c, c++: Fix up -Wunused-value on COMPLEX_EXPRs [PR97748]

The -Wunused-value warning in both C and C++ FEs (implemented
significantly differently between the two) sees the COMPLEX_EXPRs created
e.g. for complex pre/post increment and many other expressions as useless
and warns about it.

For the C warning implementation, on e.g.
COMPLEX_EXPR < ++REALPART_EXPR <x>, IMAGPART_EXPR <x>>;
would warn even on the IMAGPART_EXPR <x> there alone etc., so what works
is check if we'd warn about both operands of COMPLEX_EXPR and if yes,
warn on the whole COMPLEX_EXPR, otherwise don't warn.

The C++ warning implementation is significantly different and for that one
the only warn if both would be warned about doesn't really work,
we then miss warnings e.g. about
COMPLEX_EXPR <REALPART_EXPR <SAVE_EXPR <x>> + 1.0e+0, IMAGPART_EXPR <SAVE_EXPR <x>>> >>>>>
The patch replaces the warning_at call with call to the c-family
warn_if_unused_value function.

On the testcase which after the initial new tests contains pretty much
everything from gcc.dg/Wunused-value-1.c both approaches seem to work
nicely.

2020-11-10  Jakub Jelinek  <jakub@redhat.com>

	PR c/97748
gcc/c-family/
	* c-common.h (warn_if_unused_value): Add quiet argument defaulted
	to false.
	* c-warn.c (warn_if_unused_value): Likewise.  Pass it down
	recursively and just return true instead of warning if it is true.
	Handle COMPLEX_EXPR.
gcc/cp/
	* cvt.c (convert_to_void): Check (complain & tf_warning) in the outer
	if rather than twice times in the inner one.  Use warn_if_unused_value.
	Formatting fix.
gcc/testsuite/
	* c-c++-common/Wunused-value-1.c: New test.

diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index b80db230a6df4519213df60f41a42873c6d1aca4..94f4868915a2120ccb01a025a69eb94ca7602c42 100644 (file)
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -1365,7 +1365,7 @@ extern void warn_tautological_cmp (const op_location_t &, enum tree_code,
                                   tree, tree);
 extern void warn_logical_not_parentheses (location_t, enum tree_code, tree,
                                          tree);
-extern bool warn_if_unused_value (const_tree, location_t);
+extern bool warn_if_unused_value (const_tree, location_t, bool = false);
 extern bool strict_aliasing_warning (location_t, tree, tree);
 extern void sizeof_pointer_memaccess_warning (location_t *, tree,
                                              vec<tree, va_gc> *, tree *,

diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
index 68b093e0d467ac477aecedd771475eea58b94135..6d1f9a73e4489640b4becbae09e1e6a91a7a6de6 100644 (file)
--- a/gcc/c-family/c-warn.c
+++ b/gcc/c-family/c-warn.c
@@ -585,7 +585,7 @@ warn_logical_not_parentheses (location_t location, enum tree_code code,
    (potential) location of the expression.  */
 
 bool
-warn_if_unused_value (const_tree exp, location_t locus)
+warn_if_unused_value (const_tree exp, location_t locus, bool quiet)
 {
  restart:
   if (TREE_USED (exp) || TREE_NO_WARNING (exp))
@@ -633,7 +633,7 @@ warn_if_unused_value (const_tree exp, location_t locus)
       goto restart;
 
     case COMPOUND_EXPR:
-      if (warn_if_unused_value (TREE_OPERAND (exp, 0), locus))
+      if (warn_if_unused_value (TREE_OPERAND (exp, 0), locus, quiet))
        return true;
       /* Let people do `(foo (), 0)' without a warning.  */
       if (TREE_CONSTANT (TREE_OPERAND (exp, 1)))
@@ -648,6 +648,13 @@ warn_if_unused_value (const_tree exp, location_t locus)
        return false;
       goto warn;
 
+    case COMPLEX_EXPR:
+      /* Warn only if both operands are unused.  */
+      if (warn_if_unused_value (TREE_OPERAND (exp, 0), locus, true)
+         && warn_if_unused_value (TREE_OPERAND (exp, 1), locus, true))
+       goto warn;
+      return false;
+
     case INDIRECT_REF:
       /* Don't warn about automatic dereferencing of references, since
         the user cannot control it.  */
@@ -671,6 +678,8 @@ warn_if_unused_value (const_tree exp, location_t locus)
        return false;
 
     warn:
+      if (quiet)
+       return true;
       return warning_at (locus, OPT_Wunused_value, "value computed is not used");
     }
 }

diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index c9e7b1ff04415d142a67e0fd1fdf8a73eff0cf43..bcd7c5af81c251bc16707569b93e744042d72e79 100644 (file)
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -1568,12 +1568,13 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
          && warn_unused_value
          && !TREE_NO_WARNING (expr)
          && !processing_template_decl
-         && !cp_unevaluated_operand)
+         && !cp_unevaluated_operand
+         && (complain & tf_warning))
        {
          /* The middle end does not warn about expressions that have
             been explicitly cast to void, so we must do so here.  */
-         if (!TREE_SIDE_EFFECTS (expr)) {
-            if (complain & tf_warning)
+         if (!TREE_SIDE_EFFECTS (expr))
+           {
              switch (implicit)
                {
                  case ICV_SECOND_OF_COND:
@@ -1605,14 +1606,10 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
                  default:
                    gcc_unreachable ();
                }
-          }
+           }
          else
            {
-             tree e;
-             enum tree_code code;
-             enum tree_code_class tclass;
-
-             e = expr;
+             tree e = expr;
              /* We might like to warn about (say) "(int) f()", as the
                 cast has no effect, but the compiler itself will
                 generate implicit conversions under some
@@ -1626,21 +1623,14 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain)
              while (TREE_CODE (e) == NOP_EXPR)
                e = TREE_OPERAND (e, 0);
 
-             code = TREE_CODE (e);
-             tclass = TREE_CODE_CLASS (code);
-             if ((tclass == tcc_comparison
-                  || tclass == tcc_unary
-                  || (tclass == tcc_binary
-                      && !(code == MODIFY_EXPR
-                           || code == INIT_EXPR
-                           || code == PREDECREMENT_EXPR
-                           || code == PREINCREMENT_EXPR
-                           || code == POSTDECREMENT_EXPR
-                           || code == POSTINCREMENT_EXPR))
-                  || code == VEC_PERM_EXPR
-                  || code == VEC_COND_EXPR)
-                  && (complain & tf_warning))
-               warning_at (loc, OPT_Wunused_value, "value computed is not used");
+             enum tree_code code = TREE_CODE (e);
+             enum tree_code_class tclass = TREE_CODE_CLASS (code);
+             if (tclass == tcc_comparison
+                 || tclass == tcc_unary
+                 || tclass == tcc_binary
+                 || code == VEC_PERM_EXPR
+                 || code == VEC_COND_EXPR)
+               warn_if_unused_value (e, loc);
            }
        }
       expr = build1 (CONVERT_EXPR, void_type_node, expr);

diff --git a/gcc/testsuite/c-c++-common/Wunused-value-1.c b/gcc/testsuite/c-c++-common/Wunused-value-1.c
new file mode 100644 (file)
index 0000000..90c9d93
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wunused-value-1.c
@@ -0,0 +1,33 @@
+/* PR c/97748 */
+/* { dg-do compile } */
+/* { dg-options "-Wunused-value" } */
+
+double _Complex f ();
+double _Complex *p;
+
+double _Complex
+foo (double _Complex x)
+{
+  ++x;                 /* { dg-bogus "value computed is not used" } */
+  --x;                 /* { dg-bogus "value computed is not used" } */
+  x += 1;              /* { dg-bogus "value computed is not used" } */
+  x += 1.0iF;          /* { dg-bogus "value computed is not used" } */
+  x++;                 /* { dg-bogus "value computed is not used" } */
+  x--;                 /* { dg-bogus "value computed is not used" } */
+  x + 1;               /* { dg-warning "value computed is not used" } */
+  (void) (x + 1);      /* { dg-bogus "value computed is not used" } */
+  1 + f ();            /* { dg-warning "value computed is not used" } */
+  f () + f ();                 /* { dg-warning "value computed is not used" } */
+  f () + f (), f ();   /* { dg-warning "value computed is not used" } */
+  f ();
+  (void) f ();
+  *p++;                        /* { dg-warning "value computed is not used" } */
+  ++*p;                        /* { dg-bogus "value computed is not used" } */
+  (*p ? f () : 0);
+  ({ f (); });
+  ({ f () + 1; });
+  ({ f (); 0; });
+  ({ f () + 1; 0; });  /* { dg-warning "value computed is not used" } */
+  1 + ({ f (); });     /* { dg-warning "value computed is not used" } */
+  return x;
+}