[clang] [clang][bytecode] Check strcmp input for unknown-size arrays (PR #220835)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

<details>
<summary>Changes</summary>

The later calls to getNumElems() will fail for them.

---
Full diff: https://github.com/llvm/llvm-project/pull/220835.diff


2 Files Affected:

- (modified) clang/lib/AST/ByteCode/InterpBuiltin.cpp (+3-1) 
- (modified) clang/test/AST/ByteCode/builtin-functions.cpp (+6) 


``````````diff
diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index 91d6a32c136ac..b78ce614e290d 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -306,7 +306,9 @@ static bool interp__builtin_strcmp(InterpState &S, CodePtr OpPC,
 
   if (!A.isReadablePointerType() || !B.isReadablePointerType())
     return false;
-  if (A.isDummy() || B.isDummy())
+
+  if (A.isDummy() || B.isDummy() || A.isUnknownSizeArray() ||
+      B.isUnknownSizeArray())
     return false;
 
   bool IsWide = ID == Builtin::BIwcscmp || ID == Builtin::BIwcsncmp ||
diff --git a/clang/test/AST/ByteCode/builtin-functions.cpp b/clang/test/AST/ByteCode/builtin-functions.cpp
index 47a1c23d77799..51e1b6cde4779 100644
--- a/clang/test/AST/ByteCode/builtin-functions.cpp
+++ b/clang/test/AST/ByteCode/builtin-functions.cpp
@@ -132,6 +132,12 @@ namespace strcmp {
   static_assert(__builtin_strncmp("abaa", "abba", 0) == 0);
   static_assert(__builtin_strncmp(0, 0, 0) == 0);
   static_assert(__builtin_strncmp("abab\0banana", "abab\0canada", 100) == 0);
+
+
+  constexpr char missingInit[] = bar__; // both-error {{use of undeclared identifier 'bar__'}} \
+                                        // ref-note {{declared here}}
+  static_assert(__builtin_strcmp(missingInit, "bar") == 0, ""); // both-error {{not an integral constant expression}} \
+                                                                // ref-note {{initializer of 'missingInit' is unknown}}
 }
 
 namespace WcsCmp {

``````````

</details>


https://github.com/llvm/llvm-project/pull/220835