When compiling at -Og:

  In file included from
  /builds/xen-project/people/andyhhp/xen/xen/include/asm/domain.h:4:0,
                   from /builds/xen-project/people/andyhhp/xen/xen/include/xen/domain.h:8,
                   from /builds/xen-project/people/andyhhp/xen/xen/include/xen/sched.h:11,
                   from /builds/xen-project/people/andyhhp/xen/xen/include/xen/ioreq.h:22,
                   from common.c:23:
  common.c: In function '_shadow_prealloc':
  /builds/xen-project/people/andyhhp/xen/xen/include/xen/mm.h:252:55: error: 't' may be used uninitialized in this function [-Werror=maybe-uninitialized]
       return page != head->next ? pdx_to_page(page->list.prev) : NULL;
                                                         ^
  common.c:933:28: note: 't' was declared here
       struct page_info *sp, *t;
                              ^

I'm not certain the analysis is correct.  't' is a temporary variable, and is
clearly initialised before use in foreach_pinned_shadow().  Either way,
initialising it to NULL placates the compiler.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Tim Deegan <tim@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>
---
 xen/arch/x86/mm/shadow/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index b99ca14e71..737e6b365a 100644
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -931,7 +931,7 @@ static inline void trace_shadow_prealloc_unpin(struct domain *d, mfn_t smfn)
 static void _shadow_prealloc(struct domain *d, unsigned int pages)
 {
     struct vcpu *v;
-    struct page_info *sp, *t;
+    struct page_info *sp, *t = NULL;
     mfn_t smfn;
     int i;
 
-- 
2.11.0

At 15:01 +0100 on 19 Apr (1618844491), Andrew Cooper wrote:
> When compiling at -Og:
> 
>   In file included from
>   /builds/xen-project/people/andyhhp/xen/xen/include/asm/domain.h:4:0,
>                    from /builds/xen-project/people/andyhhp/xen/xen/include/xen/domain.h:8,
>                    from /builds/xen-project/people/andyhhp/xen/xen/include/xen/sched.h:11,
>                    from /builds/xen-project/people/andyhhp/xen/xen/include/xen/ioreq.h:22,
>                    from common.c:23:
>   common.c: In function '_shadow_prealloc':
>   /builds/xen-project/people/andyhhp/xen/xen/include/xen/mm.h:252:55: error: 't' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>        return page != head->next ? pdx_to_page(page->list.prev) : NULL;
>                                                          ^
>   common.c:933:28: note: 't' was declared here
>        struct page_info *sp, *t;
>                               ^
> 
> I'm not certain the analysis is correct.  't' is a temporary variable, and is
> clearly initialised before use in foreach_pinned_shadow().  Either way,
> initialising it to NULL placates the compiler.

Yeah, this analysis seems wrong to me too - if nothing else, why does
it not complain about the identical code in shadow_blow_tables() below?

That said, since the non-debug build doesn't complain here, presumably it
will be able to elide this dead store.

Acked-by: Tim Deegan <tim@xen.org>