[PATCH] gimple-range-op: Use widen method even for sqrt reverse op [PR126534]

Hi!

For glibc the target hook returns 0 for sqrt precision (i.e. 0.5ulp
precise), but we didn't apply the +-0.5ulp widening when doing
reverse op and so still came up with a wrong range, even with 0.5ulp
precision certain values around 25.0 result in sqrt (x) being 5.0.
Even if the hook returns non-zero, I'd say more reasonable handling
of it is that it is say worst case 1ulp (or 2ulps etc.) from the
correct result (i.e. half ulp precise), rather than just 1ulp from
mathematically exact result.

So, the following patch uses the newly added widen method to add
this 0.5ulp before we feed it into frange_arithmetic.

Ok for trunk if it passes bootstrap/regtest?

2026-08-07  Jakub Jelinek  <[email protected]>

	PR tree-optimization/126534
	* gimple-range-op.cc (cfn_sqrt::op1_range): Widen lb or ub
	by further 0.5ulp or 1ulp before squaring it.

	* gcc.dg/pr126534.c: New test.

--- gcc/gimple-range-op.cc.jj	2026-03-27 10:17:14.167330201 +0100
+++ gcc/gimple-range-op.cc	2026-08-07 17:11:25.720918034 +0200
@@ -573,7 +573,10 @@ public:
 	  frange_nextafter (TYPE_MODE (type), lb, dconstninf);
 	if (real_less (&dconst0, &lb))
 	  {
-	    REAL_VALUE_TYPE op = lb;
+	    frange wlhs (type, lb, dconstinf);
+	    wlhs.clear_nan ();
+	    wlhs.widen (type);
+	    REAL_VALUE_TYPE op = wlhs.lower_bound ();
 	    frange_arithmetic (MULT_EXPR, type, lb, op, op, dconstninf);
 	  }
 	else
@@ -587,7 +590,10 @@ public:
 	  frange_nextafter (TYPE_MODE (type), ub, dconstinf);
 	if (real_isfinite (&ub))
 	  {
-	    REAL_VALUE_TYPE op = ub;
+	    frange wlhs (type, dconstninf, ub);
+	    wlhs.clear_nan ();
+	    wlhs.widen (type);
+	    REAL_VALUE_TYPE op = wlhs.upper_bound ();
 	    frange_arithmetic (MULT_EXPR, type, ub, op, op, dconstinf);
 	  }
 	else
--- gcc/testsuite/gcc.dg/pr126534.c.jj	2026-08-07 17:16:40.246010859 +0200
+++ gcc/testsuite/gcc.dg/pr126534.c	2026-08-07 17:16:24.296208995 +0200
@@ -0,0 +1,45 @@
+/* PR tree-optimization/126534 */
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+[[gnu::noipa]] int
+foo (double x)
+{
+  double y = __builtin_sqrt (x);
+  if (y <= 5.0)
+    {
+      if (x > 25.0)
+	return 1;
+      return 2;
+    }
+  return 3;
+}
+
+[[gnu::noipa]] int
+bar (double x)
+{
+  double y = __builtin_sqrt (x);
+  if (y >= 5.0)
+    {
+      if (x < 25.0)
+	return 1;
+      return 2;
+    }
+  return 3;
+}
+
+int
+main ()
+{
+#if __DBL_MANT_DIG__ == 53 && __DBL_MAX_EXP__ == 1024 && __FLT_EVAL_METHOD__ == 0
+  volatile double x = 0x1.9000000000001p+4;
+  volatile double y = 0x1.8ffffffffffffp+4;
+  if (__builtin_sqrt (x) == 5.0 && __builtin_sqrt (y) == 5.0)
+    {
+      if (foo (0x1.9000000000001p+4) != 1)    /* 25.000000000000004 */
+	__builtin_abort ();
+      if (bar (0x1.8ffffffffffffp+4) != 1)    /* 24.999999999999996 */
+	__builtin_abort ();
+    }
+#endif
+}

	Jakub