Avoid using potentially-under-aligned page buffers.

There's a project policy against using plain "char buf[BLCKSZ]" local
or static variables as page buffers; preferred style is to palloc or
malloc each buffer to ensure it is MAXALIGN'd.  However, that policy's
been ignored in an increasing number of places.  We've apparently got
away with it so far, probably because (a) relatively few people use
platforms on which misalignment causes core dumps and/or (b) the
variables chance to be sufficiently aligned anyway.  But this is not
something to rely on.  Moreover, even if we don't get a core dump,
we might be paying a lot of cycles for misaligned accesses.

To fix, invent new union types PGAlignedBlock and PGAlignedXLogBlock
that the compiler must allocate with sufficient alignment, and use
those in place of plain char arrays.

I used these types even for variables where there's no risk of a
misaligned access, since ensuring proper alignment should make
kernel data transfers faster.  I also changed some places where
we had been palloc'ing short-lived buffers, for coding style
uniformity and to save palloc/pfree overhead.

Since this seems to be a live portability hazard (despite the lack
of field reports), back-patch to all supported versions.

Patch by me; thanks to Michael Paquier for review.

Discussion: https://postgr.es/m/[email protected]

1 files changed, 6 insertions, 6 deletions

diff --git a/contrib/bloom/blinsert.c b/contrib/bloom/blinsert.c
index 4afdea7c9a3..9f223d3b2a7 100644
--- a/contrib/bloom/blinsert.c
+++ b/contrib/bloom/blinsert.c
@@ -36,7 +36,7 @@ typedef struct
 	int64		indtuples;		/* total number of tuples indexed */
 	MemoryContext tmpCtx;		/* temporary memory context reset after each
 								 * tuple */
-	char		data[BLCKSZ];	/* cached page */
+	PGAlignedBlock data;		/* cached page */
 	int			count;			/* number of tuples in cached page */
 } BloomBuildState;
 
@@ -52,7 +52,7 @@ flushCachedPage(Relation index, BloomBuildState *buildstate)
 
 	state = GenericXLogStart(index);
 	page = GenericXLogRegisterBuffer(state, buffer, GENERIC_XLOG_FULL_IMAGE);
-	memcpy(page, buildstate->data, BLCKSZ);
+	memcpy(page, buildstate->data.data, BLCKSZ);
 	GenericXLogFinish(state);
 	UnlockReleaseBuffer(buffer);
 }
@@ -63,8 +63,8 @@ flushCachedPage(Relation index, BloomBuildState *buildstate)
 static void
 initCachedPage(BloomBuildState *buildstate)
 {
-	memset(buildstate->data, 0, BLCKSZ);
-	BloomInitPage(buildstate->data, 0);
+	memset(buildstate->data.data, 0, BLCKSZ);
+	BloomInitPage(buildstate->data.data, 0);
 	buildstate->count = 0;
 }
 
@@ -84,7 +84,7 @@ bloomBuildCallback(Relation index, HeapTuple htup, Datum *values,
 	itup = BloomFormTuple(&buildstate->blstate, &htup->t_self, values, isnull);
 
 	/* Try to add next item to cached page */
-	if (BloomPageAddItem(&buildstate->blstate, buildstate->data, itup))
+	if (BloomPageAddItem(&buildstate->blstate, buildstate->data.data, itup))
 	{
 		/* Next item was added successfully */
 		buildstate->count++;
@@ -98,7 +98,7 @@ bloomBuildCallback(Relation index, HeapTuple htup, Datum *values,
 
 		initCachedPage(buildstate);
 
-		if (!BloomPageAddItem(&buildstate->blstate, buildstate->data, itup))
+		if (!BloomPageAddItem(&buildstate->blstate, buildstate->data.data, itup))
 		{
 			/* We shouldn't be here since we're inserting to the empty page */
 			elog(ERROR, "could not add new bloom tuple to empty page");