Hi!
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>>> >>>>>
so the patch instead warns if it would warn on any of the operands.
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2020-11-09 Jakub Jelinek <ja...@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 (warn_if_unused_value_p): New function.
(convert_to_void): Use it.
gcc/testsuite/
* c-c++-common/Wunused-value-1.c: New test.
--- gcc/c-family/c-common.h.jj 2020-11-03 11:15:07.170681001 +0100
+++ gcc/c-family/c-common.h 2020-11-07 09:37:48.597233063 +0100
@@ -1362,7 +1362,7 @@ extern void warn_tautological_cmp (const
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 *,
--- gcc/c-family/c-warn.c.jj 2020-10-26 10:53:56.533885147 +0100
+++ gcc/c-family/c-warn.c 2020-11-07 09:40:51.011170825 +0100
@@ -585,7 +585,7 @@ warn_logical_not_parentheses (location_t
(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, lo
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, lo
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, lo
return false;
warn:
+ if (quiet)
+ return true;
return warning_at (locus, OPT_Wunused_value, "value computed is not
used");
}
}
--- gcc/cp/cvt.c.jj 2020-07-28 15:39:09.000000000 +0200
+++ gcc/cp/cvt.c 2020-11-08 21:02:08.306584085 +0100
@@ -1111,6 +1111,46 @@ maybe_warn_nodiscard (tree expr, impl_co
}
}
+/* Return true if -Wunused-value warning should be emitted for EXPR. */
+
+static bool
+warn_if_unused_value_p (tree 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
+ circumstances. (For example a block copy will be
+ turned into a call to "__builtin_memcpy", with a
+ conversion of the return value to an appropriate
+ type.) So, to avoid false positives, we strip
+ conversions. Do not use STRIP_NOPs because it will
+ not strip conversions to "void", as that is not a
+ mode-preserving conversion. */
+ while (TREE_CODE (expr) == NOP_EXPR)
+ expr = TREE_OPERAND (expr, 0);
+
+ enum tree_code code = TREE_CODE (expr);
+ if (code == COMPLEX_EXPR)
+ /* For COMPLEX_EXPR, return true if either operand is unused. */
+ return (warn_if_unused_value_p (TREE_OPERAND (expr, 0))
+ || warn_if_unused_value_p (TREE_OPERAND (expr, 1)));
+
+ enum tree_code_class 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)
+ return true;
+ return false;
+}
+
/* When an expression is used in a void context, its value is discarded and
no lvalue-rvalue and similar conversions happen [expr.static.cast/4,
stmt.expr/1, expr.comma/1]. This permits dereferencing an incomplete type
@@ -1606,42 +1646,8 @@ convert_to_void (tree expr, impl_conv_vo
gcc_unreachable ();
}
}
- else
- {
- tree e;
- enum tree_code code;
- enum tree_code_class tclass;
-
- 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
- circumstances. (For example a block copy will be
- turned into a call to "__builtin_memcpy", with a
- conversion of the return value to an appropriate
- type.) So, to avoid false positives, we strip
- conversions. Do not use STRIP_NOPs because it will
- not strip conversions to "void", as that is not a
- mode-preserving conversion. */
- 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");
- }
+ else if ((complain & tf_warning) && warn_if_unused_value_p (expr))
+ warning_at (loc, OPT_Wunused_value, "value computed is not used");
}
expr = build1 (CONVERT_EXPR, void_type_node, expr);
}
--- gcc/testsuite/c-c++-common/Wunused-value-1.c.jj 2020-11-07
09:51:46.407757100 +0100
+++ gcc/testsuite/c-c++-common/Wunused-value-1.c 2020-11-07
10:15:34.295574782 +0100
@@ -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;
+}
Jakub
