Simplify X / X, 0 / X and X % X

2016-11-20  Marc Glisse  <marc.glisse@inria.fr>

gcc/
	* match.pd (0 / X, X / X, X % X): New simplifications.

gcc/testsuite/
	* gcc.dg/tree-ssa/divide-5.c: New file.

From-SVN: r242636

4 files changed, 40 insertions, 4 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 23de483c..c64b349 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2016-11-20  Marc Glisse  <marc.glisse@inria.fr>
+
+	* match.pd (0 / X, X / X, X % X): New simplifications.
+
 2016-11-19  Jakub Jelinek  <jakub@redhat.com>
 
 	* config/i386/i386.c (ix86_can_inline_p): Use || instead of &
diff --git a/gcc/match.pd b/gcc/match.pd
index 79a418f..f97a998 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -140,19 +140,33 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
            || !COMPLEX_FLOAT_TYPE_P (type)))
    (negate @0)))
 
-/* Make sure to preserve divisions by zero.  This is the reason why
-   we don't simplify x / x to 1 or 0 / x to 0.  */
+/* X * 1, X / 1 -> X.  */
 (for op (mult trunc_div ceil_div floor_div round_div exact_div)
   (simplify
     (op @0 integer_onep)
     (non_lvalue @0)))
 
+/* Preserve explicit divisions by 0: the C++ front-end wants to detect
+   undefined behavior in constexpr evaluation, and assuming that the division
+   traps enables better optimizations than these anyway.  */
 (for div (trunc_div ceil_div floor_div round_div exact_div)
+ /* 0 / X is always zero.  */
+ (simplify
+  (div integer_zerop@0 @1)
+  /* But not for 0 / 0 so that we can get the proper warnings and errors.  */
+  (if (!integer_zerop (@1))
+   @0))
   /* X / -1 is -X.  */
  (simplify
    (div @0 integer_minus_onep@1)
    (if (!TYPE_UNSIGNED (type))
     (negate @0)))
+ /* X / X is one.  */
+ (simplify
+  (div @0 @0)
+  /* But not for 0 / 0 so that we can get the proper warnings and errors.  */
+  (if (!integer_zerop (@0))
+   { build_one_cst (type); }))
  /* X / abs (X) is X < 0 ? -1 : 1.  */ 
  (simplify
    (div:C @0 (abs @0))
@@ -276,8 +290,6 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
       (if (inverse)
        (mult @0 { inverse; } ))))))))
 
-/* Same applies to modulo operations, but fold is inconsistent here
-   and simplifies 0 % x to 0, only preserving literal 0 % 0.  */
 (for mod (ceil_mod floor_mod round_mod trunc_mod)
  /* 0 % X is always zero.  */
  (simplify
@@ -294,6 +306,12 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (mod @0 integer_minus_onep@1)
   (if (!TYPE_UNSIGNED (type))
    { build_zero_cst (type); }))
+ /* X % X is zero.  */
+ (simplify
+  (mod @0 @0)
+  /* But not for 0 % 0 so that we can get the proper warnings and errors.  */
+  (if (!integer_zerop (@0))
+   { build_zero_cst (type); }))
  /* (X % Y) % Y is just X % Y.  */
  (simplify
   (mod (mod@2 @0 @1) @1)
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index e83e9ea..e645366 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2016-11-20  Marc Glisse  <marc.glisse@inria.fr>
+
+	* gcc.dg/tree-ssa/divide-5.c: New file.
+
 2016-11-19  Andreas Schwab  <schwab@linux-m68k.org>
 
 	* gcc.c-torture/execute/comp-goto-1.c (insn_t): Change offset to
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/divide-5.c b/gcc/testsuite/gcc.dg/tree-ssa/divide-5.c
new file mode 100644
index 0000000..48cd638
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/divide-5.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+int f(int x){
+  int y = x;
+  int z = 0;
+  return x / y - x % y + z / y;
+}
+
+/* { dg-final { scan-tree-dump "return 1;" "optimized"} } */