[PATCH] handle non-constant character assignments in strlen (PR 86083)

[Martin Sebor <msebor at gmail dot com>]

In the long resolved pr57230 I came across a discussion of
an enhancement to the strlen pass to also handle non-const char
assignments into the middle of known strings, in addition to
assignments where the stored value is known.  Storing non-nul
into the middle of a string of known length means that its
length can be assumed to stay the same, exposing more strlen
optimization opportunities.

The attached patch implements this idea.  (I looked for a simple
function that returns true/false based on whether a SSA_NAME is
or isn't definitely non-zero but couldn't find one so I created
one though it seems that extending one of the existing functions
might be a better approach?)

In a GCC build the patch discovers 40 non-nul stores out of 70
instances where range info is available (all in libstdc++), so
it gives some, albeit very small, improvement.

Martin

PR tree-optimization/86083 - handle non-constant assignments in strlen

gcc/ChangeLog:

	PR tree-optimization/86083
	* tree-ssa-strlen.c (value_is_nonzero): New static function.
	(handle_char_store): Call it.

gcc/testsuite/ChangeLog:

	PR tree-optimization/86083
	* gcc.dg/strlenopt-44.c: New test.


Index: gcc/tree-ssa-strlen.c
===================================================================
--- gcc/tree-ssa-strlen.c	(revision 261284)
+++ gcc/tree-ssa-strlen.c	(working copy)
@@ -3126,6 +3126,27 @@ get_string_cst_length (tree rhs)
   return -1;
 }
 
+/* Return true iff EXP has a non-zero value, false otherwise.  */
+
+static bool
+value_is_nonzero (tree exp)
+{
+  if (TREE_CODE (exp) != SSA_NAME)
+    return integer_nonzerop (exp);
+
+  wide_int min, max;
+  value_range_type rng = get_range_info (exp, &min, &max);
+
+  if (rng == VR_RANGE)
+    return wi::gts_p (min, wi::zero (min.get_precision ()));
+
+  if (rng == VR_ANTI_RANGE)
+    return (wi::les_p (min, wi::zero (min.get_precision ()))
+	    && wi::les_p (wi::zero (min.get_precision ()), max));
+
+  return false;
+}
+
 /* Handle a single character store.  */
 
 static bool
@@ -3165,9 +3186,7 @@ handle_char_store (gimple_stmt_iterator *gsi)
     }
 
   bool storing_zero_p = initializer_zerop (rhs);
-  bool storing_nonzero_p = (!storing_zero_p
-			    && TREE_CODE (rhs) == INTEGER_CST
-			    && integer_nonzerop (rhs));
+  bool storing_nonzero_p = !storing_zero_p && value_is_nonzero (rhs);
   /* Set to the length of the string being assigned if known.  */
   HOST_WIDE_INT rhslen;
 
Index: gcc/testsuite/gcc.dg/strlenopt-44.c
===================================================================
--- gcc/testsuite/gcc.dg/strlenopt-44.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/strlenopt-44.c	(working copy)
@@ -0,0 +1,79 @@
+/* PR tree-optimization/86083 - handle non-constant assignments in strlen
+   { dg-do compile }
+   { dg-options "-O2 -Wall -fdump-tree-optimized" } */
+
+#include "range.h"
+#include "strlenopt.h"
+
+#define CAT(x, y) x ## y
+#define CONCAT(x, y) CAT (x, y)
+#define FAILNAME(name) CONCAT (call_ ## name ##_on_line_, __LINE__)
+
+#define FAIL(name) do {				\
+    extern void FAILNAME (name) (void);		\
+    FAILNAME (name)();				\
+  } while (0)
+
+/* Macro to emit a call to funcation named
+     call_in_true_branch_not_eliminated_on_line_NNN()
+   for each call that's expected to be eliminated.  The dg-final
+   scan-tree-dump-time directive at the bottom of the test verifies
+   that no such call appears in output.  */
+#define ASSERT_ELIM(expr)						\
+  if (!(expr)) FAIL (in_true_branch_not_eliminated); else (void)0
+
+/* Macro to emit a call to a function named
+     call_made_in_{true,false}_branch_on_line_NNN()
+   for each call that's expected to be retained.  The dg-final
+   scan-tree-dump-time directive at the bottom of the test verifies
+   that the expected number of both kinds of calls appears in output
+   (a pair for each line with the invocation of the KEEP() macro.  */
+#define ASSERT_KEEP(expr)			\
+  if (expr)					\
+    FAIL (made_in_true_branch);			\
+  else						\
+    FAIL (made_in_false_branch)
+
+
+#define ELIM(init, i, c, res)			\
+  do {						\
+    char a[] = init;				\
+    a[i] = c;					\
+    ASSERT_ELIM (strlen (a) == res);		\
+  } while (0)
+
+#define KEEP(init, i, c, res)			\
+  do {						\
+    char a[] = init;				\
+    a[i] = c;					\
+    ASSERT_KEEP (strlen (a) == res);		\
+  } while (0)
+
+void test_elim (void)
+{
+  ELIM ("1", 0, UR (1, 2), 1);
+  ELIM ("1", 0, UR (1, 127), 1);
+  ELIM ("1", 0, UR ('0', '9'), 1);
+
+  ELIM ("12", 0, UR (1, 127), 2);
+  ELIM ("12", 1, UR (1, 127), 2);
+
+  ELIM ("123", 0, UR (1, 9), 3);
+  ELIM ("123", 1, UR (10, 99), 3);
+  ELIM ("123", 2, UR (100, 127), 3);
+}
+
+#line 1000
+
+void test_keep (void)
+{
+  size_t uchar_max = (unsigned char)-1;
+
+  KEEP ("1",     0, UR (1, uchar_max + 1), 1);
+  KEEP ("1\0\3", 1, UR (1, 2), 1);
+}
+
+/* { dg-final { scan-tree-dump-times "call_in_true_branch_not_eliminated_" 0 "optimized" } }
+
+   { dg-final { scan-tree-dump-times "call_made_in_true_branch_on_line_1\[0-9\]\[0-9\]\[0-9\]" 2 "optimized" } }
+   { dg-final { scan-tree-dump-times "call_made_in_false_branch_on_line_1\[0-9\]\[0-9\]\[0-9\]" 2 "optimized" } } */