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" } } */
