c-family: One more 40752 tweak for unsigned char.

My last patch didn't fix all the failures on unsignd char targets.  We were
missing one warning because by suppressing -Wsign-conversion for the second
operand of + we missed an overflow that we want to warn about, and we
properly don't warn about unsigned / or %.

	PR testsuite/93391 - PR 40752 test fails with unsigned plain char.
	* c-warn.c (conversion_warning): Change -Wsign-conversion handling.
	* lib/target-supports.exp (check_effective_target_unsigned_char):
	New.

gcc/c-family/ChangeLog

+2020-01-23  Jason Merrill  <jason@redhat.com>
+
+	* c-warn.c (conversion_warning): Change -Wsign-conversion handling.
+
 2020-01-23  Martin Sebor  <msebor@redhat.com>
 
 	PR c/84919

gcc/c-family/c-warn.c

@@ -1315,10 +1315,14 @@ conversion_warning (location_t loc, tree type, tree expr, tree result)
 	    for (int i = 0; i < arith_ops; ++i)
 	      {
 		tree op = TREE_OPERAND (expr, i);
-		tree opr = convert (type, op);
 		/* Avoid -Wsign-conversion for (unsigned)(x + (-1)).  */
-		bool minus = TREE_CODE (expr) == PLUS_EXPR && i == 1;
-		if (unsafe_conversion_p (type, op, opr, !minus))
+		if (TREE_CODE (expr) == PLUS_EXPR && i == 1
+		    && INTEGRAL_TYPE_P (type) && TYPE_UNSIGNED (type)
+		    && TREE_CODE (op) == INTEGER_CST
+		    && tree_int_cst_sgn (op) < 0)
+		  op = fold_build1 (NEGATE_EXPR, TREE_TYPE (op), op);
+		tree opr = convert (type, op);
+		if (unsafe_conversion_p (type, op, opr, true))
 		  goto op_unsafe;
 	      }
 	    /* The operands seem safe, we might still want to warn if

gcc/testsuite/ChangeLog

+2020-01-23  Jason Merrill  <jason@redhat.com>
+
+	* lib/target-supports.exp (check_effective_target_unsigned_char):
+	New.
+
 2020-01-23  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/93376

gcc/testsuite/c-c++-common/Wconversion-pr40752a.c

@@ -14,9 +14,10 @@ void foo(char c, char c2)
   c *= 2;			/* { dg-warning "conversion" } */
   c *= c2;			/* { dg-warning "conversion" } */
   c /= 2;
-  c /= c2;			/* { dg-warning "conversion" } */
+  /* If char is unsigned we avoid promoting to int.  */
+  c /= c2;  /* { dg-warning "conversion" "" { target { ! unsigned_char } } } */
   c %= 2;
-  c %= c2;			/* { dg-warning "conversion" } */
+  c %= c2;  /* { dg-warning "conversion" "" { target { ! unsigned_char } } } */
   c = -c2;			/* { dg-warning "conversion" } */
   c = ~c2;			/* { dg-warning "conversion" } */
   c = c2++;

gcc/testsuite/lib/target-supports.exp

@@ -3115,6 +3115,14 @@ proc check_effective_target_dfprt { } {
     }]
 }
 
+# Return 1 iff target has unsigned plain 'char' by default.
+
+proc check_effective_target_unsigned_char {} {
+    return [check_no_compiler_messages unsigned_char assembly {
+	char ar[(char)-1];
+    }]
+}
+
 proc check_effective_target_powerpc_popcntb_ok { } {
     return [check_cached_effective_target powerpc_popcntb_ok {