C++: inline arithTypesMatch predicate

jbj · jbj · commit 7a3663976b25 · 2020-04-22T15:14:07.000+02:00
This predicate is effectively a Cartesian product between all enum
types. It's infeasible to compute it in full, so luckily the optimizer
has been able to apply enough magic to make it feasible. That's not a
robust solution, and it has indeed broken on at least one version of the
1.24 release candidate.

On a Chromium snapshot where I ran the LGTM suite overnight, the
`m#MistypedFunctionArguments::arithTypesMatch#bb` predicate (magic for
`arithTypesMatch`) took 170m5s. That was commit b69fdf5 from the
internal repo. I tried to reproduce it in VSCode, this time with commit
646646, but it wasn't quite as bad: the predicate took only 38 seconds.
In any case, making the problematic predicate `pragma[inline]` removes
the slow magic and makes the `MistypedFunctionArguments.ql` query
faster.
diff --git a/cpp/ql/src/Likely Bugs/Underspecified Functions/MistypedFunctionArguments.qll b/cpp/ql/src/Likely Bugs/Underspecified Functions/MistypedFunctionArguments.qll
@@ -6,6 +6,7 @@
 
 import cpp
 
+pragma[inline]
 private predicate arithTypesMatch(Type arg, Type parm) {
   arg = parm
   or
