tree-optimization/92539 - missed optimization leads to bogus -Warray-bounds

The following makes niter analysis recognize a loop with an exit
condition scanning over a STRING_CST.  This is done via enhancing
the force evaluation code rather than recognizing for example
strlen (s) as number of iterations because it allows to handle
some more cases.

STRING_CSTs are easy to handle since nothing can write to them, also
processing those should be cheap.  I've refrained from handling
anything besides char8_t.

Note to avoid the -Warray-bound dianostic we have to either early unroll
the loop (there's no final value replacement done, there's a PR
for doing this as part of CD-DCE when possibly eliding a loop),
or create a canonical IV so we can DCE the loads.  The latter is what
the patch does, also avoiding to repeatedly force-evaluate niters.
This also makes final value replacement work again since now ivcanon
is after it.

There are some testsuite adjustments needed, in particular we now
unroll some loops early, causing messages to appear in different
passes but also vectorization to now no longer happening on
outer loops.  The changes mitigate that.

	PR tree-optimization/92539
	* tree-ssa-loop-ivcanon.cc (tree_unroll_loops_completely_1):
	Also try force-evaluation if ivcanon did not yet run.
	(canonicalize_loop_induction_variables):
	When niter was computed constant by force evaluation add a
	canonical IV if we didn't unroll.
	* tree-ssa-loop-niter.cc (loop_niter_by_eval): When we
	don't find a proper PHI try if the exit condition scans
	over a STRING_CST and simulate that.

	* g++.dg/warn/Warray-bounds-pr92539.C: New testcase.
	* gcc.dg/tree-ssa/sccp-16.c: New testcase.
	* g++.dg/vect/pr87621.cc: Use larger power to avoid
	inner loop unrolling.
	* gcc.dg/vect/pr89440.c: Use larger loop bound to avoid
	inner loop unrolling.
	* gcc.dg/pr77975.c: Scan cunrolli dump and adjust.

7 files changed, 127 insertions, 10 deletions

diff --git a/gcc/testsuite/g++.dg/vect/pr87621.cc b/gcc/testsuite/g++.dg/vect/pr87621.cc
index cfc53be..bc55116 100644
--- a/gcc/testsuite/g++.dg/vect/pr87621.cc
+++ b/gcc/testsuite/g++.dg/vect/pr87621.cc
@@ -21,7 +21,7 @@ T pow(T x, unsigned int n)
 void testVec(int* x)
 {
   for (int i = 0; i < 8; ++i)
-    x[i] = pow(x[i], 10);
+    x[i] = pow(x[i], 100);
 }
 
 /* { dg-final { scan-tree-dump "OUTER LOOP VECTORIZED" "vect" { target { vect_double && vect_hw_misalign } } } } */
diff --git a/gcc/testsuite/g++.dg/warn/Warray-bounds-pr92539.C b/gcc/testsuite/g++.dg/warn/Warray-bounds-pr92539.C
new file mode 100644
index 0000000..ea506ed
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Warray-bounds-pr92539.C
@@ -0,0 +1,51 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-O2 -Warray-bounds" }
+
+static bool
+ischar(int ch)
+{
+    return (0 == (ch & ~0xff) || ~0 == (ch | 0xff)) != 0;
+}
+
+static bool eat(char const*& first, char const* last)
+{
+    if (first != last && ischar(*first)) { // { dg-bogus "bounds" }
+        ++first;
+        return true;
+    }
+    return false;
+}
+
+static bool eat_two(char const*& first, char const* last)
+{
+    auto save = first;
+    if (eat(first, last) && eat(first, last))
+        return true;
+    first = save;
+    return false;
+}
+
+static bool foo(char const*& first, char const* last)
+{
+    auto local_iterator = first;
+    int i = 0;
+    for (; i < 3; ++i)
+        if (!eat_two(local_iterator, last))
+            return false;
+    first = local_iterator;
+    return true;
+}
+
+static bool test(char const* in, bool full_match = true)
+{
+    auto last = in;
+    while (*last)
+        ++last;
+    return foo(in, last) && (!full_match || (in == last)); // { dg-bogus "bounds" }
+}
+
+int main()
+{
+    return test("aa");
+}
+
diff --git a/gcc/testsuite/gcc.dg/pr77975.c b/gcc/testsuite/gcc.dg/pr77975.c
index a187ce2..9d7aad4 100644
--- a/gcc/testsuite/gcc.dg/pr77975.c
+++ b/gcc/testsuite/gcc.dg/pr77975.c
@@ -1,8 +1,8 @@
 /* PR tree-optimization/77975 */
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-ivcanon-details" } */
+/* { dg-options "-O2 -fdump-tree-cunrolli-details" } */
 
-/* { dg-final { scan-tree-dump-times "Proved that loop 1 iterates 1 times using brute force" 1 "ivcanon" } } */
+/* { dg-final { scan-tree-dump-times "Proved that loop 1 iterates 2 times using brute force" 1 "cunrolli" } } */
 
 unsigned int
 foo (unsigned int *b)
@@ -17,7 +17,7 @@ foo (unsigned int *b)
   return a; 
 }
 
-/* { dg-final { scan-tree-dump-times "Proved that loop 1 iterates 2 times using brute force" 1 "ivcanon" } } */
+/* { dg-final { scan-tree-dump-times "Proved that loop 1 iterates 3 times using brute force" 1 "cunrolli" } } */
 
 unsigned int
 bar (unsigned int *b)
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/sccp-16.c b/gcc/testsuite/gcc.dg/tree-ssa/sccp-16.c
new file mode 100644
index 0000000..c35426f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/sccp-16.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cunrolli -fdump-tree-sccp-details" } */
+
+int foo ()
+{
+  const char *s = "Hello World!";
+  int len = 0;
+  while (s[len])
+    ++len;
+  return len;
+}
+
+/* For cunrolli the growth is too large, but it should add a canonical IV
+   and SCCP peform final value replacement.  */
+/* { dg-final { scan-tree-dump "ivtmp\[^\r\n\]*PHI\[^\r\n\]*13" "cunrolli" } } */
+/* { dg-final { scan-tree-dump "with expr: 12" "sccp" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/pr89440.c b/gcc/testsuite/gcc.dg/vect/pr89440.c
index d84527e..0604969 100644
--- a/gcc/testsuite/gcc.dg/vect/pr89440.c
+++ b/gcc/testsuite/gcc.dg/vect/pr89440.c
@@ -10,7 +10,7 @@ f (float x)
   float a = 0;
   for (i = 0; i < 4; ++i)
     {
-      for (j = 0; j < 4; ++j)
+      for (j = 0; j < 32; ++j)
 	{
 	  a += 1;
 	  x += a;
@@ -23,7 +23,7 @@ int
 main()
 {
   check_vect ();
-  if (f (1.0f) != 137.0f)
+  if (f (1.0f) != 8257.0f)
     abort ();
   return 0;
 }
diff --git a/gcc/tree-ssa-loop-ivcanon.cc b/gcc/tree-ssa-loop-ivcanon.cc
index 6e6569e..d07b3d5 100644
--- a/gcc/tree-ssa-loop-ivcanon.cc
+++ b/gcc/tree-ssa-loop-ivcanon.cc
@@ -1257,6 +1257,7 @@ canonicalize_loop_induction_variables (class loop *loop,
   bool modified = false;
   class tree_niter_desc niter_desc;
   bool may_be_zero = false;
+  bool by_eval = false;
 
   /* For unrolling allow conditional constant or zero iterations, thus
      perform loop-header copying on-the-fly.  */
@@ -1291,7 +1292,11 @@ canonicalize_loop_induction_variables (class loop *loop,
       if (try_eval
 	  && (chrec_contains_undetermined (niter)
 	      || TREE_CODE (niter) != INTEGER_CST))
-	niter = find_loop_niter_by_eval (loop, &exit);
+	{
+	  niter = find_loop_niter_by_eval (loop, &exit);
+	  if (TREE_CODE (niter) == INTEGER_CST)
+	    by_eval = true;
+	}
 
       if (TREE_CODE (niter) != INTEGER_CST)
 	exit = NULL;
@@ -1346,7 +1351,7 @@ canonicalize_loop_induction_variables (class loop *loop,
 				  innermost_cunrolli_p))
     return true;
 
-  if (create_iv
+  if ((create_iv || by_eval)
       && niter && !chrec_contains_undetermined (niter)
       && exit && just_once_each_iteration_p (loop, exit->src))
     {
@@ -1492,7 +1497,7 @@ tree_unroll_loops_completely_1 (bool may_increase_size, bool unroll_outer,
     ul = UL_NO_GROWTH;
 
   if (canonicalize_loop_induction_variables
-      (loop, false, ul, !flag_tree_loop_ivcanon, unroll_outer,
+      (loop, false, ul, !flag_tree_loop_ivcanon || cunrolli, unroll_outer,
        innermost, cunrolli))
     {
       /* If we'll continue unrolling, we need to propagate constants
diff --git a/gcc/tree-ssa-loop-niter.cc b/gcc/tree-ssa-loop-niter.cc
index a088296..de8d5ae 100644
--- a/gcc/tree-ssa-loop-niter.cc
+++ b/gcc/tree-ssa-loop-niter.cc
@@ -41,6 +41,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "cfgloop.h"
 #include "tree-chrec.h"
 #include "tree-scalar-evolution.h"
+#include "tree-data-ref.h"
 #include "tree-dfa.h"
 #include "internal-fn.h"
 #include "gimple-range.h"
@@ -3594,7 +3595,51 @@ loop_niter_by_eval (class loop *loop, edge exit)
 	{
 	  phi = get_base_for (loop, op[j]);
 	  if (!phi)
-	    return chrec_dont_know;
+	    {
+	      gassign *def;
+	      if (j == 0
+		  && (cmp == NE_EXPR || cmp == EQ_EXPR)
+		  && TREE_CODE (op[0]) == SSA_NAME
+		  && TREE_CODE (op[1]) == INTEGER_CST
+		  && (def = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (op[0])))
+		  && gimple_assign_rhs_code (def) == MEM_REF)
+		{
+		  tree mem = gimple_assign_rhs1 (def);
+		  affine_iv iv;
+		  if (TYPE_MODE (TREE_TYPE (mem)) == TYPE_MODE (char_type_node)
+		      && simple_iv (loop, loop,
+				    TREE_OPERAND (mem, 0), &iv, false)
+		      && tree_fits_uhwi_p (TREE_OPERAND (mem, 1))
+		      && tree_fits_uhwi_p (iv.step))
+		    {
+		      tree str, off;
+		      /* iv.base can be &"Foo" but also (char *)&"Foo" + 1.  */
+		      split_constant_offset (iv.base, &str, &off);
+		      STRIP_NOPS (str);
+		      if (TREE_CODE (str) == ADDR_EXPR
+			  && TREE_CODE (TREE_OPERAND (str, 0)) == STRING_CST
+			  && tree_fits_uhwi_p (off))
+			{
+			  str = TREE_OPERAND (str, 0);
+			  unsigned i = 0;
+			  for (unsigned HOST_WIDE_INT idx
+			       = (tree_to_uhwi (TREE_OPERAND (mem, 1))
+				  + tree_to_uhwi (off));
+			       idx < (unsigned)TREE_STRING_LENGTH (str)
+			       && i < MAX_ITERATIONS_TO_TRACK;
+			       idx += tree_to_uhwi (iv.step), ++i)
+			    {
+			      int res = compare_tree_int
+				(op[1], TREE_STRING_POINTER (str)[idx]);
+			      if ((cmp == NE_EXPR && res == 0)
+				  || (cmp == EQ_EXPR && res != 0))
+				return build_int_cst (unsigned_type_node, i);
+			    }
+			}
+		    }
+		}
+	      return chrec_dont_know;
+	    }
 	  val[j] = PHI_ARG_DEF_FROM_EDGE (phi, loop_preheader_edge (loop));
 	  next[j] = PHI_ARG_DEF_FROM_EDGE (phi, loop_latch_edge (loop));
 	}