[patch] Avoid generating warnings for -O2 -Wunreachable-code
Richard Guenther
richard.guenther@gmail.com
Thu Nov 5 09:45:00 GMT 2009
On Thu, Nov 5, 2009 at 10:38 AM, Eric Fisher <joefoxreal@gmail.com> wrote:
> 2009/11/4 Richard Guenther <richard.guenther@gmail.com>:
>
>> -Wunreachable-code is seriously broken, I suggest to remove its
>> implementation completely.
>>
>> Richard.
>>
>
> I have no idea about this. At least, we shouldn't count on gcc emiting
> correct warnings of -Wunreachable-code when it does optimization. So
> the testcases should only test flag -Wunreachable-code with -O0. Thus,
> one solution is just to modify the testsuite.
>
> If we plan don't support -Wunreachable-code any more, here is a patch.
The patch should retain -Wunreachable-code as a no-op
for backward compatibility. And it needs a changelog ;)
The patch is ok to apply in a week if nobody raises serious objections.
Thanks,
Richard.
> Index: doc/invoke.texi
> ===================================================================
> --- doc/invoke.texi (revision 153929)
> +++ doc/invoke.texi (working copy)
> @@ -259,7 +259,7 @@ Objective-C and Objective-C++ Dialects}.
> -Wstrict-overflow -Wstrict-overflow=@var{n} @gol
> -Wswitch -Wswitch-default -Wswitch-enum -Wsync-nand @gol
> -Wsystem-headers -Wtrigraphs -Wtype-limits -Wundef -Wuninitialized @gol
> --Wunknown-pragmas -Wno-pragmas -Wunreachable-code @gol
> +-Wunknown-pragmas -Wno-pragmas @gol
> -Wunsuffixed-float-constants -Wunused -Wunused-function @gol
> -Wunused-label -Wunused-parameter -Wno-unused-result -Wunused-value
> -Wunused-variable @gol
> -Wvariadic-macros -Wvla @gol
> @@ -4188,29 +4188,6 @@ cases where multiple declaration is vali
> @opindex Wno-nested-externs
> Warn if an @code{extern} declaration is encountered within a function.
>
> -@item -Wunreachable-code
> -@opindex Wunreachable-code
> -@opindex Wno-unreachable-code
> -Warn if the compiler detects that code will never be executed.
> -
> -This option is intended to warn when the compiler detects that at
> -least a whole line of source code will never be executed, because
> -some condition is never satisfied or because it is after a
> -procedure that never returns.
> -
> -It is possible for this option to produce a warning even though there
> -are circumstances under which part of the affected line can be executed,
> -so care should be taken when removing apparently-unreachable code.
> -
> -For instance, when a function is inlined, a warning may mean that the
> -line is unreachable in only one inlined copy of the function.
> -
> -This option is not made part of @option{-Wall} because in a debugging
> -version of a program there is often substantial code which checks
> -correct functioning of the program and is, hopefully, unreachable
> -because the program does work. Another common use of unreachable
> -code is to provide behavior which is selectable at compile-time.
> -
> @item -Winline
> @opindex Winline
> @opindex Wno-inline
> Index: testsuite/gcc.dg/Wunreachable-1.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-1.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-1.c (working copy)
> @@ -1,24 +0,0 @@
> -/* { dg-do compile } */
> -/* { dg-options "-O2 -Wunreachable-code" } */
> -
> -extern void foo (void);
> -extern void baz (void);
> -
> -void bar (int i)
> -{
> - if (i < 2)
> - {
> - baz ();
> - return;
> - }
> - else
> - {
> - if (i >= 4 && i <= 5)
> - foo ();
> - return;
> - }
> -
> - baz (); /* { dg-warning "will never be executed" "" } */
> - baz ();
> - baz ();
> -}
> Index: testsuite/gcc.dg/Wunreachable-5.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-5.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-5.c (working copy)
> @@ -1,17 +0,0 @@
> -/* PR c/10175 */
> -
> -/* { dg-do compile } */
> -/* { dg-options "-Wunreachable-code" } */
> -
> -int value;
> -
> -int main(void)
> -{
> - if (0)
> - value = 0; /* { dg-warning "will never be executed" "" } */
> - else
> - value = 1;
> -
> - return 0;
> -}
> -
> Index: testsuite/gcc.dg/Wunreachable-2.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-2.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-2.c (working copy)
> @@ -1,19 +0,0 @@
> -/* { dg-do compile } */
> -/* { dg-options "-O2 -Wunreachable-code" } */
> -
> -extern int foo (const char *);
> -extern void baz (void);
> -const char *a[] = { "one", "two" };
> -
> -void bar (void)
> -{
> - int i;
> -
> - for (i = 0; i < 2; i++) /* { dg-bogus "will never be executed" ""
> { xfail *-*-* } } */
> - if (! foo (a[i])) /* { dg-bogus "will never be executed" "" {
> xfail *-*-* } } */
> - return;
> -
> - baz (); /* { dg-bogus "will never be executed" } */
> - baz ();
> - baz ();
> -}
> Index: testsuite/gcc.dg/Wunreachable-6.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-6.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-6.c (working copy)
> @@ -1,21 +0,0 @@
> -/* PR c/11370 */
> -/* { dg-do compile } */
> -/* { dg-options "-Wunreachable-code" } */
> -
> -extern int printf (const char *, ...);
> -extern void exit (int);
> -
> -int main(int argc, char *argv[])
> -{
> - if (argc != 1)
> - exit(1);
> -
> - {
> - int ix; /* { dg-bogus "will never be executed" } */
> - ix = printf("hello\n");
> - printf("%d\n", ix);
> - }
> -
> - return 0;
> -}
> -
> Index: testsuite/gcc.dg/pr12603.c
> ===================================================================
> --- testsuite/gcc.dg/pr12603.c (revision 153929)
> +++ testsuite/gcc.dg/pr12603.c (working copy)
> @@ -1,6 +1,6 @@
> /* PR 12603: No return statement warning on function that never
> returns with -O3. */
> /* { dg-do compile } */
> -/* { dg-options "-O3 -Wall -Wextra -Wreturn-type -Wunreachable-code" } */
> +/* { dg-options "-O3 -Wall -Wextra -Wreturn-type" } */
> int
> this_function_never_returns ()
> {
> Index: testsuite/gcc.dg/Wunreachable-3.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-3.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-3.c (working copy)
> @@ -1,17 +0,0 @@
> -/* PR c/10175 */
> -/* { dg-do compile } */
> -/* { dg-options "-Wunreachable-code" } */
> -
> -int i,j;
> -int main(void)
> -{
> - if (0) {
> - i = 0; /* { dg-warning "will never be executed" "" } */
> - j = 0;
> - } else {
> - i = 1;
> - j = 1;
> - }
> -
> - return 0;
> -}
> Index: testsuite/gcc.dg/Wunreachable-7.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-7.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-7.c (working copy)
> @@ -1,21 +0,0 @@
> -/* PR c/11370 */
> -/* { dg-do compile } */
> -/* { dg-options "-O2 -Wunreachable-code" } */
> -
> -extern int printf (const char *, ...);
> -extern void exit (int);
> -
> -int main(int argc, char *argv[])
> -{
> - if (argc != 1)
> - exit(1);
> -
> - {
> - int ix; /* { dg-bogus "will never be executed" } */
> - ix = printf("hello\n");
> - printf("%d\n", ix);
> - }
> -
> - return 0;
> -}
> -
> Index: testsuite/gcc.dg/20041231-1.c
> ===================================================================
> --- testsuite/gcc.dg/20041231-1.c (revision 153929)
> +++ testsuite/gcc.dg/20041231-1.c (working copy)
> @@ -1,15 +0,0 @@
> -/* PR17544 Incorrect -Wunreachable-code warning
> - Origin: Giovanni Bajo
> -
> - In C99 we append a "return 0;" when finishing a function, but it was
> - not given a source location. The gimplifier thinks a return statement
> - needs a locus so it would add one, making the compiler generated code
> - visible to the unreachable code warning. */
> -
> -/* { dg-do compile } */
> -/* { dg-options "-std=c99 -O -Wunreachable-code" } */
> -
> -int main (void) // 1
> -{ // 2
> - return 0; // 3
> -} // 4
> Index: testsuite/gcc.dg/Wunreachable-4.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-4.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-4.c (working copy)
> @@ -1,12 +0,0 @@
> -/* PR middle-end/10336 */
> -/* { dg-options "-Wunreachable-code" } */
> -
> -void foo(int i)
> -{
> - switch(i) {
> - case 0:
> - break;
> - case 1:
> - break;
> - }
> -}
> Index: testsuite/gcc.dg/Wunreachable-8.c
> ===================================================================
> --- testsuite/gcc.dg/Wunreachable-8.c (revision 153929)
> +++ testsuite/gcc.dg/Wunreachable-8.c (working copy)
> @@ -1,20 +0,0 @@
> -/* { dg-do compile } */
> -/* { dg-options "-O2 -Wunreachable-code" } */
> -float Factorial(float X)
> -{
> - float val = 1.0;
> - int k,j;
> - for (k=1; k < 5; k++) /* { dg-bogus "will never be executed" "" {
> xfail *-*-* } } */
> - {
> - val += 1.0; /* { dg-bogus "will never be executed" "" { xfail
> *-*-* } } */
> - }
> - return (val); /* { dg-bogus "will never be executed" } */
> -}
> -
> -int main (void)
> -{
> - float result;
> - result=Factorial(2.1);
> - return (0);
> -}
> -
> Index: common.opt
> ===================================================================
> --- common.opt (revision 153929)
> +++ common.opt (working copy)
> @@ -200,10 +200,6 @@ Wuninitialized
> Common Var(warn_uninitialized) Init(-1) Warning
> Warn about uninitialized automatic variables
>
> -Wunreachable-code
> -Common Var(warn_notreached) Warning
> -Warn about code that will never be executed
> -
> Wunused
> Common Var(warn_unused) Init(0) Warning
> Enable all -Wunused- warnings
> Index: tree-cfg.c
> ===================================================================
> --- tree-cfg.c (revision 153929)
> +++ tree-cfg.c (working copy)
> @@ -1760,7 +1760,6 @@ static void
> remove_bb (basic_block bb)
> {
> gimple_stmt_iterator i;
> - source_location loc = UNKNOWN_LOCATION;
>
> if (dump_file)
> {
> @@ -1830,24 +1829,9 @@ remove_bb (basic_block bb)
> i = gsi_last_bb (bb);
> else
> gsi_prev (&i);
> -
> - /* Don't warn for removed gotos. Gotos are often removed due to
> - jump threading, thus resulting in bogus warnings. Not great,
> - since this way we lose warnings for gotos in the original
> - program that are indeed unreachable. */
> - if (gimple_code (stmt) != GIMPLE_GOTO
> - && gimple_has_location (stmt))
> - loc = gimple_location (stmt);
> }
> }
>
> - /* If requested, give a warning that the first statement in the
> - block is unreachable. We walk statements backwards in the
> - loop above, so the last statement we process is the first statement
> - in the block. */
> - if (loc > BUILTINS_LOCATION && LOCATION_LINE (loc) > 0)
> - warning_at (loc, OPT_Wunreachable_code, "will never be executed");
> -
> remove_phi_nodes_and_edges_for_unreachable_block (bb);
> bb->il.gimple = NULL;
> }
>
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