the backend code generator, will this
> work okay, or will it take a very long time, or will it run into a scenario
> of 'look, all compilers are designed on the assumption that nobody is going
> to write a million lines of code in a single function'? (Not that I'm
> planning to do so by hand either, I'm just thinking about certain scenarios
> in automatic code generation.)
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