[clang][bytecode] Fix emitting dtors of zero-sized arrays

[tbaederr]

Desc->getNumElems() returning 0 made us underflow here.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Timm Baeder (tbaederr)

Changes

Desc->getNumElems() returning 0 made us underflow here.

---

Full diff: https://github.com/llvm/llvm-project/pull/134672.diff

2 Files Affected:

• (modified) clang/lib/AST/ByteCode/Compiler.cpp (+11-9)
• (modified) clang/test/AST/ByteCode/cxx23.cpp (+12)

diff --git a/clang/lib/AST/ByteCode/Compiler.cpp b/clang/lib/AST/ByteCode/Compiler.cpp
index 021acbd798646..9e9252558b1a9 100644
--- a/clang/lib/AST/ByteCode/Compiler.cpp
+++ b/clang/lib/AST/ByteCode/Compiler.cpp
@@ -6818,15 +6818,17 @@ bool Compiler<Emitter>::emitDestruction(const Descriptor *Desc,
         return true;
     }
 
-    for (ssize_t I = Desc->getNumElems() - 1; I >= 0; --I) {
-      if (!this->emitConstUint64(I, Loc))
-        return false;
-      if (!this->emitArrayElemPtrUint64(Loc))
-        return false;
-      if (!this->emitDestruction(ElemDesc, Loc))
-        return false;
-      if (!this->emitPopPtr(Loc))
-        return false;
+    if (size_t N = Desc->getNumElems()) {
+      for (ssize_t I = N - 1; I >= 0; --I) {
+        if (!this->emitConstUint64(I, Loc))
+          return false;
+        if (!this->emitArrayElemPtrUint64(Loc))
+          return false;
+        if (!this->emitDestruction(ElemDesc, Loc))
+          return false;
+        if (!this->emitPopPtr(Loc))
+          return false;
+      }
     }
     return true;
   }
diff --git a/clang/test/AST/ByteCode/cxx23.cpp b/clang/test/AST/ByteCode/cxx23.cpp
index 6a62ac11cde79..d0ade4f5278b1 100644
--- a/clang/test/AST/ByteCode/cxx23.cpp
+++ b/clang/test/AST/ByteCode/cxx23.cpp
@@ -304,3 +304,15 @@ namespace NonLiteralDtorInParam {
                               // expected23-note {{non-constexpr function '~NonLiteral' cannot be used in a constant expression}}
   }
 }
+
+namespace ZeroSizedArray {
+  struct S {
+    constexpr ~S() {
+    }
+  };
+  constexpr int foo() {
+    S s[0];
+    return 1;
+  }
+  static_assert(foo() == 1);
+}

[efriedma-quic]

Please try to avoid implicit integer conversions where overflow is a potential issue; Desc->getNumElems() returns an unsigned, so please just make the variable unsigned as well.