[clang] 9dc4af3 - Re-land "[clang] Add early exit when checking for const init of arrays."

[Sam McCall via cfe-commits]

Author: Sam McCall
Date: 2021-12-29T13:07:30+01:00
New Revision: 9dc4af327b12dfbcf90fde1641cd649c6814bf98

URL: https://github.com/llvm/llvm-project/commit/9dc4af327b12dfbcf90fde1641cd649c6814bf98
DIFF: https://github.com/llvm/llvm-project/commit/9dc4af327b12dfbcf90fde1641cd649c6814bf98.diff

LOG: Re-land "[clang] Add early exit when checking for const init of arrays."

This reverts commit 6d09aaecdfe51e13fc64d539aa7c9a790de341d7.

The test uses ulimit and ran into problems on some bots. Run on linux only.
There's nothing platform-specific about the code we're testing, so this
should be enough to ensure correctness.

Added: 
    clang/test/SemaCXX/PR51712-large-array-constexpr-check-oom.cpp

Modified: 
    clang/lib/AST/ExprConstant.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 469339e8cd624..105cd7a3506dd 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -10680,28 +10680,55 @@ bool ArrayExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
   bool HadZeroInit = Value->hasValue();
 
   if (const ConstantArrayType *CAT = Info.Ctx.getAsConstantArrayType(Type)) {
-    unsigned N = CAT->getSize().getZExtValue();
+    unsigned FinalSize = CAT->getSize().getZExtValue();
 
     // Preserve the array filler if we had prior zero-initialization.
     APValue Filler =
       HadZeroInit && Value->hasArrayFiller() ? Value->getArrayFiller()
                                              : APValue();
 
-    *Value = APValue(APValue::UninitArray(), N, N);
-
-    if (HadZeroInit)
-      for (unsigned I = 0; I != N; ++I)
-        Value->getArrayInitializedElt(I) = Filler;
+    *Value = APValue(APValue::UninitArray(), 0, FinalSize);
+    if (FinalSize == 0)
+      return true;
 
-    // Initialize the elements.
     LValue ArrayElt = Subobject;
     ArrayElt.addArray(Info, E, CAT);
-    for (unsigned I = 0; I != N; ++I)
-      if (!VisitCXXConstructExpr(E, ArrayElt, &Value->getArrayInitializedElt(I),
-                                 CAT->getElementType()) ||
-          !HandleLValueArrayAdjustment(Info, E, ArrayElt, CAT->getElementType(),
-                                       1))
-        return false;
+    // We do the whole initialization in two passes, first for just one element,
+    // then for the whole array. It's possible we may find out we can't do const
+    // init in the first pass, in which case we avoid allocating a potentially
+    // large array. We don't do more passes because expanding array requires
+    // copying the data, which is wasteful.
+    for (const unsigned N : {1u, FinalSize}) {
+      unsigned OldElts = Value->getArrayInitializedElts();
+      if (OldElts == N)
+        break;
+
+      // Expand the array to appropriate size.
+      APValue NewValue(APValue::UninitArray(), N, FinalSize);
+      for (unsigned I = 0; I < OldElts; ++I)
+        NewValue.getArrayInitializedElt(I).swap(
+            Value->getArrayInitializedElt(I));
+      Value->swap(NewValue);
+
+      if (HadZeroInit)
+        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;
+      }
+    }
 
     return true;
   }

diff  --git a/clang/test/SemaCXX/PR51712-large-array-constexpr-check-oom.cpp b/clang/test/SemaCXX/PR51712-large-array-constexpr-check-oom.cpp
new file mode 100644
index 0000000000000..859b1ac3fb2be
--- /dev/null
+++ b/clang/test/SemaCXX/PR51712-large-array-constexpr-check-oom.cpp
@@ -0,0 +1,17 @@
+// Only run this test where ulimit is known to work well.
+// (There's nothing really platform-specific being tested, this is just ulimit).
+//
+// REQUIRES: shell
+// REQUIRES: linux
+// UNSUPPORTED: msan
+// UNSUPPORTED: asan
+//
+// RUN: ulimit -v 1048576
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -triple=x86_64 %s
+// expected-no-diagnostics
+
+// This used to require too much memory and crash with OOM.
+struct {
+  int a, b, c, d;
+} arr[1<<30];
+