[cfe-dev] Massive performance regression in clang 3.9

[Richard Smith via cfe-dev]

On 19 Oct 2016 2:44 am, "Andy Gibbs" <andyg1001 at hotmail.co.uk> wrote:

Hi,

I have observed a massive performance regression when upgrading from clang
3.7
to 3.9.  I have also checked that current tip-of-trunk seems to have this
regression too.

I have managed to isolate the patch at which the regression appeared as
r248431 on 23rd Sept 2015.

I have attached a simple test-case that demonstrates the problem rather
effectively.

Prior to r248431, this code would compile in under 20 seconds, consuming
approx. 400Mb of memory.  GCC, for comparison, takes longer at around 50
seconds on my machine but consumes only ~250Mb.

Following patch r248431 and all the way up to tip-of-trunk, clang is
terminated by the OOM handler due excessive memory usage in excess of 7
Gigabytes!!!

This seems a little unreasonable -- the test-code in question is a little
pathological maybe but is designed to exploit template memoisation, so the
excessive memory usage would imply to me that this memoisation is no longer
working.


That's not the problem. The issue is that this:

(sizeof...(Is) + Is)...

substitutes into the sizeof... subexpression once for each element of Is,
which massively amplifies any change in the behaviour of sizeof...

r248431 changes our behaviour such that sizeof... now scans its parameter's
expansion looking for unexpanded packs (which can appear if it's used
inside an alias template), making the instantiation of that fragment
quadratic in the size of the pack Is.

I'll take a look and see if we can handle this a bit better.

Is anyone else able to confirm my findings?  If so, I will raise this as a
bug
report.  Richard, I have copied you as the author of the patch in question
(hope you don't mind!).

Cheers,
Andy
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