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[PATCH] Fix unsafe function attributes for special functions (PR 71876)


Hi!

As discussed at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71876,
we have a _very_ old hack in gcc, that recognizes certain functions by
name, and inserts in some cases unsafe attributes, that don't work for
a freestanding environment.

It is unsafe to return ECF_MAY_BE_ALLOCA, ECF_LEAF and ECF_NORETURN
from special_function_p, just by the name of the function, especially
for less well known functions, like "getcontext" or "savectx", which
could easily used for something completely different.

Moreover, from the backend library we cannot check flag_hosted, or if
the function has "C" or "C++" binding.

So all these functions would get the leaf attribute, which makes it very
easy to construct some kind of wrong code examples.

This patch removes the unsafe flags, and adds them to built-in
functions instead.

The patch removes also support for some completely unknown function
names from the middle-end, because these functions were apparently in
use at some point in time, but are certainly dead by now.

It is however not possible to remove the special handling by name
altogether, because the glibc does not add the return_twice function
attribute on _setjmp, __sigsetjmp and getcontext until today; a glibc
BZ is filed at: https://sourceware.org/bugzilla/show_bug.cgi?id=20382

Without the return_twice attribute we would loose the -Wclobbered
warning, and some targets (spark, ia64, maybe others too) would even
generate wrong code.


Boot-strapped and reg-tested on x86_64-pc-linux-gnu.
OK for trunk?


Thanks
Bernd.

Attachment: changelog-pr71876.txt
Description: changelog-pr71876.txt

Index: gcc/builtin-attrs.def
===================================================================
--- gcc/builtin-attrs.def	(revision 238382)
+++ gcc/builtin-attrs.def	(working copy)
@@ -131,6 +131,8 @@ DEF_ATTR_TREE_LIST (ATTR_NORETURN_NOTHROW_LIST, AT
 			ATTR_NULL, ATTR_NOTHROW_LIST)
 DEF_ATTR_TREE_LIST (ATTR_NORETURN_NOTHROW_LEAF_LIST, ATTR_NORETURN,\
 			ATTR_NULL, ATTR_NOTHROW_LEAF_LIST)
+DEF_ATTR_TREE_LIST (ATTR_RT_NOTHROW_LEAF_LIST, ATTR_RETURNS_TWICE,\
+			ATTR_NULL, ATTR_NOTHROW_LEAF_LIST)
 DEF_ATTR_TREE_LIST (ATTR_COLD_NOTHROW_LEAF_LIST, ATTR_COLD,\
 			ATTR_NULL, ATTR_NOTHROW_LEAF_LIST)
 DEF_ATTR_TREE_LIST (ATTR_COLD_NORETURN_NOTHROW_LEAF_LIST, ATTR_COLD,\
Index: gcc/builtins.def
===================================================================
--- gcc/builtins.def	(revision 238382)
+++ gcc/builtins.def	(working copy)
@@ -796,6 +796,7 @@ DEF_EXT_LIB_BUILTIN    (BUILT_IN_FINITED32, "finit
 DEF_EXT_LIB_BUILTIN    (BUILT_IN_FINITED64, "finited64", BT_FN_INT_DFLOAT64, ATTR_CONST_NOTHROW_LEAF_LIST)
 DEF_EXT_LIB_BUILTIN    (BUILT_IN_FINITED128, "finited128", BT_FN_INT_DFLOAT128, ATTR_CONST_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_FPCLASSIFY, "fpclassify", BT_FN_INT_INT_INT_INT_INT_INT_VAR, ATTR_CONST_NOTHROW_TYPEGENERIC_LEAF)
+DEF_EXT_LIB_BUILTIN    (BUILT_IN_GETCONTEXT, "getcontext", BT_FN_INT_PTR, ATTR_RT_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_ISFINITE, "isfinite", BT_FN_INT_VAR, ATTR_CONST_NOTHROW_TYPEGENERIC_LEAF)
 DEF_GCC_BUILTIN        (BUILT_IN_ISINF_SIGN, "isinf_sign", BT_FN_INT_VAR, ATTR_CONST_NOTHROW_TYPEGENERIC_LEAF)
 DEF_C99_C90RES_BUILTIN (BUILT_IN_ISINF, "isinf", BT_FN_INT_VAR, ATTR_CONST_NOTHROW_TYPEGENERIC)
@@ -837,7 +838,7 @@ DEF_LIB_BUILTIN        (BUILT_IN_REALLOC, "realloc
 DEF_GCC_BUILTIN        (BUILT_IN_RETURN, "return", BT_FN_VOID_PTR, ATTR_NORETURN_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_RETURN_ADDRESS, "return_address", BT_FN_PTR_UINT, ATTR_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_SAVEREGS, "saveregs", BT_FN_PTR_VAR, ATTR_NULL)
-DEF_GCC_BUILTIN        (BUILT_IN_SETJMP, "setjmp", BT_FN_INT_PTR, ATTR_NOTHROW_LEAF_LIST)
+DEF_GCC_BUILTIN        (BUILT_IN_SETJMP, "setjmp", BT_FN_INT_PTR, ATTR_RT_NOTHROW_LEAF_LIST)
 DEF_EXT_LIB_BUILTIN    (BUILT_IN_STRFMON, "strfmon", BT_FN_SSIZE_STRING_SIZE_CONST_STRING_VAR, ATTR_FORMAT_STRFMON_NOTHROW_3_4)
 DEF_LIB_BUILTIN        (BUILT_IN_STRFTIME, "strftime", BT_FN_SIZE_STRING_SIZE_CONST_STRING_CONST_PTR, ATTR_FORMAT_STRFTIME_NOTHROW_3_0)
 DEF_GCC_BUILTIN        (BUILT_IN_TRAP, "trap", BT_FN_VOID, ATTR_NORETURN_NOTHROW_LEAF_LIST)
@@ -849,6 +850,7 @@ DEF_GCC_BUILTIN        (BUILT_IN_VA_END, "va_end",
 DEF_GCC_BUILTIN        (BUILT_IN_VA_START, "va_start", BT_FN_VOID_VALIST_REF_VAR, ATTR_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_VA_ARG_PACK, "va_arg_pack", BT_FN_INT, ATTR_PURE_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_VA_ARG_PACK_LEN, "va_arg_pack_len", BT_FN_INT, ATTR_PURE_NOTHROW_LEAF_LIST)
+DEF_EXT_LIB_BUILTIN    (BUILT_IN_VFORK, "vfork", BT_FN_PID, ATTR_RT_NOTHROW_LEAF_LIST)
 DEF_EXT_LIB_BUILTIN    (BUILT_IN__EXIT, "_exit", BT_FN_VOID_INT, ATTR_NORETURN_NOTHROW_LEAF_LIST)
 DEF_C99_BUILTIN        (BUILT_IN__EXIT2, "_Exit", BT_FN_VOID_INT, ATTR_NORETURN_NOTHROW_LEAF_LIST)
 
Index: gcc/calls.c
===================================================================
--- gcc/calls.c	(revision 238382)
+++ gcc/calls.c	(working copy)
@@ -474,8 +474,6 @@ emit_call_1 (rtx funexp, tree fntree ATTRIBUTE_UNU
    For example, if the function might return more than one time (setjmp), then
    set RETURNS_TWICE to a nonzero value.
 
-   Similarly set NORETURN if the function is in the longjmp family.
-
    Set MAY_BE_ALLOCA for any memory allocation function that might allocate
    space from the stack such as alloca.  */
 
@@ -491,7 +489,7 @@ special_function_p (const_tree fndecl, int flags)
     name_decl = DECL_NAME (cgraph_node::get (fndecl)->orig_decl);
 
   if (fndecl && name_decl
-      && IDENTIFIER_LENGTH (name_decl) <= 17
+      && IDENTIFIER_LENGTH (name_decl) <= 12
       /* Exclude functions not at the file scope, or not `extern',
 	 since they are not the magic functions we would otherwise
 	 think they are.
@@ -506,55 +504,21 @@ special_function_p (const_tree fndecl, int flags)
       const char *name = IDENTIFIER_POINTER (name_decl);
       const char *tname = name;
 
-      /* We assume that alloca will always be called by name.  It
-	 makes no sense to pass it as a pointer-to-function to
-	 anything that does not understand its behavior.  */
-      if (IDENTIFIER_LENGTH (name_decl) == 6
-	  && name[0] == 'a'
-	  && ! strcmp (name, "alloca"))
-	flags |= ECF_MAY_BE_ALLOCA;
-
-      /* Disregard prefix _, __, __x or __builtin_.  */
+      /* Disregard prefix _ or __.  */
       if (name[0] == '_')
 	{
-	  if (name[1] == '_'
-	      && name[2] == 'b'
-	      && !strncmp (name + 3, "uiltin_", 7))
-	    tname += 10;
-	  else if (name[1] == '_' && name[2] == 'x')
-	    tname += 3;
-	  else if (name[1] == '_')
+	  if (name[1] == '_')
 	    tname += 2;
 	  else
 	    tname += 1;
 	}
 
-      if (tname[0] == 's')
-	{
-	  if ((tname[1] == 'e'
-	       && (! strcmp (tname, "setjmp")
-		   || ! strcmp (tname, "setjmp_syscall")))
-	      || (tname[1] == 'i'
-		  && ! strcmp (tname, "sigsetjmp"))
-	      || (tname[1] == 'a'
-		  && ! strcmp (tname, "savectx")))
-	    flags |= ECF_RETURNS_TWICE | ECF_LEAF;
-
-	  if (tname[1] == 'i'
-	      && ! strcmp (tname, "siglongjmp"))
-	    flags |= ECF_NORETURN;
-	}
-      else if ((tname[0] == 'q' && tname[1] == 's'
-		&& ! strcmp (tname, "qsetjmp"))
-	       || (tname[0] == 'v' && tname[1] == 'f'
-		   && ! strcmp (tname, "vfork"))
-	       || (tname[0] == 'g' && tname[1] == 'e'
-		   && !strcmp (tname, "getcontext")))
-	flags |= ECF_RETURNS_TWICE | ECF_LEAF;
-
-      else if (tname[0] == 'l' && tname[1] == 'o'
-	       && ! strcmp (tname, "longjmp"))
-	flags |= ECF_NORETURN;
+      /* ECF_RETURNS_TWICE is safe even for -ffrestanding.  */
+      if (! strcmp (tname, "setjmp")
+	  || ! strcmp (tname, "sigsetjmp")
+	  || ! strcmp (tname, "vfork")
+	  || ! strcmp (tname, "getcontext"))
+	flags |= ECF_RETURNS_TWICE;
     }
 
   if (DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL)

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