[clang] [clang][Interp] Zero-init remaining string literal elements (PR #66862)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr updated https://github.com/llvm/llvm-project/pull/66862

>From f2b9dec7ccf73258eb74c1823e4a954a459b5d51 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Wed, 20 Sep 2023 08:18:51 +0200
Subject: [PATCH] [clang][Interp] Zero-init remaining string literal elements

---
 clang/lib/AST/Interp/ByteCodeExprGen.cpp | 21 +++++++++++++++++++--
 clang/test/AST/Interp/arrays.cpp         | 21 +++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/clang/lib/AST/Interp/ByteCodeExprGen.cpp b/clang/lib/AST/Interp/ByteCodeExprGen.cpp
index e813d4fa651ceaf..c2b107cef7c9708 100644
--- a/clang/lib/AST/Interp/ByteCodeExprGen.cpp
+++ b/clang/lib/AST/Interp/ByteCodeExprGen.cpp
@@ -858,8 +858,8 @@ bool ByteCodeExprGen<Emitter>::VisitStringLiteral(const StringLiteral *E) {
 
   // If the initializer string is too long, a diagnostic has already been
   // emitted. Read only the array length from the string literal.
-  unsigned N =
-      std::min(unsigned(CAT->getSize().getZExtValue()), E->getLength());
+  unsigned ArraySize = CAT->getSize().getZExtValue();
+  unsigned N = std::min(ArraySize, E->getLength());
   size_t CharWidth = E->getCharByteWidth();
 
   for (unsigned I = 0; I != N; ++I) {
@@ -878,6 +878,23 @@ bool ByteCodeExprGen<Emitter>::VisitStringLiteral(const StringLiteral *E) {
       llvm_unreachable("unsupported character width");
     }
   }
+
+  // Fill up the rest of the char array with NUL bytes.
+  for (unsigned I = N; I != ArraySize; ++I) {
+    if (CharWidth == 1) {
+      this->emitConstSint8(0, E);
+      this->emitInitElemSint8(I, E);
+    } else if (CharWidth == 2) {
+      this->emitConstUint16(0, E);
+      this->emitInitElemUint16(I, E);
+    } else if (CharWidth == 4) {
+      this->emitConstUint32(0, E);
+      this->emitInitElemUint32(I, E);
+    } else {
+      llvm_unreachable("unsupported character width");
+    }
+  }
+
   return true;
 }
 
diff --git a/clang/test/AST/Interp/arrays.cpp b/clang/test/AST/Interp/arrays.cpp
index 5640f57f6aeb826..73d918e75db0fb5 100644
--- a/clang/test/AST/Interp/arrays.cpp
+++ b/clang/test/AST/Interp/arrays.cpp
@@ -371,3 +371,24 @@ namespace ArrayInitLoop {
                            // expected-note {{15 == 6}}
 #endif
 }
+
+namespace StringZeroFill {
+  struct A {
+    char c[6];
+  };
+  constexpr A a = { "abc" };
+  static_assert(a.c[0] == 'a', "");
+  static_assert(a.c[1] == 'b', "");
+  static_assert(a.c[2] == 'c', "");
+  static_assert(a.c[3] == '\0', "");
+  static_assert(a.c[4] == '\0', "");
+  static_assert(a.c[5] == '\0', "");
+
+  constexpr char b[6] = "foo";
+  static_assert(b[0] == 'f', "");
+  static_assert(b[1] == 'o', "");
+  static_assert(b[2] == 'o', "");
+  static_assert(b[3] == '\0', "");
+  static_assert(b[4] == '\0', "");
+  static_assert(b[5] == '\0', "");
+}