[clang][bytecode] Fix builtin_memchr with non-0 start index (#131633)

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -2045,8 +2045,10 @@ static bool interp__builtin_memchr(InterpState &S, CodePtr OpPC,
       (ID == Builtin::BIstrchr || ID == Builtin::BI__builtin_strchr);
 
   size_t Index = Ptr.getIndex();
+  size_t Step = 0;
   for (;;) {
-    const Pointer &ElemPtr = Index > 0 ? Ptr.atIndex(Index) : Ptr;
+    const Pointer &ElemPtr =
+        (Index + Step) > 0 ? Ptr.atIndex(Index + Step) : Ptr;
 
     if (!CheckLoad(S, OpPC, ElemPtr))
       return false;
@@ -2060,8 +2062,8 @@ static bool interp__builtin_memchr(InterpState &S, CodePtr OpPC,
     if (StopAtZero && V == 0)
       break;
 
-    ++Index;
-    if (MaxLength && Index == MaxLength->getZExtValue())
+    ++Step;
+    if (MaxLength && Step == MaxLength->getZExtValue())
       break;
   }
 

clang/test/AST/ByteCode/builtin-functions.cpp

@@ -1459,6 +1459,11 @@ namespace Memchr {
   constexpr bool b = !memchr("hello", 'h', 3); // both-error {{constant expression}} \
                                                // both-note {{non-constexpr function 'memchr' cannot be used in a constant expression}}
 
+  constexpr bool f() {
+    const char *c = "abcdef";
+    return __builtin_char_memchr(c + 1, 'f', 1) == nullptr;
+  }
+  static_assert(f());
 }
 
 namespace Strchr {