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C++: Fix performance issue on cpp/comma-before-misleading-indentation #10958

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merged 2 commits into from
Oct 26, 2022
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C++: Fix performance issue on cpp/comma-before-misleading-indentation #10958

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6f77e14
C++: Fix rare performance issue on cpp/comma-before-misleading-indent…
geoffw0 Oct 24, 2022
257748d
C++: Rename predicate.
geoffw0 Oct 25, 2022
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41 changes: 31 additions & 10 deletions cpp/ql/src/Best Practices/Likely Errors/CommaBeforeMisleadingIndentation.ql
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Original file line number Diff line number Diff line change
Expand Up @@ -16,15 +16,36 @@
import cpp
import semmle.code.cpp.commons.Exclusions

/** Gets the sub-expression of 'e' with the earliest-starting Location */
/**
* Gets a child of `e`, including conversions but excluding call arguments.
*/
pragma[inline]
Expr getAChildWithConversions(Expr e) {
result.getParentWithConversions() = e and
not result = any(Call c).getAnArgument()
}

/**
* Gets the left-most column position of any transitive child of `e` (including
* conversions but excluding call arguments).
*/
int getCandidateColumn(Expr e) {
result = e.getLocation().getStartColumn() or
result = getCandidateColumn(getAChildWithConversions(e))
}

/**
* Gets the transitive child of `e` (including conversions but excluding call
* arguments) at the left-most column position, preferring less deeply nested
* expressions if there is a choice.
*/
Expr normalizeExpr(Expr e) {
result =
min(Expr child |
child.getParentWithConversions*() = e.getFullyConverted() and
not child.getParentWithConversions*() = any(Call c).getAnArgument()
|
child order by child.getLocation().getStartColumn(), count(child.getParentWithConversions*())
)
e.getLocation().getStartColumn() = min(getCandidateColumn(e)) and
result = e
or
not e.getLocation().getStartColumn() = min(getCandidateColumn(e)) and
result = normalizeExpr(getAChildWithConversions(e)) and
result.getLocation().getStartColumn() = min(getCandidateColumn(e))
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Is this final conjunct necessary to prevent the normalizeExpr call on line 47 from having multiple results? If so, could we get rid of this final conjunct by wrapping line 47 in a min that orders by the start location? (Or does this run into non-monotonicity issues?

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I believe line 48 is necessary to ensure we get a global minimum Expr, not just the minimum under one particular subtree (child of e). It doesn't appear to affect any of the tests if I remove it, but I think it would be wrong to do so.

Wrapping line 47 in min does indeed hit non-monotonicity issues (which seem to plague this particular predicate).

}

predicate isParenthesized(CommaExpr ce) {
Expand All @@ -43,8 +64,8 @@ from CommaExpr ce, Expr left, Expr right, Location leftLoc, Location rightLoc
where
ce.fromSource() and
not isFromMacroDefinition(ce) and
left = normalizeExpr(ce.getLeftOperand()) and
right = normalizeExpr(ce.getRightOperand()) and
left = normalizeExpr(ce.getLeftOperand().getFullyConverted()) and
right = normalizeExpr(ce.getRightOperand().getFullyConverted()) and
leftLoc = left.getLocation() and
rightLoc = right.getLocation() and
not isParenthesized(ce) and
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