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

[Marc Glisse <marc dot glisse at inria dot fr>]

Hello,

since we were discussing this recently...

The condition is copied from the existing 0 % X case, visible in the 
context of the diff.

As far as I understand, the main case where we do not want to optimize is 
during constexpr evaluation in the C++ front-end (it wants to detect the 
undefined behavior), and with late folding I think this means we only need 
to care about an explicit 0/0, not about X/X where X would become 0 after 
the simplification.

And later, if we do have something like X/0, we could handle it the same 
way as we currently handle *(char*)0, insert a trap after that instruction 
and clear the following code, which likely gives better code than 
replacing 0/0 with 1.

Bootstrap+regtest on powerpc64le-unknown-linux-gnu.

2016-11-07  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.

--
Marc Glisse

Index: gcc/match.pd
===================================================================
--- gcc/match.pd	(revision 241856)
+++ gcc/match.pd	(working copy)
@@ -133,33 +133,47 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (non_lvalue @0)))
 
 /* Transform x * -1.0 into -x.  */
 (simplify
  (mult @0 real_minus_onep)
   (if (!HONOR_SNANS (type)
        && (!HONOR_SIGNED_ZEROS (type)
            || !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))
    (if (INTEGRAL_TYPE_P (type)
 	&& TYPE_OVERFLOW_UNDEFINED (type))
     (cond (lt @0 { build_zero_cst (type); })
           { build_minus_one_cst (type); } { build_one_cst (type); })))
  /* X / -X is -1.  */
  (simplify
    (div:C @0 (negate @0))
@@ -269,38 +283,42 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 	&& !real_zerop (@1))
     (with
      { tree tem = const_binop (RDIV_EXPR, type, build_one_cst (type), @1); }
      (if (tem)
       (mult @0 { tem; } )))
     (if (cst != COMPLEX_CST)
      (with { tree inverse = exact_inverse (type, @1); }
       (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
   (mod 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 always zero.  */
  (simplify
   (mod @0 integer_onep)
   { build_zero_cst (type); })
  /* X % -1 is zero.  */
  (simplify
   (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)
   @2)
  /* From extract_muldiv_1: (X * C1) % C2 is zero if C1 is a multiple of C2.  */
  (simplify
   (mod (mult @0 INTEGER_CST@1) INTEGER_CST@2)
   (if (ANY_INTEGRAL_TYPE_P (type)
        && TYPE_OVERFLOW_UNDEFINED (type)
        && wi::multiple_of_p (@1, @2, TYPE_SIGN (type)))
Index: gcc/testsuite/gcc.dg/tree-ssa/divide-5.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/divide-5.c	(revision 0)
+++ gcc/testsuite/gcc.dg/tree-ssa/divide-5.c	(working copy)
@@ -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"} } */