net/mlx5: introduce Tx burst routine template

net/mlx5: introduce Tx burst routine template

Mellanox NICs support the wide set of Tx offloads. The supported
offloads are reported by the mlx5 PMD in rte_eth_dev_info
tx_offload_capa field.
An application may choose any combination of supported offloads
and configure the device appropriately. Some of Tx offloads may be
not requested by application, or ever all of them may be omitted.
Most of the Tx offloads require some code branches in tx_burst routine
to support ones. If Tx offload is not requested the tx_burst routine
code may be significantly simplified and consume less CPU cycles.

For example, if application does not engage TSO offload this code
can be omitted, if multi-segment packet is not supposed the tx_burst
may assume single mbuf packets only, etc.

Currently, the mlx5 PMD implements multiple tx_burst subroutines
for most common combinations of requested Tx offloads, each branch
has its own dedicated implementation. It is not very easy to update,
support and develop such kind of code - multiple branches impose
the multiple points to process. Also many of frequently requested
offload combinations are not supported yet. That leads to selecting of
not completely matching tx_burst routine and harms the performance.

This patch introduces the new approach for tx_burst code. It is proposed
to develop the unified template for tx_burst routine, which supports
all the Tx offloads and takes the compile time defined parameter
describing the supposed set of supported offloads. On the base
of this template, the compiler is able to generate multiple tx_burst
routines highly optimized for the statically specified set of
Tx offloads.
Next, in runtime, at Tx queue configuration the best matching optimized
implementation of tx_burst is chosen.

This patch intentionally omits the template internal implementation,
but just introduces the template itself to emboss the approach of
the multiple specially tuned tx_burst routines.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>