| Message ID | 1455582057-27565-9-git-send-email-eblake@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Eric Blake <eblake@redhat.com> writes: > By sticking the next pointer first, we don't need a union with > 64-bit padding for smaller types. On 32-bit platforms, this > can reduce the size of uint8List from 16 bytes (or 12, depending > on whether 64-bit ints can tolerate 4-byte alignment) down to 8. > It has no effect on 64-bit platforms (where alignment still > dictates a 16-byte struct); but fewer anonymous unions is still > a win in my book. > > It requires visit_next_list() to gain a size parameter, to know > what size element to allocate; comparable to the size parameter > of visit_start_struct(). > > I debated about going one step further, to allow for fewer casts, > by doing: > typedef GenericList GenericList; > struct GenericList { > GenericList *next; > }; > struct FooList { > GenericList base; > Foo value; > }; > so that you convert to 'GenericList *' by '&foolist->base', and > back by 'container_of(generic, GenericList, base)' (as opposed to > the existing '(GenericList *)foolist' and '(FooList *)generic'). > But doing that would require hoisting the declaration of > GenericList prior to inclusion of qapi-types.h, rather than its > current spot in visitor.h; it also makes iteration a bit more > verbose through 'foolist->base.next' instead of 'foolist->next'. > > Signed-off-by: Eric Blake <eblake@redhat.com> > > --- > v10: hoist earlier in series > v9: no change > v8: rebase to earlier changes > v7: new patch; probably too invasive to be worth it, especially if > we can't prove that our current size for uint8List is a bottleneck > --- > include/qapi/visitor.h | 14 +++++++------- > include/qapi/visitor-impl.h | 2 +- > scripts/qapi-types.py | 5 +---- > scripts/qapi-visit.py | 2 +- > qapi/qapi-visit-core.c | 4 ++-- > qapi/opts-visitor.c | 4 ++-- > qapi/qapi-dealloc-visitor.c | 3 ++- > qapi/qmp-input-visitor.c | 5 +++-- > qapi/qmp-output-visitor.c | 3 ++- > qapi/string-input-visitor.c | 4 ++-- > qapi/string-output-visitor.c | 2 +- > 11 files changed, 24 insertions(+), 24 deletions(-) > > diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h > index 5e581dc..c131a32 100644 > --- a/include/qapi/visitor.h > +++ b/include/qapi/visitor.h > @@ -19,13 +19,13 @@ > #include "qapi/error.h" > #include <stdlib.h> > > -typedef struct GenericList > -{ > - union { > - void *value; > - uint64_t padding; > - }; > + > +/* This struct is layout-compatible with all other *List structs > + * created by the qapi generator. It is used as a typical > + * singly-linked list. */ > +typedef struct GenericList { > struct GenericList *next; > + char padding[]; > } GenericList; > > void visit_start_struct(Visitor *v, const char *name, void **obj, > @@ -36,7 +36,7 @@ void visit_start_implicit_struct(Visitor *v, void **obj, size_t size, > void visit_end_implicit_struct(Visitor *v); > > void visit_start_list(Visitor *v, const char *name, Error **errp); > -GenericList *visit_next_list(Visitor *v, GenericList **list); > +GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size); > void visit_end_list(Visitor *v); > > /** > diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h > index ea252f8..7905a28 100644 > --- a/include/qapi/visitor-impl.h > +++ b/include/qapi/visitor-impl.h > @@ -29,7 +29,7 @@ struct Visitor > > void (*start_list)(Visitor *v, const char *name, Error **errp); > /* Must be set */ > - GenericList *(*next_list)(Visitor *v, GenericList **list); > + GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size); > /* Must be set */ > void (*end_list)(Visitor *v); > > diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py > index 7b0dca8..83f230a 100644 > --- a/scripts/qapi-types.py > +++ b/scripts/qapi-types.py > @@ -26,11 +26,8 @@ def gen_array(name, element_type): > return mcgen(''' > > struct %(c_name)s { > - union { > - %(c_type)s value; > - uint64_t padding; > - }; > %(c_name)s *next; > + %(c_type)s value; > }; > ''', > c_name=c_name(name), c_type=element_type.c_type()) > diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py > index 177dfc4..9ff0337 100644 > --- a/scripts/qapi-visit.py > +++ b/scripts/qapi-visit.py > @@ -129,7 +129,7 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error > } > > for (prev = (GenericList **)obj; > - !err && (i = visit_next_list(v, prev)) != NULL; > + !err && (i = visit_next_list(v, prev, sizeof(**obj))) != NULL; > prev = &i) { > %(c_name)s *native_i = (%(c_name)s *)i; > visit_type_%(c_elt_type)s(v, NULL, &native_i->value, &err); > diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c > index f856286..6fa66f1 100644 > --- a/qapi/qapi-visit-core.c > +++ b/qapi/qapi-visit-core.c > @@ -50,9 +50,9 @@ void visit_start_list(Visitor *v, const char *name, Error **errp) > v->start_list(v, name, errp); > } > > -GenericList *visit_next_list(Visitor *v, GenericList **list) > +GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size) > { Lost since v9: assert(size). In my review of v9, I suggested tightening the assertion to size >= GenericList. > - return v->next_list(v, list); > + return v->next_list(v, list, size); > } > > void visit_end_list(Visitor *v) > diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c > index ae5b955..73e4ace 100644 > --- a/qapi/opts-visitor.c > +++ b/qapi/opts-visitor.c > @@ -215,7 +215,7 @@ opts_start_list(Visitor *v, const char *name, Error **errp) > > > static GenericList * > -opts_next_list(Visitor *v, GenericList **list) > +opts_next_list(Visitor *v, GenericList **list, size_t size) > { > OptsVisitor *ov = to_ov(v); > GenericList **link; > @@ -258,7 +258,7 @@ opts_next_list(Visitor *v, GenericList **list) > abort(); > } > > - *link = g_malloc0(sizeof **link); > + *link = g_malloc0(size); > return *link; > } > > diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c > index 2659d3f..6667e8c 100644 > --- a/qapi/qapi-dealloc-visitor.c > +++ b/qapi/qapi-dealloc-visitor.c > @@ -100,7 +100,8 @@ static void qapi_dealloc_start_list(Visitor *v, const char *name, Error **errp) > qapi_dealloc_push(qov, NULL); > } > > -static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp) > +static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp, > + size_t size) > { > GenericList *list = *listp; > QapiDeallocVisitor *qov = to_qov(v); > diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c > index 2f48b95..2621660 100644 > --- a/qapi/qmp-input-visitor.c > +++ b/qapi/qmp-input-visitor.c > @@ -165,7 +165,8 @@ static void qmp_input_start_list(Visitor *v, const char *name, Error **errp) > qmp_input_push(qiv, qobj, errp); > } > > -static GenericList *qmp_input_next_list(Visitor *v, GenericList **list) > +static GenericList *qmp_input_next_list(Visitor *v, GenericList **list, > + size_t size) > { > QmpInputVisitor *qiv = to_qiv(v); > GenericList *entry; > @@ -184,7 +185,7 @@ static GenericList *qmp_input_next_list(Visitor *v, GenericList **list) > return NULL; > } > > - entry = g_malloc0(sizeof(*entry)); > + entry = g_malloc0(size); > if (first) { > *list = entry; > } else { > diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c > index f47eefa..d44c676 100644 > --- a/qapi/qmp-output-visitor.c > +++ b/qapi/qmp-output-visitor.c > @@ -126,7 +126,8 @@ static void qmp_output_start_list(Visitor *v, const char *name, Error **errp) > qmp_output_push(qov, list); > } > > -static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp) > +static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp, > + size_t size) > { > GenericList *list = *listp; > QmpOutputVisitor *qov = to_qov(v); > diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c > index 18b9339..59eb5dc 100644 > --- a/qapi/string-input-visitor.c > +++ b/qapi/string-input-visitor.c > @@ -139,7 +139,7 @@ start_list(Visitor *v, const char *name, Error **errp) > } > } > > -static GenericList *next_list(Visitor *v, GenericList **list) > +static GenericList *next_list(Visitor *v, GenericList **list, size_t size) > { > StringInputVisitor *siv = to_siv(v); > GenericList **link; > @@ -173,7 +173,7 @@ static GenericList *next_list(Visitor *v, GenericList **list) > link = &(*list)->next; > } > > - *link = g_malloc0(sizeof **link); > + *link = g_malloc0(size); > return *link; > } > > diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c > index b980bd3..c2e5c5b 100644 > --- a/qapi/string-output-visitor.c > +++ b/qapi/string-output-visitor.c > @@ -276,7 +276,7 @@ start_list(Visitor *v, const char *name, Error **errp) > sov->head = true; > } > > -static GenericList *next_list(Visitor *v, GenericList **list) > +static GenericList *next_list(Visitor *v, GenericList **list, size_t size) > { > StringOutputVisitor *sov = to_sov(v); > GenericList *ret = NULL; Doing this patch earlier in the series halved its size :)
diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h index 5e581dc..c131a32 100644 --- a/include/qapi/visitor.h +++ b/include/qapi/visitor.h @@ -19,13 +19,13 @@ #include "qapi/error.h" #include <stdlib.h> -typedef struct GenericList -{ - union { - void *value; - uint64_t padding; - }; + +/* This struct is layout-compatible with all other *List structs + * created by the qapi generator. It is used as a typical + * singly-linked list. */ +typedef struct GenericList { struct GenericList *next; + char padding[]; } GenericList; void visit_start_struct(Visitor *v, const char *name, void **obj, @@ -36,7 +36,7 @@ void visit_start_implicit_struct(Visitor *v, void **obj, size_t size, void visit_end_implicit_struct(Visitor *v); void visit_start_list(Visitor *v, const char *name, Error **errp); -GenericList *visit_next_list(Visitor *v, GenericList **list); +GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size); void visit_end_list(Visitor *v); /** diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h index ea252f8..7905a28 100644 --- a/include/qapi/visitor-impl.h +++ b/include/qapi/visitor-impl.h @@ -29,7 +29,7 @@ struct Visitor void (*start_list)(Visitor *v, const char *name, Error **errp); /* Must be set */ - GenericList *(*next_list)(Visitor *v, GenericList **list); + GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size); /* Must be set */ void (*end_list)(Visitor *v); diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py index 7b0dca8..83f230a 100644 --- a/scripts/qapi-types.py +++ b/scripts/qapi-types.py @@ -26,11 +26,8 @@ def gen_array(name, element_type): return mcgen(''' struct %(c_name)s { - union { - %(c_type)s value; - uint64_t padding; - }; %(c_name)s *next; + %(c_type)s value; }; ''', c_name=c_name(name), c_type=element_type.c_type()) diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py index 177dfc4..9ff0337 100644 --- a/scripts/qapi-visit.py +++ b/scripts/qapi-visit.py @@ -129,7 +129,7 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error } for (prev = (GenericList **)obj; - !err && (i = visit_next_list(v, prev)) != NULL; + !err && (i = visit_next_list(v, prev, sizeof(**obj))) != NULL; prev = &i) { %(c_name)s *native_i = (%(c_name)s *)i; visit_type_%(c_elt_type)s(v, NULL, &native_i->value, &err); diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c index f856286..6fa66f1 100644 --- a/qapi/qapi-visit-core.c +++ b/qapi/qapi-visit-core.c @@ -50,9 +50,9 @@ void visit_start_list(Visitor *v, const char *name, Error **errp) v->start_list(v, name, errp); } -GenericList *visit_next_list(Visitor *v, GenericList **list) +GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size) { - return v->next_list(v, list); + return v->next_list(v, list, size); } void visit_end_list(Visitor *v) diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c index ae5b955..73e4ace 100644 --- a/qapi/opts-visitor.c +++ b/qapi/opts-visitor.c @@ -215,7 +215,7 @@ opts_start_list(Visitor *v, const char *name, Error **errp) static GenericList * -opts_next_list(Visitor *v, GenericList **list) +opts_next_list(Visitor *v, GenericList **list, size_t size) { OptsVisitor *ov = to_ov(v); GenericList **link; @@ -258,7 +258,7 @@ opts_next_list(Visitor *v, GenericList **list) abort(); } - *link = g_malloc0(sizeof **link); + *link = g_malloc0(size); return *link; } diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c index 2659d3f..6667e8c 100644 --- a/qapi/qapi-dealloc-visitor.c +++ b/qapi/qapi-dealloc-visitor.c @@ -100,7 +100,8 @@ static void qapi_dealloc_start_list(Visitor *v, const char *name, Error **errp) qapi_dealloc_push(qov, NULL); } -static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp) +static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp, + size_t size) { GenericList *list = *listp; QapiDeallocVisitor *qov = to_qov(v); diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c index 2f48b95..2621660 100644 --- a/qapi/qmp-input-visitor.c +++ b/qapi/qmp-input-visitor.c @@ -165,7 +165,8 @@ static void qmp_input_start_list(Visitor *v, const char *name, Error **errp) qmp_input_push(qiv, qobj, errp); } -static GenericList *qmp_input_next_list(Visitor *v, GenericList **list) +static GenericList *qmp_input_next_list(Visitor *v, GenericList **list, + size_t size) { QmpInputVisitor *qiv = to_qiv(v); GenericList *entry; @@ -184,7 +185,7 @@ static GenericList *qmp_input_next_list(Visitor *v, GenericList **list) return NULL; } - entry = g_malloc0(sizeof(*entry)); + entry = g_malloc0(size); if (first) { *list = entry; } else { diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c index f47eefa..d44c676 100644 --- a/qapi/qmp-output-visitor.c +++ b/qapi/qmp-output-visitor.c @@ -126,7 +126,8 @@ static void qmp_output_start_list(Visitor *v, const char *name, Error **errp) qmp_output_push(qov, list); } -static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp) +static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp, + size_t size) { GenericList *list = *listp; QmpOutputVisitor *qov = to_qov(v); diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c index 18b9339..59eb5dc 100644 --- a/qapi/string-input-visitor.c +++ b/qapi/string-input-visitor.c @@ -139,7 +139,7 @@ start_list(Visitor *v, const char *name, Error **errp) } } -static GenericList *next_list(Visitor *v, GenericList **list) +static GenericList *next_list(Visitor *v, GenericList **list, size_t size) { StringInputVisitor *siv = to_siv(v); GenericList **link; @@ -173,7 +173,7 @@ static GenericList *next_list(Visitor *v, GenericList **list) link = &(*list)->next; } - *link = g_malloc0(sizeof **link); + *link = g_malloc0(size); return *link; } diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c index b980bd3..c2e5c5b 100644 --- a/qapi/string-output-visitor.c +++ b/qapi/string-output-visitor.c @@ -276,7 +276,7 @@ start_list(Visitor *v, const char *name, Error **errp) sov->head = true; } -static GenericList *next_list(Visitor *v, GenericList **list) +static GenericList *next_list(Visitor *v, GenericList **list, size_t size) { StringOutputVisitor *sov = to_sov(v); GenericList *ret = NULL;
By sticking the next pointer first, we don't need a union with 64-bit padding for smaller types. On 32-bit platforms, this can reduce the size of uint8List from 16 bytes (or 12, depending on whether 64-bit ints can tolerate 4-byte alignment) down to 8. It has no effect on 64-bit platforms (where alignment still dictates a 16-byte struct); but fewer anonymous unions is still a win in my book. It requires visit_next_list() to gain a size parameter, to know what size element to allocate; comparable to the size parameter of visit_start_struct(). I debated about going one step further, to allow for fewer casts, by doing: typedef GenericList GenericList; struct GenericList { GenericList *next; }; struct FooList { GenericList base; Foo value; }; so that you convert to 'GenericList *' by '&foolist->base', and back by 'container_of(generic, GenericList, base)' (as opposed to the existing '(GenericList *)foolist' and '(FooList *)generic'). But doing that would require hoisting the declaration of GenericList prior to inclusion of qapi-types.h, rather than its current spot in visitor.h; it also makes iteration a bit more verbose through 'foolist->base.next' instead of 'foolist->next'. Signed-off-by: Eric Blake <eblake@redhat.com> --- v10: hoist earlier in series v9: no change v8: rebase to earlier changes v7: new patch; probably too invasive to be worth it, especially if we can't prove that our current size for uint8List is a bottleneck --- include/qapi/visitor.h | 14 +++++++------- include/qapi/visitor-impl.h | 2 +- scripts/qapi-types.py | 5 +---- scripts/qapi-visit.py | 2 +- qapi/qapi-visit-core.c | 4 ++-- qapi/opts-visitor.c | 4 ++-- qapi/qapi-dealloc-visitor.c | 3 ++- qapi/qmp-input-visitor.c | 5 +++-- qapi/qmp-output-visitor.c | 3 ++- qapi/string-input-visitor.c | 4 ++-- qapi/string-output-visitor.c | 2 +- 11 files changed, 24 insertions(+), 24 deletions(-)