Fix loop-to-assume transformation in goto-symex

[tautschnig]

The previous implementation (from e672e0d) would not account for
loop heads with side effects.

Fixes: #5450

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[martin-cs]

Yep; that looks like the kind of fix that is needed. Are there already tests for the kind of loop it is supposed to target?

[smowton]

Are you sure your test case exercises the new code? I'd expect the first loop to leave count == 5 in the constant propagator. Then the second loop (the empty do-while) would use the empty-loop special case, but the third loop would probably be skipped by plain constant propagation evaluating the conditional jump statically before we consider if it's an empty loop?

[smowton]

(Either way it'd be worth adding comments to the test-case indicating what you expect to happen)

[hannes-steffenhagen-diffblue]

@smowton This definitely fixes the reported issue

[hannes-steffenhagen-diffblue]

LGTM (the way I understood this this previously replaced all empty or seemingly “empty” loops with assume false, and now only does if the loop condition can be simplified to true?).

[smowton]

OK, if you're sure it works as advertised

[tautschnig]

OK, if you're sure it works as advertised

Well, now at least it is being tested...

[codecov]

Codecov Report

Merging #5455 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5455   +/-   ##
========================================
  Coverage    68.23%   68.23%           
========================================
  Files         1178     1178           
  Lines        97588    97590    +2     
========================================
+ Hits         66589    66592    +3     
+ Misses       30999    30998    -1     

Flag	Coverage Δ
#cproversmt2	42.81% <100.00%> (+<0.01%)	⬆️
#regression	65.40% <100.00%> (+<0.01%)	⬆️
#unit	32.23% <0.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
src/goto-symex/symex_goto.cpp	97.00% <100.00%> (+0.39%)	⬆️

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