Use std::forward_list instead of std::map in irept by default

[tautschnig]

This reduces the size of an irept by 5 pointers (i.e., 40 bytes on
64-bit systems). On SV-COMP's ReachSafety-ECA with this change we can
perform 3819.81 symex_step calls per second, compared to 2752.28 calls
per second with the std::map configuration. The performance improvements
are spread across various irept-related operations.

This is a repeat of #4458 and re-introduces what #4729 just reverted. It must only be merged after #4724 is in.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 4ea27a0).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113878030

[tautschnig]

This would need a timeout limit increase for the CodeBuild job to pass.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 61b0bc3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114067829

[smowton]

Shall we do this then?

[codecov]

Codecov Report

Merging #4731 (235dde8) into develop (a503f5c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4731   +/-   ##
========================================
  Coverage    69.31%   69.32%           
========================================
  Files         1241     1242    +1     
  Lines       100381   100418   +37     
========================================
+ Hits         69580    69615   +35     
- Misses       30801    30803    +2     

Flag	Coverage Δ
cproversmt2	43.09% <ø> (+0.01%)	⬆️
regression	66.22% <ø> (+<0.01%)	⬆️
unit	32.26% <ø> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/util/irep.cpp	97.41% <ø> (-0.03%)	⬇️
src/util/irep.h	95.38% <ø> (ø)
src/util/irep_hash_container.cpp	63.88% <ø> (ø)
src/util/irep_serialization.cpp	92.39% <ø> (ø)
src/util/lispirep.cpp	70.00% <ø> (+0.76%)	⬆️
src/util/merge_irep.cpp	33.33% <ø> (ø)
src/util/type.h	100.00% <ø> (ø)
.../strings/string_constraint_generator_constants.cpp	63.33% <0.00%> (-2.19%)	⬇️
... and 9 more

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[martin-cs]

Sounds reasonable.

[tautschnig]

@peterschrammel (or anyone else with direct access to CodeBuild logs): am I right to assume that the "cbmc" CodeBuild job just times out? I believe this is down to it running with -D_GLIBCXX_DEBUG, which makes the tests take 2 hours of wall clock time on my machine (compared to 400 seconds in a Release build).

[peterschrammel]

Yes, it times out.

Screenshot 2020-11-12 at 10 47 47

[tautschnig]

Yes, it times out.

Screenshot 2020-11-12 at 10 47 47

Thank you @peterschrammel! How long does this build job take without the changes in this PR, i.e., on about any other PR? I am not too surprised that -D_GLIBCXX_DEBUG adds a lot of overhead here, but at the same time I don't want to go ahead and merge this PR just to make the job then fail on all other PRs as well (even when it's not longer marked "Required").

[peterschrammel]

You should actually have access. Ping me if you forgot your credentials.