From: Josh Triplett on
On Sat, Jul 10, 2010 at 10:39:22AM +0200, Jiri Slaby wrote:
> Gcc 4.5 defines
> extern void __builtin_unreachable(void);
> so, add it also to sparse.
>
> Signed-off-by: Jiri Slaby <jslaby(a)suse.cz>
> ---
> lib.c | 1 +
> 1 files changed, 1 insertions(+), 0 deletions(-)
>
> diff --git a/lib.c b/lib.c
> index a218bfc..ae6a20c 100644
> --- a/lib.c
> +++ b/lib.c
> @@ -740,6 +740,7 @@ void declare_builtin_functions(void)
> add_pre_buffer ("extern char * __builtin___strncpy_chk(char *, const char *, __SIZE_TYPE__, __SIZE_TYPE__);\n");
> add_pre_buffer ("extern int __builtin___vsprintf_chk(char *, int, __SIZE_TYPE__, const char *, __builtin_va_list);\n");
> add_pre_buffer ("extern int __builtin___vsnprintf_chk(char *, __SIZE_TYPE__, int, __SIZE_TYPE__, const char *, __builtin_va_list ap);\n");
> + add_pre_buffer ("extern void __builtin_unreachable(void);\n");

__builtin_unreachable has special semantics beyond just a function.
This definition will suffice to allow compilation, but
__builtin_unreachable should have the same effect in sparse that it does
in GCC: mark the point (and the remainder of the basic block) as
unreachable. Something like the mechanism used for handling noreturn
would work here as well; declaring the function to have attribute
noreturn would probably have almost the right semantics.

- Josh Triplett
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From: Chris Li on
On Sat, Jul 10, 2010 at 2:07 AM, Josh Triplett <josh(a)joshtriplett.org> wrote:
> __builtin_unreachable has special semantics beyond just a function.
> This definition will suffice to allow compilation, but
> __builtin_unreachable should have the same effect in sparse that it does
> in GCC: mark the point (and the remainder of the basic block) as
> unreachable. �Something like the mechanism used for handling noreturn
> would work here as well; declaring the function to have attribute
> noreturn would probably have almost the right semantics.
>

The attribute noreturn will apply to the whole function. The function
NEVER returns.
__builtin_unreachable only apply to current basic block. e.g. some
error handling path like panic. The function can still return a value on the
normal path. It has different meaning than attribute noreturn. So I don't think
automatically give the function noreturn attribute is the right thing to do.

I will apply the patch until we got better way to handle this.

Chris
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From: Josh Triplett on
On Tue, Jul 13, 2010 at 12:52:48AM -0700, Chris Li wrote:
> On Sat, Jul 10, 2010 at 2:07 AM, Josh Triplett <josh(a)joshtriplett.org> wrote:
> > __builtin_unreachable has special semantics beyond just a function.
> > This definition will suffice to allow compilation, but
> > __builtin_unreachable should have the same effect in sparse that it does
> > in GCC: mark the point (and the remainder of the basic block) as
> > unreachable. �Something like the mechanism used for handling noreturn
> > would work here as well; declaring the function to have attribute
> > noreturn would probably have almost the right semantics.
> >
>
> The attribute noreturn will apply to the whole function. The function
> NEVER returns.
> __builtin_unreachable only apply to current basic block. e.g. some
> error handling path like panic. The function can still return a value on the
> normal path. It has different meaning than attribute noreturn. So I don't think
> automatically give the function noreturn attribute is the right thing to do.

No, I didn't mean that using __builtin_unreachable should mark the
function calling it as noreturn. I meant that as an approximation to
the right behavior, __builtin_unreachable *itself* could have attribute
noreturn.

- Josh Triplett
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From: Chris Li on
On Tue, Jul 13, 2010 at 11:12 AM, Josh Triplett <josh(a)joshtriplett.org> wrote:
> No, I didn't mean that using __builtin_unreachable should mark the
> function calling it as noreturn. �I meant that as an approximation to
> the right behavior, __builtin_unreachable *itself* could have attribute
> noreturn.

Ah, that make sense.

Chris
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