Build user-space with different gcc optimization levels

This fix resolves warnings reported during compiling of user-space
libraries with different gcc optimization levels.

Tested with gcc versions: 4.9.2 (Debian), and 6.1.1 (Fedora).
The patch enables use of following opt levels: O0, O1, O2, O3, Og, Os, Ofast.

List of warnings:

[GCC 4.9.2 -Os]
libzfs_sendrecv.c:3726:26: error: 'clp' may be used uninitialized in this function [-Werror=maybe-uninitialized]

[GCC 4.9.2 -Og]
fs_fletcher.c:323:26: error: 'idx' may be used uninitialized in this function [-Werror=maybe-uninitialized]
dsl_dataset.c:1290:12: error: 'atp' may be used uninitialized in this function [-Werror=maybe-uninitialized]

[GCC 4.9.2 -Ofast]
u8_textprep.c:1310:9: error: 'tc[3ul]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
u8_textprep.c:177:23: error: 'u8t[0ul]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
dsl_dataset.c:2089:37: error: ‘hds’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
dsl_dataset.c:3216:2: error: ‘ds’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
dsl_dataset.c:1591:2: error: ‘ds’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
dsl_dataset.c:3341:2: error: ‘ds’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
vdev_raidz.c:1153:8: error: 'dcount[2]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
vdev_raidz.c:1167:17: error: 'dst[2]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
kernel.c:1005:2: error: ‘resid’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
libzfs_dataset.c:2826:8: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
libzfs_dataset.c:3056:35: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
libzfs_dataset.c:1584:13: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
libzfs_dataset.c:3056:35: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
libzfs_dataset.c:1792:66: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
libzfs_dataset.c:3986:35: error: ‘val’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

[GCC 6.1.1]
Resolved in PR #4907

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4937

module/unicode/u8_textprep.c

@@ -1060,7 +1060,7 @@ do_composition(size_t uv, uchar_t *s, uchar_t *comb_class, uchar_t *start,
 	uchar_t *disp, size_t last, uchar_t **os, uchar_t *oslast)
 {
 	uchar_t t[U8_STREAM_SAFE_TEXT_MAX + 1];
-	uchar_t tc[U8_MB_CUR_MAX];
+	uchar_t tc[U8_MB_CUR_MAX] = { '\0' };
 	uint8_t saved_marks[U8_MAX_CHARS_A_SEQ];
 	size_t saved_marks_count;
 	uchar_t *p;
@@ -1396,7 +1396,7 @@ collect_a_seq(size_t uv, uchar_t *u8s, uchar_t **source, uchar_t *slast,
 	uchar_t comb_class[U8_MAX_CHARS_A_SEQ];
 	uchar_t disp[U8_MAX_CHARS_A_SEQ];
 	uchar_t start[U8_MAX_CHARS_A_SEQ];
-	uchar_t u8t[U8_MB_CUR_MAX];
+	uchar_t u8t[U8_MB_CUR_MAX] = { '\0' };
 	uchar_t uts[U8_STREAM_SAFE_TEXT_MAX + 1];
 	uchar_t tc;
 	size_t last;

module/zcommon/zfs_fletcher.c

@@ -294,7 +294,7 @@ int
 fletcher_4_impl_set(const char *val)
 {
 	const fletcher_4_ops_t *ops;
-	enum fletcher_selector idx;
+	enum fletcher_selector idx = FLETCHER_FASTEST;
 	size_t val_len;
 	unsigned i;
 

module/zfs/dsl_dataset.c

@@ -1270,7 +1270,7 @@ dsl_dataset_snapshot_check(void *arg, dmu_tx_t *tx)
 	    pair != NULL; pair = nvlist_next_nvpair(ddsa->ddsa_snaps, pair)) {
 		int error = 0;
 		dsl_dataset_t *ds;
-		char *name, *atp;
+		char *name, *atp = NULL;
 		char dsname[ZFS_MAX_DATASET_NAME_LEN];
 
 		name = nvpair_name(pair);
@@ -1584,7 +1584,7 @@ dsl_dataset_snapshot_tmp_sync(void *arg, dmu_tx_t *tx)
 {
 	dsl_dataset_snapshot_tmp_arg_t *ddsta = arg;
 	dsl_pool_t *dp = dmu_tx_pool(tx);
-	dsl_dataset_t *ds;
+	dsl_dataset_t *ds = NULL;
 
 	VERIFY0(dsl_dataset_hold(dp, ddsta->ddsta_fsname, FTAG, &ds));
 
@@ -2081,7 +2081,7 @@ dsl_dataset_rename_snapshot_sync(void *arg, dmu_tx_t *tx)
 {
 	dsl_dataset_rename_snapshot_arg_t *ddrsa = arg;
 	dsl_pool_t *dp = dmu_tx_pool(tx);
-	dsl_dataset_t *hds;
+	dsl_dataset_t *hds = NULL;
 
 	VERIFY0(dsl_dataset_hold(dp, ddrsa->ddrsa_fsname, FTAG, &hds));
 	ddrsa->ddrsa_tx = tx;
@@ -3209,7 +3209,7 @@ dsl_dataset_set_refquota_sync(void *arg, dmu_tx_t *tx)
 {
 	dsl_dataset_set_qr_arg_t *ddsqra = arg;
 	dsl_pool_t *dp = dmu_tx_pool(tx);
-	dsl_dataset_t *ds;
+	dsl_dataset_t *ds = NULL;
 	uint64_t newval;
 
 	VERIFY0(dsl_dataset_hold(dp, ddsqra->ddsqra_name, FTAG, &ds));
@@ -3336,7 +3336,7 @@ dsl_dataset_set_refreservation_sync(void *arg, dmu_tx_t *tx)
 {
 	dsl_dataset_set_qr_arg_t *ddsqra = arg;
 	dsl_pool_t *dp = dmu_tx_pool(tx);
-	dsl_dataset_t *ds;
+	dsl_dataset_t *ds = NULL;
 
 	VERIFY0(dsl_dataset_hold(dp, ddsqra->ddsqra_name, FTAG, &ds));
 	dsl_dataset_set_refreservation_sync_impl(ds,

module/zfs/vdev_raidz.c

@@ -1103,8 +1103,8 @@ vdev_raidz_matrix_reconstruct(raidz_map_t *rm, int n, int nmissing,
 	int i, j, x, cc, c;
 	uint8_t *src;
 	uint64_t ccount;
-	uint8_t *dst[VDEV_RAIDZ_MAXPARITY];
-	uint64_t dcount[VDEV_RAIDZ_MAXPARITY];
+	uint8_t *dst[VDEV_RAIDZ_MAXPARITY] = { NULL };
+	uint64_t dcount[VDEV_RAIDZ_MAXPARITY] = { 0 };
 	uint8_t log = 0;
 	uint8_t val;
 	int ll;