[clang] [clang][bytecode] Check builtin_memcpy() for non-block pointers (PR #174184)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/174184

This pretty hard to produce in C++ but easy in C.

Fixes #171609

>From cecb72441ee0e2886deb8f058d7a59f770430fb1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Fri, 2 Jan 2026 09:10:48 +0100
Subject: [PATCH] [clang][bytecode] Check builtin_memcpy() for non-block
 pointers

This pretty hard to produce in C++ but easy in C.

Fixes #171609
---
 clang/lib/AST/ByteCode/InterpBuiltin.cpp | 15 +++++++++++++--
 clang/test/AST/ByteCode/builtins.c       |  1 +
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index 57d5f0ae6eed3..65101174247d1 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -64,6 +64,18 @@ static APSInt popToAPSInt(InterpState &S, QualType T) {
   return popToAPSInt(S.Stk, *S.getContext().classify(T));
 }
 
+/// Check for common reasons a pointer can't be read from, which
+/// are usually not diagnosed in a builtin function.
+static bool isReadable(const Pointer &P) {
+  if (P.isDummy())
+    return false;
+  if (!P.isBlockPointer())
+    return false;
+  if (!P.isLive())
+    return false;
+  return true;
+}
+
 /// Pushes \p Val on the stack as the type given by \p QT.
 static void pushInteger(InterpState &S, const APSInt &Val, QualType QT) {
   assert(QT->isSignedIntegerOrEnumerationType() ||
@@ -1794,8 +1806,7 @@ static bool interp__builtin_memcpy(InterpState &S, CodePtr OpPC,
     return false;
   }
 
-  // Can't read from dummy pointers.
-  if (DestPtr.isDummy() || SrcPtr.isDummy())
+  if (!isReadable(DestPtr) || !isReadable(SrcPtr))
     return false;
 
   if (DestPtr.getType()->isIncompleteType()) {
diff --git a/clang/test/AST/ByteCode/builtins.c b/clang/test/AST/ByteCode/builtins.c
index a51260cd3431f..5be5455ab8813 100644
--- a/clang/test/AST/ByteCode/builtins.c
+++ b/clang/test/AST/ByteCode/builtins.c
@@ -17,3 +17,4 @@ int structStrlen(void) {
   return 1;
 }
 
+void f() { __builtin_memcpy(f, f, 1); }