[PATCH] Fix PR optimize/3508

[Roger Sayle <roger at eyesopen dot com>]

Ok, "Fix" may be a bit strong, and the word "Implement" may be much
more appropriate.  Whilst checking GNATS for PRs I could fix, I came
across this one, which is more of a request than a problem report.

This optimizes builtin memcmp for the case where all the arguments
are known constants at compile time.  The patch has been tested by
"make bootstrap" and "make -k check" on i686-pc-linux-gnu with no
new regressions.  The patch also contains a new test case.


Ok for the mainline to close PR 3508?



2001-12-27  Roger Sayle <roger@eyesopen.com>
	* builtins.c (expand_builtin_memcmp): Optimize memcmp built-in
	when all arguments are known constant.  Fixes PR opt/3508.
	* gcc.c-torture/execute/string-opt-16.c: New testcase.


*** gcc/gcc/builtins.c	Sun Dec 16 23:12:34 2001
--- patch6/gcc/builtins.c	Thu Dec 27 11:14:18 2001
*************** expand_builtin_memcmp (exp, arglist, tar
*** 2252,2257 ****
--- 2252,2258 ----
       enum machine_mode mode;
  {
    tree arg1, arg2, len;
+   const char *p1, *p2;

    if (!validate_arglist (arglist,
  		      POINTER_TYPE, POINTER_TYPE, INTEGER_TYPE, VOID_TYPE))
*************** expand_builtin_memcmp (exp, arglist, tar
*** 2269,2274 ****
--- 2270,2288 ----
        expand_expr (arg1, const0_rtx, VOIDmode, EXPAND_NORMAL);
        expand_expr (arg2, const0_rtx, VOIDmode, EXPAND_NORMAL);
        return const0_rtx;
+     }
+
+   p1 = c_getstr (arg1);
+   p2 = c_getstr (arg2);
+
+   /* If all arguments are constant, and the value of len is not greater
+      than the lengths of arg1 and arg2, evaluate at compile-time.  */
+   if (host_integerp (len, 1) && p1 && p2
+       && compare_tree_int (len, strlen (p1)+1) <= 0
+       && compare_tree_int (len, strlen (p2)+1) <= 0)
+     {
+       const int r = memcmp (p1, p2, tree_low_cst (len, 1));
+       return (r < 0 ? constm1_rtx : (r > 0 ? const1_rtx : const0_rtx));
      }

    /* If len parameter is one, return an expression corresponding to
*** /dev/null	Tue May  5 14:32:27 1998
--- string-opt-16.c	Thu Dec 27 11:07:26 2001
***************
*** 0 ****
--- 1,45 ----
+ /* Copyright (C) 2001  Free Software Foundation.
+
+    Ensure that builtin memcmp operations when all three arguments
+    are constant is optimized and performs correctly.  Taken from
+    PR optimize/3508.
+
+    Written by Roger Sayle, 12/26/2001.  */
+
+ extern void abort (void);
+ extern void link_error (void);
+
+ typedef __SIZE_TYPE__ size_t;
+ extern int memcmp (const void *, const void *, size_t);
+
+ int
+ main (int argc)
+ {
+   if (memcmp ("abcd", "efgh", 4) >= 0)
+      link_error ();
+   if (memcmp ("abcd", "abcd", 4) != 0)
+      link_error ();
+   if (memcmp ("efgh", "abcd", 4) <= 0)
+      link_error ();
+   return 0;
+ }
+
+ #ifdef __OPTIMIZE__
+ /* When optimizing, all the above cases should be transformed into
+    something else.  So any remaining calls to the original function
+    should abort.  */
+ static int
+ memcmp (const void *s1, const void *s2, size_t len)
+ {
+   abort ();
+ }
+ #else
+ /* When not optimizing, the above tests may generate references to
+    the function link_error, but should never actually call it.  */
+ static void
+ link_error ()
+ {
+   abort ();
+ }
+ #endif
+


Roger
--
Roger Sayle,                         E-mail: roger@eyesopen.com
OpenEye Scientific Software,         WWW: http://www.eyesopen.com/
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