Work around gcc 4.6.0 bug that breaks WAL replay.

ReadRecord's habit of using both direct references to tmpRecPtr and
references to *RecPtr (which is pointing at tmpRecPtr) triggers an
optimization bug in gcc 4.6.0, which apparently has forgotten about
aliasing rules.  Avoid the compiler bug, and make the code more readable
to boot, by getting rid of the direct references.  Improve the comments
while at it.

Back-patch to all supported versions, in case they get built with 4.6.0.

Tom Lane, with some cosmetic suggestions from Alex Hunsaker

src/backend/access/transam/xlog.c

@@ -3728,23 +3728,32 @@ ReadRecord(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt)
 		RecPtr = &tmpRecPtr;
 
 		/*
-		 * Align recptr to next page if no more records can fit on the current
-		 * page.
+		 * RecPtr is pointing to end+1 of the previous WAL record.  We must
+		 * advance it if necessary to where the next record starts.  First,
+		 * align to next page if no more records can fit on the current page.
 		 */
 		if (XLOG_BLCKSZ - (RecPtr->xrecoff % XLOG_BLCKSZ) < SizeOfXLogRecord)
-		{
-			NextLogPage(tmpRecPtr);
-			/* We will account for page header size below */
-		}
+			NextLogPage(*RecPtr);
 
-		if (tmpRecPtr.xrecoff >= XLogFileSize)
+		/* Check for crossing of xlog segment boundary */
+		if (RecPtr->xrecoff >= XLogFileSize)
 		{
-			(tmpRecPtr.xlogid)++;
-			tmpRecPtr.xrecoff = 0;
+			(RecPtr->xlogid)++;
+			RecPtr->xrecoff = 0;
 		}
+
+		/*
+		 * If at page start, we must skip over the page header.  But we can't
+		 * do that until we've read in the page, since the header size is
+		 * variable.
+		 */
 	}
 	else
 	{
+		/*
+		 * In this case, the passed-in record pointer should already be
+		 * pointing to a valid record starting position.
+		 */
 		if (!XRecOffIsValid(RecPtr->xrecoff))
 			ereport(PANIC,
 					(errmsg("invalid record offset at %X/%X",
@@ -3773,11 +3782,13 @@ retry:
 	if (targetRecOff == 0)
 	{
 		/*
-		 * Can only get here in the continuing-from-prev-page case, because
-		 * XRecOffIsValid eliminated the zero-page-offset case otherwise. Need
-		 * to skip over the new page's header.
+		 * At page start, so skip over page header.  The Assert checks that
+		 * we're not scribbling on caller's record pointer; it's OK because we
+		 * can only get here in the continuing-from-prev-record case, since
+		 * XRecOffIsValid rejected the zero-page-offset case otherwise.
 		 */
-		tmpRecPtr.xrecoff += pageHeaderSize;
+		Assert(RecPtr == &tmpRecPtr);
+		RecPtr->xrecoff += pageHeaderSize;
 		targetRecOff = pageHeaderSize;
 	}
 	else if (targetRecOff < pageHeaderSize)

src/include/access/xlog_internal.h

@@ -154,13 +154,13 @@ typedef XLogLongPageHeaderData *XLogLongPageHeader;
 /* Align a record pointer to next page */
 #define NextLogPage(recptr) \
 	do {	\
-		if (recptr.xrecoff % XLOG_BLCKSZ != 0)	\
-			recptr.xrecoff +=	\
-				(XLOG_BLCKSZ - recptr.xrecoff % XLOG_BLCKSZ);	\
-		if (recptr.xrecoff >= XLogFileSize) \
+		if ((recptr).xrecoff % XLOG_BLCKSZ != 0)	\
+			(recptr).xrecoff +=	\
+				(XLOG_BLCKSZ - (recptr).xrecoff % XLOG_BLCKSZ);	\
+		if ((recptr).xrecoff >= XLogFileSize) \
 		{	\
-			(recptr.xlogid)++;	\
-			recptr.xrecoff = 0; \
+			((recptr).xlogid)++;	\
+			(recptr).xrecoff = 0; \
 		}	\
 	} while (0)