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Mostly-cosmetic improvements in memory chunk header alignment coding.

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Add commentary about what we're doing and why.  Apply the method used for
padding in GenerationChunk to AllocChunkData, replacing the rather ad-hoc
solution used in commit 7e3aa03b4.  Reorder fields in GenerationChunk so
that the padding calculation will work even if sizeof(size_t) is different
from sizeof(void *) --- likely that will never happen, but we don't need
the assumption if we do it like this.  Improve static assertions about
alignment.

In passing, fix a couple of oversights in the "large chunk" path in
GenerationAlloc().

Discussion: https://postgr.es/m/E1eHa4J-0006hI-Q8@gemulon.postgresql.org
This commit is contained in:
Tom Lane
2017-11-24 15:50:22 -05:00
parent cc3c4af4a9
commit f65d21b258
3 changed files with 62 additions and 42 deletions
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21
src/backend/utils/mmgr/slab.c
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@@ -91,12 +91,18 @@ typedef struct SlabBlock
/*
* SlabChunk
* The prefix of each piece of memory in an SlabBlock
* The prefix of each piece of memory in a SlabBlock
*
* Note: to meet the memory context APIs, the payload area of the chunk must
* be maxaligned, and the "slab" link must be immediately adjacent to the
* payload area (cf. GetMemoryChunkContext). Since we support no machines on
* which MAXALIGN is more than twice sizeof(void *), this happens without any
* special hacking in this struct declaration. But there is a static
* assertion below that the alignment is done correctly.
*/
typedef struct SlabChunk
{
/* block owning this chunk */
void *block;
SlabBlock *block; /* block owning this chunk */
SlabContext *slab; /* owning context */
/* there must not be any padding to reach a MAXALIGN boundary here! */
} SlabChunk;
@@ -190,8 +196,11 @@ SlabContextCreate(MemoryContext parent,
Size freelistSize;
SlabContext *slab;
/* Assert we padded SlabChunk properly */
StaticAssertStmt(sizeof(SlabChunk) == MAXALIGN(sizeof(SlabChunk)),
"sizeof(SlabChunk) is not maxaligned");
StaticAssertStmt(offsetof(SlabChunk, slab) + sizeof(MemoryContext) ==
MAXALIGN(sizeof(SlabChunk)),
sizeof(SlabChunk),
"padding calculation in SlabChunk is wrong");
/* Make sure the linked list node fits inside a freed chunk */
@@ -199,7 +208,7 @@ SlabContextCreate(MemoryContext parent,
chunkSize = sizeof(int);
/* chunk, including SLAB header (both addresses nicely aligned) */
fullChunkSize = MAXALIGN(sizeof(SlabChunk) + MAXALIGN(chunkSize));
fullChunkSize = sizeof(SlabChunk) + MAXALIGN(chunkSize);
/* Make sure the block can store at least one chunk. */
if (blockSize - sizeof(SlabBlock) < fullChunkSize)
@@ -443,7 +452,7 @@ SlabAlloc(MemoryContext context, Size size)
/* Prepare to initialize the chunk header. */
VALGRIND_MAKE_MEM_UNDEFINED(chunk, sizeof(SlabChunk));
chunk->block = (void *) block;
chunk->block = block;
chunk->slab = slab;
#ifdef MEMORY_CONTEXT_CHECKING