[PATCH] Canonicalize constant multiplies in division

[Wilco Dijkstra]

This patch implements some of the optimizations discussed in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71026.

Canonicalize x / (C1 * y) into (x * C2) / y.

This moves constant multiplies out of the RHS of a division in order
to allow further simplifications (such as (C1 * x) / (C2 * y) ->
(C3 * x) / y) and to enable more reciprocal CSEs.

OK for commit?

ChangeLog
2017-10-17  Wilco Dijkstra  <wdijk...@arm.com>  
            Jackson Woodruff  <jackson.woodr...@arm.com>

    gcc/
        PR 71026/tree-optimization
        * match.pd: Canonicalize constant multiplies in division.

    gcc/testsuite/
        PR 71026/tree-optimization
        * gcc.dg/cse_recip.c: New test.
--

diff --git a/gcc/match.pd b/gcc/match.pd
index 
ade851f78fb9ac6ce03b752f63e03f3b5a19cda9..532fabf51ce8a45d54147a3ae0b3917e22b1a4d0
 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -342,10 +342,20 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (negate @0)))
 
 (if (flag_reciprocal_math)
- /* Convert (A/B)/C to A/(B*C)  */
+ /* Convert (A/B)/C to A/(B*C). */
  (simplify
   (rdiv (rdiv:s @0 @1) @2)
-   (rdiv @0 (mult @1 @2)))
+  (rdiv @0 (mult @1 @2)))
+
+ /* Canonicalize x / (C1 * y) to (x * C2) / y.  */
+ (if (optimize)
+  (simplify
+   (rdiv @0 (mult @1 REAL_CST@2))
+   (if (!real_zerop (@1))
+    (with
+     { tree tem = const_binop (RDIV_EXPR, type, build_one_cst (type), @2); }
+     (if (tem)
+      (rdiv (mult @0 { tem; } ) @1))))))
 
  /* Convert A/(B/C) to (A/B)*C  */
  (simplify
@@ -628,15 +638,6 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
     (if (tem)
      (rdiv { tem; } @1)))))
 
-/* Convert C1/(X*C2) into (C1/C2)/X  */
-(simplify
- (rdiv REAL_CST@0 (mult @1 REAL_CST@2))
-  (if (flag_reciprocal_math)
-   (with
-    { tree tem = const_binop (RDIV_EXPR, type, @0, @2); }
-    (if (tem)
-     (rdiv { tem; } @1)))))
-
 /* Simplify ~X & X as zero.  */
 (simplify
  (bit_and:c (convert? @0) (convert? (bit_not @0)))
diff --git a/gcc/testsuite/gcc.dg/cse_recip.c b/gcc/testsuite/gcc.dg/cse_recip.c
new file mode 100644
index 
0000000000000000000000000000000000000000..20ed529c33ebecc911fb540a8b2b597bba0023e6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cse_recip.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-Ofast -fdump-tree-optimized" } */
+
+void
+cse_recip (float x, float y, float *a)
+{
+  a[0] = y / (5 * x);
+  a[1] = y / (3 * x);
+  a[2] = y / x;
+}
+
+/* { dg-final { scan-tree-dump-times " / " 1 "optimized" } } */