Over the years, we got several PRs that suggested to add a warning that would
warn about unreachable statements between `switch (cond)' and the first case.
In some cases our -Wuninitialized warning can detect such a case, but mostly
not. This patch is an attempt to add a proper warning about this peculiar
case. I chose to not warn about declarations between switch and the first
case, because we use that in the codebase and I think this kind of use is
fine. As it's usually the case, some obscure cases cropped up during testing,
this time those were Duff's devices, so I had to go the extra mile to handle
them specially.
This is a C FE part only; I'd like to hear some feedback first before I plunge
into the C++ FE.
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2016-05-10 Marek Polacek <pola...@redhat.com>
PR c/49859
* c.opt (Wswitch-unreachable): New option.
* c-parser.c: Include tree-iterator.h.
(c_parser_switch_statement): Implement -Wswitch-unreachable warning.
* doc/invoke.texi: Document -Wswitch-unreachable.
* gcc.dg/Wjump-misses-init-1.c: Use -Wno-switch-unreachable.
* gcc.dg/nested-func-1.c: Likewise.
* gcc.dg/pr67784-4.c: Likewise.
* gcc.dg/Wswitch-unreachable-1.c: New test.
* gcc.dg/c99-vla-jump-5.c (f): Add dg-warning.
* gcc.dg/switch-warn-1.c (foo1): Likewise.
* c-c++-common/goacc/sb-2.c: Likewise.
* gcc.dg/gomp/block-10.c: Likewise.
* gcc.dg/gomp/block-9.c: Likewise.
* gcc.dg/gomp/target-1.c: Likewise.
* gcc.dg/gomp/target-2.c: Likewise.
* gcc.dg/gomp/taskgroup-1.c: Likewise.
* gcc.dg/gomp/teams-1.c: Likewise.
diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index bdc6ee0..8b6fdbb 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -634,6 +634,11 @@ Wswitch-bool
C ObjC C++ ObjC++ Var(warn_switch_bool) Warning Init(1)
Warn about switches with boolean controlling expression.
+Wswitch-unreachable
+C ObjC C++ ObjC++ Var(warn_switch_unreachable) Warning Init(1)
+Warn about statements between switch's controlling expression and the first
+case.
+
Wtemplates
C++ ObjC++ Var(warn_templates) Warning
Warn on primary template declaration.
diff --git gcc/c/c-parser.c gcc/c/c-parser.c
index 6523c08..bdf8e8e 100644
--- gcc/c/c-parser.c
+++ gcc/c/c-parser.c
@@ -58,6 +58,7 @@ along with GCC; see the file COPYING3. If not see
#include "c-family/c-indentation.h"
#include "gimple-expr.h"
#include "context.h"
+#include "tree-iterator.h"
/* We need to walk over decls with incomplete struct/union/enum types
after parsing the whole translation unit.
@@ -5605,7 +5606,30 @@ c_parser_switch_statement (c_parser *parser, bool *if_p)
c_start_case (switch_loc, switch_cond_loc, expr, explicit_cast_p);
save_break = c_break_label;
c_break_label = NULL_TREE;
+ location_t first_loc = (c_parser_next_token_is (parser, CPP_OPEN_BRACE)
+ ? c_parser_peek_2nd_token (parser)->location
+ : c_parser_peek_token (parser)->location);
body = c_parser_c99_block_statement (parser, if_p);
+ tree first = expr_first (TREE_CODE (body) == BIND_EXPR
+ ? BIND_EXPR_BODY (body) : body);
+ /* Statements between `switch' and the first case are never executed. */
+ if (warn_switch_unreachable
+ && first != NULL_TREE
+ && TREE_CODE (first) != CASE_LABEL_EXPR
+ && TREE_CODE (first) != LABEL_EXPR)
+ {
+ if (TREE_CODE (first) == DECL_EXPR
+ && DECL_INITIAL (DECL_EXPR_DECL (first)) == NULL_TREE)
+ /* Don't warn for a declaration, but warn for an initialization. */;
+ else if (TREE_CODE (first) == GOTO_EXPR
+ && TREE_CODE (GOTO_DESTINATION (first)) == LABEL_DECL
+ && DECL_ARTIFICIAL (GOTO_DESTINATION (first)))
+ /* Don't warn for compiler-generated gotos. These occur in Duff's
+ devices, for example. */;
+ else
+ warning_at (first_loc, OPT_Wswitch_unreachable,
+ "statement will never be executed");
+ }
c_finish_case (body, ce.original_type);
if (c_break_label)
{
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index a54a0af..8f9c186 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -297,7 +297,8 @@ Objective-C and Objective-C++ Dialects}.
-Wsuggest-attribute=@r{[}pure@r{|}const@r{|}noreturn@r{|}format@r{]} @gol
-Wsuggest-final-types @gol -Wsuggest-final-methods -Wsuggest-override @gol
-Wmissing-format-attribute -Wsubobject-linkage @gol
--Wswitch -Wswitch-default -Wswitch-enum -Wswitch-bool -Wsync-nand @gol
+-Wswitch -Wswitch-default -Wswitch-enum -Wswitch-bool @gol
+-Wswitch-unreachable -Wsync-nand @gol
-Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs @gol
-Wtype-limits -Wundef @gol
-Wuninitialized -Wunknown-pragmas -Wunsafe-loop-optimizations @gol
@@ -4135,6 +4136,39 @@ switch ((int) (a == 4))
@end smallexample
This warning is enabled by default for C and C++ programs.
+@item -Wswitch-unreachable
+@opindex Wswitch-unreachable
+@opindex Wno-switch-unreachable
+Warn whenever a @code{switch} statement contains statements between the
+controlling expression and the first case label, which will never be
+executed. For example:
+@smallexample
+@group
+switch (cond)
+ @{
+ i = 15;
+ @dots{}
+ case 5:
+ @dots{}
+ @}
+@end group
+@end smallexample
+@option{-Wswitch-unreachable} will not warn if the statement between the
+controlling expression and the first case label is just a declaration:
+@smallexample
+@group
+switch (cond)
+ @{
+ int i;
+ @dots{}
+ case 5:
+ i = 5;
+ @dots{}
+ @}
+@end group
+@end smallexample
+This warning is enabled by default for C and C++ programs.
+
@item -Wsync-nand @r{(C and C++ only)}
@opindex Wsync-nand
@opindex Wno-sync-nand
diff --git gcc/testsuite/c-c++-common/goacc/sb-2.c
gcc/testsuite/c-c++-common/goacc/sb-2.c
index a6760ec..e986af3 100644
--- gcc/testsuite/c-c++-common/goacc/sb-2.c
+++ gcc/testsuite/c-c++-common/goacc/sb-2.c
@@ -4,19 +4,19 @@ void foo(int i)
{
switch (i) // { dg-error "invalid entry to OpenACC structured block" }
{
- #pragma acc parallel
+ #pragma acc parallel // { dg-warning "statement will never be executed" }
{ case 0:; }
}
switch (i) // { dg-error "invalid entry to OpenACC structured block" }
{
- #pragma acc kernels
+ #pragma acc kernels // { dg-warning "statement will never be executed" }
{ case 0:; }
}
switch (i) // { dg-error "invalid entry to OpenACC structured block" }
{
- #pragma acc data
+ #pragma acc data // { dg-warning "statement will never be executed" }
{ case 0:; }
}
}
diff --git gcc/testsuite/gcc.dg/Wjump-misses-init-1.c
gcc/testsuite/gcc.dg/Wjump-misses-init-1.c
index 86117f1..71809be 100644
--- gcc/testsuite/gcc.dg/Wjump-misses-init-1.c
+++ gcc/testsuite/gcc.dg/Wjump-misses-init-1.c
@@ -1,5 +1,5 @@
/* { dg-do compile } */
-/* { dg-options "-Wjump-misses-init" } */
+/* { dg-options "-Wjump-misses-init -Wno-switch-unreachable" } */
int
f1 (int a)
{
diff --git gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c
gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c
index e69de29..ec620d3 100644
--- gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c
+++ gcc/testsuite/gcc.dg/Wswitch-unreachable-1.c
@@ -0,0 +1,135 @@
+/* PR c/49859 */
+/* { dg-do compile } */
+/* { dg-options "-Wswitch-unreachable" } */
+
+extern void foo (int);
+extern int j;
+
+void
+fn0 (int i)
+{
+ switch (i)
+ {
+ int k;
+ case 1:
+ k = 11;
+ foo (k);
+ }
+
+ switch (i)
+ j = 10; /* { dg-warning "statement will never be executed" } */
+
+ switch (i)
+ ;
+
+ switch (i)
+ {
+ int t = 10; /* { dg-warning "statement will never be executed" } */
+ default:
+ foo (t);
+ }
+
+ switch (i)
+ {
+ j = 12; /* { dg-warning "statement will never be executed" } */
+ default:
+ foo (j);
+ }
+
+ int o;
+ switch (i)
+ {
+ o = 333; /* { dg-warning "statement will never be executed" } */
+ case 4: break;
+ default:
+ foo (o);
+ }
+
+ switch (i)
+ switch (j) /* { dg-warning "statement will never be executed" } */
+ {
+ o = 42; /* { dg-warning "statement will never be executed" } */
+ case 8:;
+ }
+
+ switch (i)
+ {
+ int l = 3; /* { dg-warning "statement will never be executed" } */
+ o = 5;
+ j = 7;
+ ++l;
+ }
+
+ switch (i)
+ if (j != 3) /* { dg-warning "statement will never be executed" } */
+ foo (j);
+
+ switch (i)
+ while (1)
+ foo (0);
+
+ switch (i)
+ while (i > 5) { }
+
+ switch (i)
+ goto X; /* { dg-warning "statement will never be executed" } */
+X:
+
+ switch (i)
+ do
+ foo (1);
+ while (1);
+
+ switch (i)
+ for (;;)
+ foo (-1);
+
+ switch (i)
+ default:
+ j = 6;
+
+ switch (i)
+ default:
+ j = sizeof (struct { int i; });
+
+ switch (i)
+ {
+ typedef int T;
+ case 3:
+ {
+ T x = 5;
+ foo (x);
+ }
+ }
+
+ switch (i)
+ {
+ static int g;
+ default:
+ foo (g);
+ }
+
+ switch (i)
+ {
+L:
+ j = 16;
+ default:
+ if (j < 5)
+ goto L;
+ break;
+ }
+
+ switch (i)
+ {
+ int A[i]; /* { dg-warning "statement will never be executed" } */
+ default: /* { dg-error "switch jumps into scope" } */
+ break;
+ }
+
+ switch (i)
+ {
+ int A[3];
+ default:
+ break;
+ }
+}
diff --git gcc/testsuite/gcc.dg/c99-vla-jump-5.c
gcc/testsuite/gcc.dg/c99-vla-jump-5.c
index fc5e04d..b5b85fd 100644
--- gcc/testsuite/gcc.dg/c99-vla-jump-5.c
+++ gcc/testsuite/gcc.dg/c99-vla-jump-5.c
@@ -15,7 +15,7 @@ void
f (int a, int b)
{
switch (a) {
- int v[b];
+ int v[b]; /* { dg-warning "statement will never be executed" } */
case 2: /* { dg-error "switch jumps into scope of identifier with variably
modified type" } */
default: /* { dg-error "switch jumps into scope of identifier with variably
modified type" } */
switch (a)
diff --git gcc/testsuite/gcc.dg/gomp/block-10.c
gcc/testsuite/gcc.dg/gomp/block-10.c
index 69ae3c0..29c2d91 100644
--- gcc/testsuite/gcc.dg/gomp/block-10.c
+++ gcc/testsuite/gcc.dg/gomp/block-10.c
@@ -5,28 +5,28 @@ void foo(int i)
int j;
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp parallel
+ #pragma omp parallel // { dg-warning "statement will never be executed" }
{ case 0:; }
}
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp for
+ #pragma omp for // { dg-warning "statement will never be executed" }
for (j = 0; j < 10; ++ j)
{ case 1:; }
}
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp critical
+ #pragma omp critical // { dg-warning "statement will never be executed" }
{ case 2:; }
}
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp master
+ #pragma omp master // { dg-warning "statement will never be executed" }
{ case 3:; }
}
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp sections
+ #pragma omp sections // { dg-warning "statement will never be executed" }
{ case 4:;
#pragma omp section
{ case 5:; }
@@ -34,7 +34,7 @@ void foo(int i)
}
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp ordered
+ #pragma omp ordered // { dg-warning "statement will never be executed" }
{ default:; }
}
}
diff --git gcc/testsuite/gcc.dg/gomp/block-9.c
gcc/testsuite/gcc.dg/gomp/block-9.c
index 2fae3de..202599f 100644
--- gcc/testsuite/gcc.dg/gomp/block-9.c
+++ gcc/testsuite/gcc.dg/gomp/block-9.c
@@ -5,7 +5,7 @@ void foo(int i)
int j;
switch (i) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp parallel
+ #pragma omp parallel // { dg-warning "statement will never be executed" }
{ case 0:; }
#pragma omp for
for (j = 0; j < 10; ++ j)
diff --git gcc/testsuite/gcc.dg/gomp/target-1.c
gcc/testsuite/gcc.dg/gomp/target-1.c
index aaa6a14..6bc5eb9 100644
--- gcc/testsuite/gcc.dg/gomp/target-1.c
+++ gcc/testsuite/gcc.dg/gomp/target-1.c
@@ -23,7 +23,7 @@ foo (int x)
switch (x) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp target
+ #pragma omp target // { dg-warning "statement will never be executed" }
{ case 0:; }
}
}
diff --git gcc/testsuite/gcc.dg/gomp/target-2.c
gcc/testsuite/gcc.dg/gomp/target-2.c
index 3a7afc4..c5c38d8 100644
--- gcc/testsuite/gcc.dg/gomp/target-2.c
+++ gcc/testsuite/gcc.dg/gomp/target-2.c
@@ -23,7 +23,7 @@ foo (int x, int y)
switch (x) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp target data map(tofrom: y)
+ #pragma omp target data map(tofrom: y) // { dg-warning "statement will never
be executed" }
{ case 0:; }
}
}
diff --git gcc/testsuite/gcc.dg/gomp/taskgroup-1.c
gcc/testsuite/gcc.dg/gomp/taskgroup-1.c
index 1997e0c..f39b7ef 100644
--- gcc/testsuite/gcc.dg/gomp/taskgroup-1.c
+++ gcc/testsuite/gcc.dg/gomp/taskgroup-1.c
@@ -23,7 +23,7 @@ foo (int x)
switch (x) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp taskgroup
+ #pragma omp taskgroup // { dg-warning "statement will never be executed" }
{ case 0:; }
}
}
diff --git gcc/testsuite/gcc.dg/gomp/teams-1.c
gcc/testsuite/gcc.dg/gomp/teams-1.c
index ad5b100..db7f50b 100644
--- gcc/testsuite/gcc.dg/gomp/teams-1.c
+++ gcc/testsuite/gcc.dg/gomp/teams-1.c
@@ -23,7 +23,7 @@ foo (int x)
switch (x) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp target teams
+ #pragma omp target teams // { dg-warning "statement will never be executed" }
{ case 0:; }
}
}
@@ -54,7 +54,7 @@ bar (int x)
switch (x) // { dg-error "invalid entry to OpenMP structured block" }
{
- #pragma omp target
+ #pragma omp target // { dg-warning "statement will never be executed" }
#pragma omp teams
{ case 0:; }
}
diff --git gcc/testsuite/gcc.dg/nested-func-1.c
gcc/testsuite/gcc.dg/nested-func-1.c
index cb26e89..2052a6f 100644
--- gcc/testsuite/gcc.dg/nested-func-1.c
+++ gcc/testsuite/gcc.dg/nested-func-1.c
@@ -1,7 +1,7 @@
/* Test for proper errors for break and continue in nested functions. */
/* Origin: Joseph Myers <j...@polyomino.org.uk> */
/* { dg-do compile } */
-/* { dg-options "" } */
+/* { dg-options "-Wno-switch-unreachable" } */
void
foo (int a)
diff --git gcc/testsuite/gcc.dg/pr67784-4.c gcc/testsuite/gcc.dg/pr67784-4.c
index 81a43fd..5462080 100644
--- gcc/testsuite/gcc.dg/pr67784-4.c
+++ gcc/testsuite/gcc.dg/pr67784-4.c
@@ -1,6 +1,6 @@
/* PR c/67784 */
/* { dg-do compile } */
-/* { dg-options "" } */
+/* { dg-options "-Wno-switch-unreachable" } */
typedef int T;
diff --git gcc/testsuite/gcc.dg/switch-warn-1.c
gcc/testsuite/gcc.dg/switch-warn-1.c
index 04ca4e3..58fbd7d 100644
--- gcc/testsuite/gcc.dg/switch-warn-1.c
+++ gcc/testsuite/gcc.dg/switch-warn-1.c
@@ -11,7 +11,7 @@ foo1 (unsigned char i)
{
switch (i)
{
- case -1: /* { dg-warning "case label value is less than minimum value
for type" } */
+ case -1: /* { dg-warning "case label value is less than minimum value
for type|statement will never be executed" } */
return 1;
case 256: /* { dg-warning "case label value exceeds maximum value for
type" } */
return 2;
Marek
