Fold comparisons between sqrt and zero

[Richard Sandiford]

Richard Biener <richard.guenther@gmail.com> writes:
> On Mon, Oct 26, 2015 at 10:36 AM, Richard Sandiford
> <rdsandiford@googlemail.com> wrote:
>> An upcoming patch adds a fold from hypot(x,x) to fabs(x)*sqrt(2).
>> This is unusual in that it could trigger in the gimplifier but would
>> require new SSA names to be created.  This patch makes sure that we
>> don't try to create new SSA names when we're not yet in SSA form.
>>
>> Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.
>> OK to install?
>
> Please use
>
>  if (gimple_in_ssa_p (cfun))
>   res = make_ssa_name (type);
>  else
>   res = create_tmp_reg (type);
>
> Ok with that change.

Thanks, committed in that form.  I retested the series on top of it and
the create_tmp_reg causes the later signbit patch (not yet committed)
to regress on:

  signbit(sqrt(x))

which 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.

I don't really like patches like this since they'll probably help no
real code, but still...

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.
OK to install?

Thanks,
Richard


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

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

diff --git a/gcc/match.pd b/gcc/match.pd
index 26491d2..b8e6b46 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
        {
diff --git a/gcc/testsuite/gcc.dg/torture/builtin-sqrt-cmp-1.c b/gcc/testsuite/gcc.dg/torture/builtin-sqrt-cmp-1.c
new file mode 100644
index 0000000..3f4a708
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/builtin-sqrt-cmp-1.c
@@ -0,0 +1,53 @@
+/* { dg-do link } */
+/* { dg-options "-ffast-math" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" } { "" } } */
+
+extern double sqrt (double);
+extern float sqrtf (float);
+extern long double sqrtl (long double);
+
+/* All references to link_error should go away at compile-time.  */
+extern void link_error (void);
+
+#define TEST_ONE(SUFFIX, TYPE)			\
+  void __attribute__ ((noinline, noclone))	\
+  test##SUFFIX (TYPE f, int *res)		\
+  {						\
+    TYPE sqrt_res = sqrt##SUFFIX (f);		\
+    res[0] = sqrt_res < 0;			\
+    if (res[0])					\
+      link_error ();				\
+    res[1] = sqrt_res <= 0;			\
+    if (res[1] != (f == 0))			\
+      link_error ();				\
+    res[2] = (sqrt_res == 0);			\
+    if (res[2] != (f == 0))			\
+      link_error ();				\
+    res[3] = (sqrt_res != 0);			\
+    if (res[3] != (f != 0))			\
+      link_error ();				\
+    res[4] = (sqrt_res > 0);			\
+    if (res[4] != (f > 0))			\
+      link_error ();				\
+    res[5] = (sqrt_res >= 0);			\
+    if (!res[5])				\
+      link_error ();				\
+  }
+
+volatile float f;
+volatile double d;
+volatile long double ld;
+
+TEST_ONE (f, float)
+TEST_ONE (, double)
+TEST_ONE (l, long double)
+
+int
+main ()
+{
+  int res[6];
+  testf (f, res);
+  test (d, res);
+  testl (ld, res);
+  return 0;
+}