[PATCH][GCC] Simplification of 1U << (31 - x)

[Sudi Das]

Hi all

This is a fix for PR 80131 
Currently the code A << (B - C) is not simplified.
However at least a more specific case of 1U << (C -x) where C = precision(type) 
- 1 can be simplified to (1 << C) >> x.

This is done by adding a new simplification rule in match.pd

So for a test case :

unsigned int f1(unsigned int i)
{
  return 1U << (31 - i);
}

We see a gimple dump of 

f1 (unsigned int i)
{
  unsigned int D.3121;

  D.3121 = 2147483648 >> i;
  return D.3121;
}

instead of 

f1 (unsigned int i)
{
  unsigned int D.3121;

  _1 = 31 - i;
  D.3121 = 1 << _1;
  return D.3121;
}


Add a new test case and checked for regressions on bootstrapped 
aarch64-none-linux-gnu.
Ok for stage 1?

Thanks
Sudi

2017-03-23  Sudakshina Das  <sudi....@arm.com>

        PR middle-end/80131
        * match.pd: Simplify 1 << (C - x) where C = precision (type) - 1.

2017-03-23  Sudakshina Das  <sudi....@arm.com>

        PR middle-end/80131
        * testsuite/gcc.dg/pr80131-1.c: New Test.
diff --git a/gcc/match.pd b/gcc/match.pd
index 7b96800..be20fb7 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -508,6 +508,19 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
        && tree_nop_conversion_p (type, TREE_TYPE (@1)))
    (lshift @0 @2)))
 
+/* Fold (1 << (C - x)) where C = precision(type) - 1
+   into ((1 << C) >> x). */
+(simplify
+ (lshift integer_onep@0 (minus INTEGER_CST@1 @2))
+  (if (INTEGRAL_TYPE_P (type)
+       && TYPE_PRECISION (type) <= HOST_BITS_PER_WIDE_INT
+       && tree_to_uhwi (@1) == (unsigned)(TYPE_PRECISION (type) - 1))
+   (if (TYPE_UNSIGNED(type))
+     (rshift (lshift @0 @1) @2)
+   (with
+    { tree utype = unsigned_type_for (type); }
+    (convert:type (rshift (lshift (convert:utype @0) @1) @2))))))
+
 /* Fold (C1/X)*C2 into (C1*C2)/X.  */
 (simplify
  (mult (rdiv@3 REAL_CST@0 @1) REAL_CST@2)
diff --git a/gcc/testsuite/gcc.dg/pr80131-1.c b/gcc/testsuite/gcc.dg/pr80131-1.c
new file mode 100644
index 0000000..2bb6ff3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr80131-1.c
@@ -0,0 +1,30 @@
+/* { dg-do compile } */
+/* { dg-options "-fdump-tree-gimple" } */
+
+/* Checks the simplification of:
+   1 << (C - x) to (1 << C) >> x, where C = precision (type) - 1
+   f1 is not simplified but f2, f3 and f4 are. */
+
+__INT64_TYPE__ f1 (__INT64_TYPE__ i)
+{
+  return (__INT64_TYPE__)1 << (31 - i);
+}
+
+__INT64_TYPE__ f2 (__INT64_TYPE__ i)
+{
+  return (__INT64_TYPE__)1 << (63 - i);
+}
+
+__UINT64_TYPE__ f3 (__INT64_TYPE__ i)
+{
+  return (__UINT64_TYPE__)1 << (63 - i);
+}
+
+__INT32_TYPE__ f4 (__INT32_TYPE__ i)
+{
+  return (__INT32_TYPE__)1 << (31 - i);
+}
+
+/* { dg-final { scan-tree-dump-times "= 31 -"  1 "gimple" } } */
+/* { dg-final { scan-tree-dump-times "9223372036854775808 >>" 2 "gimple" } } */
+/* { dg-final { scan-tree-dump "2147483648 >>" "gimple" } } */