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C PATCH to add -Wswitch-unreachable (PR c/49859)


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  <polacek@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 <jsm@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


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