Zero initialize uses of instr_time about to trigger compiler warnings

These are all not necessary from a correctness POV. However, in the near
future instr_time will be simplified to an int64, at which point gcc would
otherwise start to warn about the changed places.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20230116023639.rn36vf6ajqmfciua@awork3.anarazel.de

src/backend/access/transam/xlog.c

@@ -2191,6 +2191,8 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
 				/* Measure I/O timing to write WAL data */
 				if (track_wal_io_timing)
 					INSTR_TIME_SET_CURRENT(start);
+				else
+					INSTR_TIME_SET_ZERO(start);
 
 				pgstat_report_wait_start(WAIT_EVENT_WAL_WRITE);
 				written = pg_pwrite(openLogFile, from, nleft, startoffset);
@@ -8151,6 +8153,8 @@ issue_xlog_fsync(int fd, XLogSegNo segno, TimeLineID tli)
 	/* Measure I/O timing to sync the WAL file */
 	if (track_wal_io_timing)
 		INSTR_TIME_SET_CURRENT(start);
+	else
+		INSTR_TIME_SET_ZERO(start);
 
 	pgstat_report_wait_start(WAIT_EVENT_WAL_SYNC);
 	switch (sync_method)

src/backend/storage/buffer/bufmgr.c

@@ -1017,6 +1017,8 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
 
 			if (track_io_timing)
 				INSTR_TIME_SET_CURRENT(io_start);
+			else
+				INSTR_TIME_SET_ZERO(io_start);
 
 			smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
 
@@ -2902,6 +2904,8 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln)
 
 	if (track_io_timing)
 		INSTR_TIME_SET_CURRENT(io_start);
+	else
+		INSTR_TIME_SET_ZERO(io_start);
 
 	/*
 	 * bufToWrite is either the shared buffer or a copy, as appropriate.

src/backend/storage/file/buffile.c

@@ -446,6 +446,8 @@ BufFileLoadBuffer(BufFile *file)
 
 	if (track_io_timing)
 		INSTR_TIME_SET_CURRENT(io_start);
+	else
+		INSTR_TIME_SET_ZERO(io_start);
 
 	/*
 	 * Read whatever we can get, up to a full bufferload.
@@ -525,6 +527,8 @@ BufFileDumpBuffer(BufFile *file)
 
 		if (track_io_timing)
 			INSTR_TIME_SET_CURRENT(io_start);
+		else
+			INSTR_TIME_SET_ZERO(io_start);
 
 		bytestowrite = FileWrite(thisfile,
 								 file->buffer.data + wpos,

src/backend/storage/ipc/latch.c

@@ -1401,6 +1401,8 @@ WaitEventSetWait(WaitEventSet *set, long timeout,
 		Assert(timeout >= 0 && timeout <= INT_MAX);
 		cur_timeout = timeout;
 	}
+	else
+		INSTR_TIME_SET_ZERO(start_time);
 
 	pgstat_report_wait_start(wait_event_info);
 

src/bin/psql/common.c

@@ -1276,6 +1276,8 @@ DescribeQuery(const char *query, double *elapsed_msec)
 
 	if (timing)
 		INSTR_TIME_SET_CURRENT(before);
+	else
+		INSTR_TIME_SET_ZERO(before);
 
 	/*
 	 * To parse the query but not execute it, we prepare it, using the unnamed
@@ -1406,6 +1408,8 @@ ExecQueryAndProcessResults(const char *query,
 
 	if (timing)
 		INSTR_TIME_SET_CURRENT(before);
+	else
+		INSTR_TIME_SET_ZERO(before);
 
 	if (pset.bind_flag)
 		success = PQsendQueryParams(pset.db, query, pset.bind_nparams, NULL, (const char * const *) pset.bind_params, NULL, NULL, 0);
@@ -1702,6 +1706,8 @@ ExecQueryUsingCursor(const char *query, double *elapsed_msec)
 
 	if (timing)
 		INSTR_TIME_SET_CURRENT(before);
+	else
+		INSTR_TIME_SET_ZERO(before);
 
 	/* if we're not in a transaction, start one */
 	if (PQtransactionStatus(pset.db) == PQTRANS_IDLE)