[postgis-tickets] [PostGIS] #4368: Reorder struct layouts to minimize padding

[PostGIS]

#4368: Reorder struct layouts to minimize padding
-------------------------+---------------------------
 Reporter:  Algunenano   |      Owner:  Algunenano
     Type:  enhancement  |     Status:  assigned
 Priority:  medium       |  Milestone:  PostGIS 3.0.0
Component:  postgis      |    Version:  trunk
 Keywords:               |
-------------------------+---------------------------
 The current LWGEOM structs have too much padding which makes them bigger
 than what they could be.

 For example, currently they look like this:
 {{{
 typedef struct
 {
     uint8_t type;
     uint8_t flags;
     GBOX *bbox;
     int32_t srid;
     void *data;
 }
 LWGEOM;
 }}}

 When you build them in a 64 bit machine which I'm considering the default,
 the struct looks like this:
 {{{
 {
     uint8_t type;     /* 1 byte */
     uint8_t flags;    /* 1 byte */
                       /* 6 bytes padding */
     GBOX *bbox;       /* 8 bytes */
     int32_t srid;     /* 4 bytes */
                       /* 4 bytes padding */
     void *data;       /* 8 bytes */
 }
 }}}

 For a total of 32 bytes.

 I'm proposing to use this instead:
 {{{
 {
         GBOX *bbox;    /* 8 bytes */
         void *data;    /* 8 bytes */
         int32_t srid;  /* 4 bytes */
         uint8_t type;  /* 1 byte */
         uint8_t flags; /* 1 byte */
         char pad[2];   /* 2 bytes of padding */
 }
 }}}

 So the new way uses 24 bytes instead (25% less). I expect this to have a
 impact in big multigeometries as it reduces their memory footprint which
 should improve performance. OTOH, I want to test it first as changing the
 position of certain elements might be harmful in certain scenarios.

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/4368>
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