[clang] Short-circuit trivial constructors when evaluating arrays

VisitCXXConstructExpr() will later do something similar, but for large arrays, we should try to do it once an not for every element. Fixes #56774 Differential Revision: https://reviews.llvm.org/D130791
llvm · Aug 3, 2022 · 11e52ec · 11e52ec
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diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -54,6 +54,9 @@ Bug Fixes
   `Issue 56800 <https://github.com/llvm/llvm-project/issues/56800>`_.
 - Fix `#56772 <https://github.com/llvm/llvm-project/issues/56772>`_ - invalid
   destructor names were incorrectly accepted on template classes.
+- Improve compile-times with large dynamic array allocations with trivial
+  constructors. This fixes
+  `Issue 56774<https://github.com/llvm/llvm-project/issues/56774>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
@@ -10833,6 +10833,9 @@ bool ArrayExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
     if (FinalSize == 0)
       return true;
 
+    bool HasTrivialConstructor = CheckTrivialDefaultConstructor(
+        Info, E->getExprLoc(), E->getConstructor(),
+        E->requiresZeroInitialization());
     LValue ArrayElt = Subobject;
     ArrayElt.addArray(Info, E, CAT);
     // We do the whole initialization in two passes, first for just one element,
@@ -10856,19 +10859,26 @@ bool ArrayExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
         for (unsigned I = OldElts; I < N; ++I)
           Value->getArrayInitializedElt(I) = Filler;
 
-      // Initialize the elements.
-      for (unsigned I = OldElts; I < N; ++I) {
-        if (!VisitCXXConstructExpr(E, ArrayElt,
-                                   &Value->getArrayInitializedElt(I),
-                                   CAT->getElementType()) ||
-            !HandleLValueArrayAdjustment(Info, E, ArrayElt,
-                                         CAT->getElementType(), 1))
-          return false;
-        // When checking for const initilization any diagnostic is considered
-        // an error.
-        if (Info.EvalStatus.Diag && !Info.EvalStatus.Diag->empty() &&
-            !Info.keepEvaluatingAfterFailure())
-          return false;
+      if (HasTrivialConstructor && N == FinalSize) {
+        // If we have a trivial constructor, only evaluate it once and copy
+        // the result into all the array elements.
+        APValue &FirstResult = Value->getArrayInitializedElt(0);
+        for (unsigned I = OldElts; I < FinalSize; ++I)
+          Value->getArrayInitializedElt(I) = FirstResult;
+      } else {
+        for (unsigned I = OldElts; I < N; ++I) {
+          if (!VisitCXXConstructExpr(E, ArrayElt,
+                                     &Value->getArrayInitializedElt(I),
+                                     CAT->getElementType()) ||
+              !HandleLValueArrayAdjustment(Info, E, ArrayElt,
+                                           CAT->getElementType(), 1))
+            return false;
+          // When checking for const initilization any diagnostic is considered
+          // an error.
+          if (Info.EvalStatus.Diag && !Info.EvalStatus.Diag->empty() &&
+              !Info.keepEvaluatingAfterFailure())
+            return false;
+        }
       }
     }