Fix PR43419: pow->sqrt not honoring signed zeros

[Michael Matz]

Hello,

On Thu, 18 Mar 2010, Michael Matz wrote:

> > According to N1256, F.9.4.4:
> > 
> >   pow(±0, y) returns +0 for y > 0 and not an odd integer.
> > 
> > So, pow(-0.0, 0.5) should return +0. But sqrt(-0.0) should return -0
> > according to the IEEE 754 standard (and F.9.4.5 from ISO C99).
> 
> But unfortunately you are right, this expansion can only be done for 
> -fno-signed-zeros.

Like so.

Regstrapping on x86_64-linux in progress, it's not a regression from 4.4 
but fairly low risk, so okay for trunk if that passes?


Ciao,
Michael.
-- 
	PR middle-end/43419
	* builtins.c (expand_builtin_pow): Don't transform pow(x, 0.5)
	into sqrt(x) if we need to preserve signed zeros.

testsuite/
	* gcc.dg/pr43419.c: New testcase.

Index: builtins.c
===================================================================
--- builtins.c	(revision 157518)
+++ builtins.c	(working copy)
@@ -2980,7 +2980,10 @@ expand_builtin_pow (tree exp, rtx target
 	  && ((flag_unsafe_math_optimizations
 	       && optimize_insn_for_speed_p ()
 	       && powi_cost (n/2) <= POWI_MAX_MULTS)
-	      || n == 1))
+	      /* Even the c==0.5 case can not be done unconditionally
+	         when we need to preserve signed zeros, as
+		 pow (-0, 0.5) is +0, while sqrt(-0) is -0.  */
+	      || (!HONOR_SIGNED_ZEROS (mode) && n == 1)))
 	{
 	  tree call_expr = build_call_nofold (fn, 1, narg0);
 	  /* Use expand_expr in case the newly built call expression
Index: testsuite/gcc.dg/pr43419.c
===================================================================
--- testsuite/gcc.dg/pr43419.c	(revision 0)
+++ testsuite/gcc.dg/pr43419.c	(revision 0)
@@ -0,0 +1,19 @@
+/* { dg-do run } */
+/* { dg-options "-O1" } */
+/* { dg-add-options ieee } */
+#include <math.h>
+
+extern void abort (void);
+void __attribute__((noinline)) f (double x)
+{
+  double pluszero = pow (x, 0.5);
+  double minuszero = sqrt (x);
+  if (signbit (pluszero) == signbit (minuszero))
+    abort ();
+}
+
+int main(void)
+{
+  f (-0.0);
+  return 0;
+}