c-family: Fix up -W*conversion on bitwise &/|/^ [PR101537]

The following testcases emit a bogus -Wconversion warning.  This is because
conversion_warning function doesn't handle BIT_*_EXPR (only unsafe_conversion_p
that is called during the default: case, and that one doesn't handle
SAVE_EXPRs added because the unsigned char & or | operands promoted to int
have side-effects and =| or =& is used.

The patch handles BIT_IOR_EXPR/BIT_XOR_EXPR like the last 2 operands of
COND_EXPR by recursing on the two operands, if either of them doesn't fit
into the narrower type, complain.  BIT_AND_EXPR too, but first it needs to
handle some special cases that unsafe_conversion_p does, namely when one
of the two operands is a constant.

This fixes completely the pr101537.c test and for C also pr103881.c
and doesn't regress anything in the testsuite, for C++ pr103881.c still
emits the bogus warnings.
This is because while the C FE emits in that case a SAVE_EXPR that
conversion_warning can handle already, C++ FE emits
TARGET_EXPR <D.whatever, ...>, something | D.whatever
etc. and conversion_warning handles COMPOUND_EXPR by "recursing" on the
rhs.  To handle that case, we'd need for TARGET_EXPR on the lhs remember
in some hash map the mapping from D.whatever to the TARGET_EXPR and when
we see D.whatever, use corresponding TARGET_EXPR initializer instead.

2022-01-11  Jakub Jelinek  <jakub@redhat.com>

	PR c/101537
	PR c/103881
gcc/c-family/
	* c-warn.c (conversion_warning): Handle BIT_AND_EXPR, BIT_IOR_EXPR
	and BIT_XOR_EXPR.
gcc/testsuite/
	* c-c++-common/pr101537.c: New test.
	* c-c++-common/pr103881.c: New test.

gcc/c-family/c-warn.c

@@ -1304,6 +1304,34 @@ conversion_warning (location_t loc, tree type, tree expr, tree result)
 		|| conversion_warning (loc, type, op2, result));
       }
 
+    case BIT_AND_EXPR:
+      if (TREE_CODE (expr_type) == INTEGER_TYPE
+	  && TREE_CODE (type) == INTEGER_TYPE)
+	for (int i = 0; i < 2; ++i)
+	  {
+	    tree op = TREE_OPERAND (expr, i);
+	    if (TREE_CODE (op) != INTEGER_CST)
+	      continue;
+
+	    /* If one of the operands is a non-negative constant
+	       that fits in the target type, then the type of the
+	       other operand does not matter.  */
+	    if (int_fits_type_p (op, c_common_signed_type (type))
+		&& int_fits_type_p (op, c_common_unsigned_type (type)))
+	      return false;
+
+	    /* If constant is unsigned and fits in the target
+	       type, then the result will also fit.  */
+	    if (TYPE_UNSIGNED (TREE_TYPE (op)) && int_fits_type_p (op, type))
+	      return false;
+	  }
+      /* FALLTHRU */
+    case BIT_IOR_EXPR:
+    case BIT_XOR_EXPR:
+      return (conversion_warning (loc, type, TREE_OPERAND (expr, 0), result)
+	      || conversion_warning (loc, type, TREE_OPERAND (expr, 1),
+				     result));
+
     default_:
     default:
       conversion_kind = unsafe_conversion_p (type, expr, result, true);

gcc/testsuite/c-c++-common/pr101537.c

@@ -0,0 +1,26 @@
+/* PR c/101537 */
+/* { dg-do compile } */
+/* { dg-options "-Wconversion" } */
+
+int
+foo ()
+{
+  int aaa = 1;
+  unsigned char bbb = 0;
+  bbb |= aaa ? 1 : 0;
+  return bbb;
+}
+
+int
+bar (unsigned char x, int f)
+{
+  x |= f ? 1 : 0;
+  return x;
+}
+
+int
+baz (unsigned char x, int f)
+{
+  x = x | f ? 1 : 0;
+  return x;
+}

gcc/testsuite/c-c++-common/pr103881.c

@@ -0,0 +1,20 @@
+/* PR c/103881 */
+/* { dg-do compile } */
+/* { dg-options "-Wconversion" } */
+
+unsigned char bar (void);
+
+void
+foo (void)
+{
+  unsigned char t = 0;
+  t |= bar ();
+  t |= bar () & bar ();	/* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+  t &= bar () & bar ();	/* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+  t = bar () & bar ();
+
+  unsigned char a = bar ();
+  t |= a & a;
+  t |= bar () & a;		/* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+  t |= a & bar ();		/* { dg-bogus "conversion from 'int' to 'unsigned char' may change value" "" { xfail c++ } } */
+}