Re: [PR] mm: Add pending configuration for mm node struct and precding [nuttx]

[via GitHub]

W-M-R commented on code in PR #15484:
URL: https://github.com/apache/nuttx/pull/15484#discussion_r1912647164


##########
mm/mm_heap/mm.h:
##########
@@ -173,7 +183,16 @@ typedef size_t mmsize_t;
 
 struct mm_allocnode_s
 {
+#ifdef CONFIG_MM_NODE_PENDING
+  union
+  {
+    mmsize_t preceding;                     /* Physical preceding chunk size */

Review Comment:
   It is invalid to do aligned_data(MM_ALIGN) on precding alone.
   ```
   #include <stdio.h>
   
   
   struct st0
   {
       char a;
       char b;
   }__attribute__((aligned(16)));
   
   struct st1
   {
       char a __attribute__((aligned(16)));
       char b;
   };
   
   struct st2
   {
       char a;
       char b __attribute__((aligned(16)));
   };
   
   struct st3
   {
       char a __attribute__((aligned(16)));
       char b __attribute__((aligned(16)));
       char d[15];
   }__attribute__((aligned(16)));
   
   struct st4
   {
       union
       {
           char a;
           char a_align[16];
       };
   
       union
       {
           char b;
           char b_align[16];
       };
   };
   
   #define OFFSET(struct, member) (&(((struct *)0)->member))
   
   #define PRINTF_STRUCT(struct) \
       printf("%s: ", #struct); \
       printf("size: %d\n", sizeof(struct)); \
       printf("member a: %d\n", OFFSET(struct, a)); \
       printf("member b: %d\n", OFFSET(struct, b));
   
   int main()
   {
       PRINTF_STRUCT(struct st0);
       PRINTF_STRUCT(struct st1);
       PRINTF_STRUCT(struct st2);
       PRINTF_STRUCT(struct st3);
       PRINTF_STRUCT(struct st4);
   }
   
   
   ```
   ```
   struct st0: size: 16
   member a: 0
   member b: 1
   struct st1: size: 16
   member a: 0
   member b: 1
   struct st2: size: 32
   member a: 0
   member b: 16
   struct st3: size: 32
   member a: 0
   member b: 16
   struct st4: size: 32
   member a: 0
   member b: 16
   ```
   My understanding is that the alignment of __attribute__((aligned(16))) is 
more about the alignment of the size of the entire structure, and the alignment 
of a single member is limited to the alignment of access, not size.



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