On Fri, Nov 24, 2017 at 10:59:53PM +0100, Jakub Jelinek wrote:
> The testcase below has a useless break; that causes a bogus -Wreturn-type
> warning. The C++ FE already has code to avoid adding a BREAK_STMT
> after a return or similar sequence that is known not to return.
> The following patch extends block_may_fallthrough to also return false
> for SWITCH_STMTs that can't fall through.
>
> Those are ones with non-empty body where the whole body can't fallthrough,
> additionally they need to have a default: case label (or cover the whole
> range of values, but that is not what this patch can compute, that would
> be too big duplication of the gimplification processing) and no BREAK_STMT.
>
> For the default: case label we need to look in all SWITCH_BODY children
> except for nested SWITCH_STMTs, for BREAK_STMTs also not in
> {FOR,DO,WHILE}_BODY.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Actually, thinking about it some more, maybe it would be more efficient
to gather this information during construction of the SWITCH_STMT in some
new flag on the tree, so cxx_block_may_fallthru would just:
case SWITCH_STMT:
if (SWITCH_STMT_CANT_FALLTHRU_P (stmt)
&& SWITCH_STMT_BODY (stmt)
&& !block_may_fallthru (SWITCH_STMT_BODY (stmt)))
return false;
return true;
and
/* Set if the body of a switch stmt contains a default: case label
and does not contain any break; stmts, thus if SWITCH_STMT_BODY
is not empty and doesn't fallthru, then the whole switch stmt
can't. */
#define SWITCH_STMT_CANT_FALLTHRU_P(NODE) \
TREE_LANG_FLAG_0 (SWITCH_STMT_CHECK (NODE))
Seems the C++ FE already has switch_stack, so we could just add there
a has_default_p, has_break_stmt_p and inside_loop_p flags and both
inside of templates and outside in finish_case_label, in
finish_break_stmt if actually adding BREAK_STMT and when entering
a body of a FOR/DO/WHILE loop tweak those flags. Seems switch_stack
is also maintained during pt.c, but we should compute it both during
parsing and during pt.c (start with the bit clear on a new SWITCH_STMT).
Thoughts on this?
Guess for the C FE we should similarly handle SWITCH_EXPR, though a break;
in there is a goto outside of the switch, so all we need to note is a
default: case label.
Jakub
