Re: [PATCH] gcc compiler option -Og warnings fix

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-04-01 14:20, Keith Wiles:
> The new compiler option -Og causes a few warning on variables
> being used before being set warnings.

Sometimes the compiler is wrong. It seems this option makes it
even wronger. Why not use -Wno-error with -Og?

More details below:

>  lib/librte_eal/linuxapp/eal/eal_pci_uio.c | 2 +-
>  lib/librte_lpm/rte_lpm6.c                 | 1 +
>  lib/librte_vhost/vhost_rxtx.c             | 4 ++--

There are also some warnings in mlx drivers, solved with patch below:

--- a/drivers/net/mlx4/mlx4.c
+++ b/drivers/net/mlx4/mlx4.c
@@ -5415,7 +5415,7 @@ mlx4_pci_devinit(struct rte_pci_driver *pci_drv, struct rte_pci_device *pci_dev)
        int err = 0;
        struct ibv_context *attr_ctx = NULL;
        struct ibv_device_attr device_attr;
-       unsigned int vf;
+       unsigned int vf = 0;
        int idx;
        int i;
 
--- a/drivers/net/mlx5/mlx5.c
+++ b/drivers/net/mlx5/mlx5.c
@@ -260,8 +260,8 @@ mlx5_pci_devinit(struct rte_pci_driver *pci_drv, struct rte_pci_device *pci_dev)
        int err = 0;
        struct ibv_context *attr_ctx = NULL;
        struct ibv_device_attr device_attr;
-       unsigned int vf;
-       unsigned int mps;
+       unsigned int vf = 0;
+       unsigned int mps = 0;
        int idx;
        int i;

> --- a/lib/librte_eal/linuxapp/eal/eal_pci_uio.c
> +++ b/lib/librte_eal/linuxapp/eal/eal_pci_uio.c
> @@ -152,7 +152,7 @@ pci_get_uio_dev(struct rte_pci_device *dev, char *dstbuf,
>  			   unsigned int buflen, int create)
>  {
>  	struct rte_pci_addr *loc = &dev->addr;
> -	unsigned int uio_num;
> +	unsigned int uio_num = 0;

This one is OK to fix.

> --- a/lib/librte_lpm/rte_lpm6.c
> +++ b/lib/librte_lpm/rte_lpm6.c
> @@ -381,6 +381,7 @@ add_step(struct rte_lpm6 *lpm, struct rte_lpm6_tbl_entry *tbl,
>  	int8_t bitshift;
>  	uint8_t bits_covered;
>  
> +	*tbl_next = NULL;
>  	/*
>  	 * Calculate index to the table based on the number and position
>  	 * of the bytes being inspected in this step.

It would be more logical to set this variable in the right condition branch:
--- a/lib/librte_lpm/rte_lpm6.c
+++ b/lib/librte_lpm/rte_lpm6.c
@@ -429,6 +429,7 @@ add_step(struct rte_lpm6 *lpm, struct rte_lpm6_tbl_entry *tbl,
                        }
                }
 
+               *tbl_next = NULL;
                return 0;
        }

> --- a/lib/librte_vhost/vhost_rxtx.c
> +++ b/lib/librte_vhost/vhost_rxtx.c
> @@ -295,7 +295,7 @@ virtio_dev_rx(struct virtio_net *dev, uint16_t queue_id,
>  	for (i = 0; i < count; i++) {
>  		uint16_t desc_idx = desc_indexes[i];
>  		uint16_t used_idx = (res_start_idx + i) & (vq->size - 1);
> -		uint32_t copied;
> +		uint32_t copied = 0;

This variable is not used if copy_mbuf_to_desc fails, so it is always
initialised before being used.
We can workaround the silly compiler while avoiding a performance hit
by resetting the variable only in the error case of copy_mbuf_to_desc:

--- a/lib/librte_vhost/vhost_rxtx.c
+++ b/lib/librte_vhost/vhost_rxtx.c
@@ -147,8 +147,10 @@ copy_mbuf_to_desc(struct virtio_net *dev, struct vhost_virtqueue *vq,
        struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, 0, 0, 0, 0, 0}, 0};
 
        desc = &vq->desc[desc_idx];
-       if (unlikely(desc->len < vq->vhost_hlen))
+       if (unlikely(desc->len < vq->vhost_hlen)) {
+               *copied = 0;
                return -1;
+       }

>  		err = copy_mbuf_to_desc(dev, vq, pkts[i], desc_idx, &copied);
> @@ -531,7 +531,7 @@ virtio_dev_merge_rx(struct virtio_net *dev, uint16_t queue_id,
>  {
>  	struct vhost_virtqueue *vq;
>  	uint32_t pkt_idx = 0, nr_used = 0;
> -	uint16_t start, end;
> +	uint16_t start = 0, end = 0;

I don't understand this one because the variables are not used if
reserve_avail_buf_mergeable fails.
I don't see any smart workaround.
Huawei, Yuanhan, can we expect a little slowdown with this change?