[ifcombine] propagate signbit mask to XOR right-hand operand

If a single-bit bitfield takes up the sign bit of a storage unit,
comparing the corresponding bitfield between two objects loads the
storage units, XORs them, converts the result to signed char, and
compares it with zero: ((signed char)(a.<byte> ^ c.<byte>) >= 0).

fold_truth_andor_for_ifcombine recognizes the compare with zero as a
sign bit test, then it decomposes the XOR into an equality test.

The problem is that, after this decomposition, that figures out the
width of the accessed fields, we apply the sign bit mask to the
left-hand operand of the compare, but we failed to also apply it to
the right-hand operand when both were taken from the same XOR.

This patch fixes that.


for  gcc/ChangeLog

	PR tree-optimization/118409
	* gimple-fold.cc (fold_truth_andor_for_ifcombine): Apply the
	signbit mask to the right-hand XOR operand too.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118409
	* gcc.dg/field-merge-20.c: New.

gcc/gimple-fold.cc

@@ -8270,6 +8270,16 @@ fold_truth_andor_for_ifcombine (enum tree_code code, tree truth_type,
 	ll_and_mask = sign;
       else
 	ll_and_mask &= sign;
+      if (l_xor)
+	{
+	  if (!lr_and_mask.get_precision ())
+	    lr_and_mask = sign;
+	  else
+	    lr_and_mask &= sign;
+	  if (l_const.get_precision ())
+	    l_const &= wide_int::from (lr_and_mask,
+				       l_const.get_precision (), UNSIGNED);
+	}
     }
 
   if (rsignbit)
@@ -8279,6 +8289,16 @@ fold_truth_andor_for_ifcombine (enum tree_code code, tree truth_type,
 	rl_and_mask = sign;
       else
 	rl_and_mask &= sign;
+      if (r_xor)
+	{
+	  if (!rr_and_mask.get_precision ())
+	    rr_and_mask = sign;
+	  else
+	    rr_and_mask &= sign;
+	  if (r_const.get_precision ())
+	    r_const &= wide_int::from (rr_and_mask,
+				       r_const.get_precision (), UNSIGNED);
+	}
     }
 
   /* If either comparison code is not correct for our logical operation,

gcc/testsuite/gcc.dg/field-merge-20.c

@@ -0,0 +1,64 @@
+/* { dg-do run } */
+/* { dg-options "-O1" } */
+
+/* tree-optimization/118409 */
+
+/* Check that tests involving a sign bit of a storage unit are handled
+   correctly.  The compares are turned into xor tests by earlier passes, and
+   ifcombine has to propagate the sign bit mask to the right hand of the
+   compare extracted from the xor, otherwise we'll retain unwanted bits for the
+   compare.  */
+
+typedef struct {
+    int p : __CHAR_BIT__;
+    int d : 1;
+    int b : __CHAR_BIT__ - 2;
+    int e : 1;
+} g;
+
+g a = {.d = 1, .e = 1}, c = {.b = 1, .d = 1, .e = 1};
+
+__attribute__((noipa))
+int f1 ()
+{
+  if (a.d == c.d
+      && a.e == c.e)
+    return 0;
+  return -1;
+}
+
+__attribute__((noipa))
+int f2 ()
+{
+  if (a.d != c.d
+      || a.e != c.e)
+    return -1;
+  return 0;
+}
+
+__attribute__((noipa))
+int f3 ()
+{
+  if (c.d == a.d
+      && c.e == a.e)
+    return 0;
+  return -1;
+}
+
+__attribute__((noipa))
+int f4 ()
+{
+  if (c.d != a.d
+      || c.e != a.e)
+    return -1;
+  return 0;
+}
+
+int main() {
+  if (f1 () < 0
+      || f2 () < 0
+      || f3 () < 0
+      || f4 () < 0)
+    __builtin_abort();
+  return 0;
+}