[gcc r15-11443] range-op-float: Fix up inf handling in other reverse ops [PR126464]

[Jakub Jelinek]

https://gcc.gnu.org/g:46b25706b7af8f8843c3dd88bbf59c5b44de0ac1

commit r15-11443-g46b25706b7af8f8843c3dd88bbf59c5b44de0ac1
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jul 31 08:56:03 2026 +0200

    range-op-float: Fix up inf handling in other reverse ops [PR126464]
    
    On Thu, Jul 30, 2026 at 09:31:17AM +0200, Richard Biener wrote:
    > > The following testcase is miscompiled since my r16-1108 change.
    > > The problem is if we handle a reverse of a narrowing float to float cast
    > > (in the example there are double -> float and long double -> double
    > > cast) and the lhs range is [-inf, -inf] or [+inf, +inf] (note, regardless
    > > of whether some NaNs are allowed or not, so not necessarily
    > > lhs.known_isinf ()), then handling that range in the wider type also
    > > as [-inf, -inf] or [+inf, +inf] is wrong, e.g. for the double -> float
    > > conversion, [-inf, -0x0.ffffff8p+128] double range could map to just
    > > that [-inf, -inf].  We have already float_widen_lhs_range function
    > > but that just extends the range by +/-1ulp or 0.5ulp if the bounds
    > > are finite.  If the range isn't singleton (except for optional NaN),
    > > then the minimum (or maximum) finite is already in the range, so this just
    > > extends the case where they are singleton.
    > > I don't know how to portably figure out that 0x0.ffffff8p+128 for
    > > double -> float (especially when in float_widen_lhs_range we don't know
    > > yet the wider type), so the patch just uses the +/-1ulp extension (i.e.
    > > [-inf, min_finite] or [+inf, max_finite] case.
    
    On a second thought, this actually isn't specific to just reverse of
    narrowing float to float casts, it is a problem for any other reverse binary
    ops too.
    
    E.g. the following testcase is miscompiled at -O2 since r13-3926-gd4c2f1d376da
    (but works with -O0).  The lhs of the addition is [-inf, -inf], one of its
    operand is [-1e304, -1e300] and we think the other operand has to be
    [-inf, -inf].  That is obviously wrong, even much larger operands can result
    in -inf, anything below -DBL_MAX + -1e300 where x + -1e300 doesn't round to
    -DBL_MAX or higher but to -inf.
    
    So, the following patch just widens lb of +inf and ub of -inf by 1ulp for
    all callers (and thus doesn't need the also_inf argument.
    
    2026-07-31  Jakub Jelinek  <jakub@redhat.com>
    
            PR tree-optimization/126464
            * range-op-float.cc (float_widen_lhs_range): Remove also_inf
            argument, replace its uses as if it was always true.
    
            * gcc.dg/torture/pr126464.c: New test.
    
    Reviewed-by: Richard Biener <rguenth@suse.de>
    (cherry picked from commit 6f32166346de2d1c0f6253e4fe37e8a6edac655d)

Diff:
---
 gcc/range-op-float.cc                   | 10 +++++-----
 gcc/testsuite/gcc.dg/torture/pr126464.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/gcc/range-op-float.cc b/gcc/range-op-float.cc
index 9e024f3c6560..b8cb2b5097d1 100644
--- a/gcc/range-op-float.cc
+++ b/gcc/range-op-float.cc
@@ -2366,14 +2366,14 @@ zero_to_inf_range (REAL_VALUE_TYPE &lb, REAL_VALUE_TYPE &ub, int signbit_known)
    in each direction.  See PR109008 for more details.  */
 
 static frange
-float_widen_lhs_range (tree type, const frange &lhs, bool also_inf = false)
+float_widen_lhs_range (tree type, const frange &lhs)
 {
   frange ret = lhs;
   if (lhs.known_isnan ())
     return ret;
   REAL_VALUE_TYPE lb = lhs.lower_bound ();
   REAL_VALUE_TYPE ub = lhs.upper_bound ();
-  if (real_isfinite (&lb) || (also_inf && !real_isneg (&lb)))
+  if (real_isfinite (&lb) || !real_isneg (&lb))
     {
       frange_nextafter (TYPE_MODE (type), lb, dconstninf);
       if (real_isinf (&lb))
@@ -2389,7 +2389,7 @@ float_widen_lhs_range (tree type, const frange &lhs, bool also_inf = false)
 	}
       if (!flag_rounding_math
 	  && !MODE_COMPOSITE_P (TYPE_MODE (type))
-	  && (!also_inf || real_isfinite (&lhs.lower_bound ())))
+	  && real_isfinite (&lhs.lower_bound ()))
 	{
 	  /* If not -frounding-math nor IBM double double, actually widen
 	     just by 0.5ulp rather than 1ulp.  */
@@ -2398,7 +2398,7 @@ float_widen_lhs_range (tree type, const frange &lhs, bool also_inf = false)
 	  real_arithmetic (&lb, RDIV_EXPR, &tem, &dconst2);
 	}
     }
-  if (real_isfinite (&ub) || (also_inf && real_isneg (&ub)))
+  if (real_isfinite (&ub) || real_isneg (&ub))
     {
       frange_nextafter (TYPE_MODE (type), ub, dconstinf);
       if (real_isinf (&ub))
@@ -2409,7 +2409,7 @@ float_widen_lhs_range (tree type, const frange &lhs, bool also_inf = false)
 	}
       if (!flag_rounding_math
 	  && !MODE_COMPOSITE_P (TYPE_MODE (type))
-	  && (!also_inf || real_isfinite (&lhs.upper_bound ())))
+	  && real_isfinite (&lhs.upper_bound ()))
 	{
 	  /* If not -frounding-math nor IBM double double, actually widen
 	     just by 0.5ulp rather than 1ulp.  */
diff --git a/gcc/testsuite/gcc.dg/torture/pr126464.c b/gcc/testsuite/gcc.dg/torture/pr126464.c
new file mode 100644
index 000000000000..c0dced1eedc0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr126464.c
@@ -0,0 +1,28 @@
+/* PR tree-optimization/126464 */
+/* { dg-do run } */
+
+#if __DBL_MANT_DIG__ == 53 && __DBL_MAX_10_EXP__ == 308 \
+    && __DBL_HAS_INFINITY__ && __FLT_EVAL_METHOD__ == 0
+[[gnu::noipa]] double
+foo (double x, double y)
+{
+  if (y >= -1e304 && y <= -1e300)
+    {
+      if (x + y == -__builtin_inf ())
+	return x * 0.5;
+    }
+  return x;
+}
+#endif
+
+int
+main ()
+{
+#if __DBL_MANT_DIG__ == 53 && __DBL_MAX_10_EXP__ == 308 \
+    && __DBL_HAS_INFINITY__ && __FLT_EVAL_METHOD__ == 0
+  if (foo (-__builtin_inf (), -1e303) != -__builtin_inf ())
+    __builtin_abort ();
+  if (foo (-1.797693134862e308, -1e303) != -1.797693134862e308 / 2.0)
+    __builtin_abort ();
+#endif
+}