[clang] [analyzer] Workaround for unintended slowdown (scope increase) (PR #136720)

[Gábor Horváth via cfe-commits]

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@@ -2856,8 +2873,29 @@ void ExprEngine::processBranch(
       // conflicts with the widen-loop analysis option (which is off by
       // default). If we intend to support and stabilize the loop widening,
       // we must ensure that it 'plays nicely' with this logic.
-      if (!SkipTrueBranch || AMgr.options.ShouldWidenLoops)
+      if (!SkipTrueBranch || AMgr.options.ShouldWidenLoops) {
         Builder.generateNode(StTrue, true, PredN);
+      } else if (AMgr.options.LegacyInliningPrevention) {
+        // FIXME: There is an ancient and very arbitrary heuristic in
+        // `ExprEngine::processCFGBlockEntrance` which prevents all further
+        // inlining of a function if it finds an execution path within that
+        // function which reaches the `MaxBlockVisitOnPath` limit (a/k/a
+        // `analyzer-max-loop`, by default four iterations in a loop). Adding
+        // this "don't assume third iteration" logic significantly increased
+        // the analysis runtime on some inputs because less functions were
+        // arbitrarily excluded from being inlined, so more entrypoints used
+        // up their full allocated budget. As a hacky compensation for this,
+        // here we apply the "should not inline" mark in cases when the loop
+        // could potentially reach the `MaxBlockVisitOnPath` limit without the
+        // "don't assume third iteration" logic. This slightly overcompensates
+        // (activates if the third iteration can be entered, and will not
+        // recognize cases where the fourth iteration would't be completed), but
+        // should be good enough for practical purposes.
+        if (const LocationContext *LC = getInlinedLocationContext(Pred, G)) {
+          Engine.FunctionSummaries->markShouldNotInline(
+              LC->getStackFrame()->getDecl());
+        }
+      }
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Xazax-hun wrote:

Also I am not sure we should have legacy in the name of something that is enabled by default. 

https://github.com/llvm/llvm-project/pull/136720