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C++: Suppress ExprHasNoEffect on template code #2399

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geoffw0 merged 1 commit into from
Nov 21, 2019
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C++: Suppress ExprHasNoEffect on template code #2399

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1 change: 1 addition & 0 deletions cpp/ql/src/Likely Bugs/Likely Typos/ExprHasNoEffect.ql
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Expand Up @@ -84,6 +84,7 @@ where
not peivc.getEnclosingFunction().isDefaulted() and
not exists(Macro m | peivc = m.getAnInvocation().getAnExpandedElement()) and
not peivc.isFromTemplateInstantiation(_) and

@geoffw0 geoffw0 Nov 20, 2019

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Is this line still contributing positively?

@rdmarsh2 rdmarsh2 Nov 20, 2019

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Yes - it handles cases where a call in a void context has a pure target in some instantiations and a target with side effects in other instantiations. Ideally we'd handle that with a forall, but I don't think we have any way of matching the corresponding elements in different instantiations.

@jbj jbj Nov 21, 2019

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The unusual thing here is that we're excluding both uninstantiated templates and instantiated templates. That means we're excluding all template code. We usually exclude either one or the other, but in this case there has been false positives in both types of code.

We could try something elaborate like counting the number of potential alerts in the template and its instantiations and reporting an alert only if those counts match, but I don't think this query is important enough for us to spend time on that.

@geoffw0 geoffw0 Nov 21, 2019

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OK, sounds like this is a reasonable solution then.

not peivc.isFromUninstantiatedTemplate(_) and

@geoffw0 geoffw0 Nov 20, 2019

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I'm a bit surprised we didn't already have this exception, given the number of other exceptions!

parent = peivc.getParent() and
not parent.isInMacroExpansion() and
not parent instanceof PureExprInVoidContext and
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2 changes: 0 additions & 2 deletions cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/ExprHasNoEffect.expected
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Expand Up @@ -25,8 +25,6 @@
| test.c:25:5:25:16 | ... ? ... : ... | This expression has no effect. | test.c:25:5:25:16 | ... ? ... : ... | |
| test.c:26:15:26:16 | 32 | This expression has no effect. | test.c:26:15:26:16 | 32 | |
| test.c:27:9:27:10 | 33 | This expression has no effect. | test.c:27:9:27:10 | 33 | |
| test.cpp:24:3:24:3 | call to operator++ | This expression has no effect (because $@ has no external side effects). | test.cpp:9:14:9:23 | operator++ | operator++ |
| test.cpp:25:3:25:3 | call to operator++ | This expression has no effect (because $@ has no external side effects). | test.cpp:9:14:9:23 | operator++ | operator++ |
| test.cpp:62:5:62:5 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:47:14:47:22 | operator= | operator= |
| test.cpp:65:5:65:5 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:55:7:55:7 | operator= | operator= |
| volatile.c:9:5:9:5 | c | This expression has no effect. | volatile.c:9:5:9:5 | c | |
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4 changes: 2 additions & 2 deletions cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/test.cpp
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Expand Up @@ -21,8 +21,8 @@ class MyTemplateClass
MyIterator arg1, arg2;
_It arg3;

++arg1; // pure, does nothing
++arg2; // pure, does nothing
++arg1; // pure, does nothing [NOT DETECTED]
++arg2; // pure, does nothing [NOT DETECTED]
++arg3; // not pure in all cases (when _It is int this has a side-effect)

return arg2;
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