Fold comparisons between sqrt and zero

The expression:

      signbit(sqrt(x))

is always 0 for -ffast-math.  The signbit fold first converts it to:

      sqrt(x) < 0

and whether we realise that this is false depends on a race between two
folders: the sqrt comparison folder, which wants to convert it to

      x < 0*0

and the generic tree_expr_nonnegative_p rule for ... < 0, which would
give the hoped-for 0.

The sqrt code already handles comparisons with negative values specially,
so this patch simply extends that idea to comparisons with zero.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* match.pd: Handle sqrt(x) cmp 0 specially.

gcc/testsuite/
	* gcc.dg/torture/builtin-sqrt-cmp-1.c: New test.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@229422 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 26491d27840..b8e6b4643c8 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -1973,6 +1973,25 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 	{ constant_boolean_node (true, type); })
        /* sqrt(x) > y is the same as x >= 0, if y is negative.  */
        (ge @0 { build_real (TREE_TYPE (@0), dconst0); })))
+     (if (real_equal (TREE_REAL_CST_PTR (@1), &dconst0))
+      (switch
+       /* sqrt(x) < 0 is always false.  */
+       (if (cmp == LT_EXPR)
+	{ constant_boolean_node (false, type); })
+       /* sqrt(x) >= 0 is always true if we don't care about NaNs.  */
+       (if (cmp == GE_EXPR && !HONOR_NANS (@0))
+	{ constant_boolean_node (true, type); })
+       /* sqrt(x) <= 0 -> x == 0.  */
+       (if (cmp == LE_EXPR)
+	(eq @0 @1))
+       /* Otherwise sqrt(x) cmp 0 -> x cmp 0.  Here cmp can be >=, >,
+          == or !=.  In the last case:
+
+	    (sqrt(x) != 0) == (NaN != 0) == true == (x != 0)
+
+	  if x is negative or NaN.  Due to -funsafe-math-optimizations,
+	  the results for other x follow from natural arithmetic.  */
+       (cmp @0 @1)))
      (if (cmp == GT_EXPR || cmp == GE_EXPR)
       (with
        {