[libc] Independent strcat/strncat/stpcpy (#142643)

The previous implementations called other entrypoints. This patch fixes
strcat, strncat, and stpcpy to be properly independent.

Author: michaelrj-google

libc/src/string/CMakeLists.txt

@@ -87,7 +87,6 @@ add_entrypoint_object(
   HDRS
     stpcpy.h
   DEPENDS
-    .mempcpy
     .string_utils
 )
 
@@ -108,7 +107,6 @@ add_entrypoint_object(
   HDRS
     strcat.h
   DEPENDS
-    .strcpy
     .string_utils
     libc.include.llvm-libc-types.size_t
 )
@@ -265,7 +263,6 @@ add_entrypoint_object(
   HDRS
     strncat.h
   DEPENDS
-    .strncpy
     .string_utils
     libc.include.llvm-libc-types.size_t
 )

libc/src/string/stpcpy.cpp

@@ -8,7 +8,6 @@
 
 #include "src/string/stpcpy.h"
 #include "src/__support/macros/config.h"
-#include "src/string/mempcpy.h"
 #include "src/string/string_utils.h"
 
 #include "src/__support/common.h"
@@ -18,8 +17,8 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(char *, stpcpy,
                    (char *__restrict dest, const char *__restrict src)) {
   size_t size = internal::string_length(src) + 1;
-  char *result =
-      reinterpret_cast<char *>(LIBC_NAMESPACE::mempcpy(dest, src, size));
+  __builtin_memcpy(dest, src, size);
+  char *result = dest + size;
 
   if (result != nullptr)
     return result - 1;

libc/src/string/strcat.cpp

@@ -9,7 +9,6 @@
 #include "src/string/strcat.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/null_check.h"
-#include "src/string/strcpy.h"
 #include "src/string/string_utils.h"
 
 #include "src/__support/common.h"
@@ -21,9 +20,11 @@ LLVM_LIBC_FUNCTION(char *, strcat,
   LIBC_CRASH_ON_NULLPTR(dest);
   LIBC_CRASH_ON_NULLPTR(src);
   size_t dest_length = internal::string_length(dest);
-  size_t src_length = internal::string_length(src);
-  LIBC_NAMESPACE::strcpy(dest + dest_length, src);
-  dest[dest_length + src_length] = '\0';
+  size_t i;
+  for (i = 0; src[i] != '\0'; ++i)
+    dest[dest_length + i] = src[i];
+
+  dest[dest_length + i] = '\0';
   return dest;
 }
 

libc/src/string/strncat.cpp

@@ -10,7 +10,6 @@
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/null_check.h"
 #include "src/string/string_utils.h"
-#include "src/string/strncpy.h"
 
 #include "src/__support/common.h"
 
@@ -23,11 +22,12 @@ LLVM_LIBC_FUNCTION(char *, strncat,
     LIBC_CRASH_ON_NULLPTR(dest);
     LIBC_CRASH_ON_NULLPTR(src);
   }
-  size_t src_length = internal::string_length(src);
-  size_t copy_amount = src_length > count ? count : src_length;
   size_t dest_length = internal::string_length(dest);
-  LIBC_NAMESPACE::strncpy(dest + dest_length, src, copy_amount);
-  dest[dest_length + copy_amount] = '\0';
+  size_t i;
+  for (i = 0; i < count && src[i] != '\0'; ++i)
+    dest[dest_length + i] = src[i];
+
+  dest[dest_length + i] = '\0';
   return dest;
 }