Re: [dpdk-dev] [PATCH] Minor fixes in rte_common.h file.

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ravi Kerur
> Sent: Tuesday, December 16, 2014 4:47 PM
> To: Neil Horman
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] Minor fixes in rte_common.h file.
> 
> On Sat, Dec 13, 2014 at 2:39 AM, Neil Horman <nhorman@tuxdriver.com> wrote:
> >
> > On Fri, Dec 12, 2014 at 03:04:34PM -0800, r k wrote:
> > > Subject: [PATCH] Minor fixes in rte_common.h file.
> > >
> > > Fix rte_is_power_of_2 since 0 is not.
> > > Avoid branching instructions in RTE_MAX and RTE_MIN.
> > >
> > > Signed-off-by: Ravi Kerur <rkerur@gmail.com>
> > > ---
> > >  lib/librte_eal/common/include/rte_common.h | 6 +++---
> > >  lib/librte_pmd_e1000/igb_pf.c              | 4 ++--
> > >  lib/librte_pmd_ixgbe/ixgbe_pf.c            | 4 ++--
> > >  3 files changed, 7 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/lib/librte_eal/common/include/rte_common.h
> > > b/lib/librte_eal/common/include/rte_common.h
> > > index 921b91f..e163f35 100644
> > > --- a/lib/librte_eal/common/include/rte_common.h
> > > +++ b/lib/librte_eal/common/include/rte_common.h
> > > @@ -203,7 +203,7 @@ extern int RTE_BUILD_BUG_ON_detected_error;  static
> > > inline int  rte_is_power_of_2(uint32_t n)  {
> > > -       return ((n-1) & n) == 0;
> > > +       return n && !(n & (n - 1));
> > >  }
> > >
> > >  /**
> > > @@ -259,7 +259,7 @@ rte_align64pow2(uint64_t v)  #define RTE_MIN(a, b)
> > ({ \
> > >                 typeof (a) _a = (a); \
> > >                 typeof (b) _b = (b); \
> > > -               _a < _b ? _a : _b; \
> > > +                _b ^ ((_a ^ _b) & -(_a < _b)); \
> > Are you sure this is actually faster than the branch version?  What about
> > using
> > a cmov instead?
> >
> >
> <rk> i am pretty sure modified code is faster than branching. I remember
> cmov had performance issues esp. on Pentuim-4 not sure how new intel cpu's
> perform.

I also think most modern compilers are smart enough to avoid any branching here and will use cmov instead.
And we are way ahead of Pentium 4 times these days.
Konstantin

> 
> >         })
> > >
> > >  /**
> > > @@ -268,7 +268,7 @@ rte_align64pow2(uint64_t v)  #define RTE_MAX(a, b)
> > ({ \
> > >                 typeof (a) _a = (a); \
> > >                 typeof (b) _b = (b); \
> > > -               _a > _b ? _a : _b; \
> > > +               _a ^ ((_a ^ _b) & -(_a < _b)); \
> > Same as above
> >
> > <rk> Same as above.
> 
> > >         })
> > >
> > >  /*********** Other general functions / macros ********/ diff --git
> > > a/lib/librte_pmd_e1000/igb_pf.c b/lib/librte_pmd_e1000/igb_pf.c index
> > > bc3816a..546499c 100644
> > > --- a/lib/librte_pmd_e1000/igb_pf.c
> > > +++ b/lib/librte_pmd_e1000/igb_pf.c
> > > @@ -321,11 +321,11 @@ igb_vf_set_mac_addr(struct rte_eth_dev *dev,
> > uint32_t
> > > vf, uint32_t *msgbuf)  static int  igb_vf_set_multicast(struct
> > rte_eth_dev
> > > *dev, __rte_unused uint32_t vf, uint32_t *msgbuf)  {
> > > -       int i;
> > > +       int16_t i;
> > >         uint32_t vector_bit;
> > >         uint32_t vector_reg;
> > >         uint32_t mta_reg;
> > > -       int entries = (msgbuf[0] & E1000_VT_MSGINFO_MASK) >>
> > > +       int32_t entries = (msgbuf[0] & E1000_VT_MSGINFO_MASK) >>
> > >                 E1000_VT_MSGINFO_SHIFT;
> > NAK, this has nothing to do with the included changelog
> >
> 
> <rk> It does, it causes compilation errors such as
> 
> /root/dpdk-new/dpdk/lib/librte_pmd_e1000/igb_pf.c: In function
> \u2018igb_pf_mbx_process\u2019:
> /root/dpdk-new/dpdk/lib/librte_pmd_e1000/igb_pf.c:350:23: error: array
> subscript is above array bounds [-Werror=array-bounds]
>    vfinfo->vf_mc_hashes[i] = hash_list[i];
>                        ^
> cc1: all warnings being treated as errors
> 
> Also it is always better to use explicit int definitions esp. for 64bit
> systems.
> 
> 
> 
> >
> > >         uint16_t *hash_list = (uint16_t *)&msgbuf[1];
> > >         struct e1000_hw *hw =
> > > E1000_DEV_PRIVATE_TO_HW(dev->data->dev_private);
> > > diff --git a/lib/librte_pmd_ixgbe/ixgbe_pf.c
> > > b/lib/librte_pmd_ixgbe/ixgbe_pf.c index 51da1fd..426caf9 100644
> > > --- a/lib/librte_pmd_ixgbe/ixgbe_pf.c
> > > +++ b/lib/librte_pmd_ixgbe/ixgbe_pf.c
> > > @@ -390,7 +390,7 @@ ixgbe_vf_set_multicast(struct rte_eth_dev *dev,
> > > __rte_unused uint32_t vf, uint32
> > >         struct ixgbe_hw *hw =
> > > IXGBE_DEV_PRIVATE_TO_HW(dev->data->dev_private);
> > >         struct ixgbe_vf_info *vfinfo =
> > >                 *(IXGBE_DEV_PRIVATE_TO_P_VFDATA(dev->data->dev_private));
> > > -       int nb_entries = (msgbuf[0] & IXGBE_VT_MSGINFO_MASK) >>
> > > +       int32_t nb_entries = (msgbuf[0] & IXGBE_VT_MSGINFO_MASK) >>
> > >                 IXGBE_VT_MSGINFO_SHIFT;
> > ditto
> > >         uint16_t *hash_list = (uint16_t *)&msgbuf[1];
> > >         uint32_t mta_idx;
> > > @@ -399,7 +399,7 @@ ixgbe_vf_set_multicast(struct rte_eth_dev *dev,
> > > __rte_unused uint32_t vf, uint32
> > >         const uint32_t IXGBE_MTA_BIT_SHIFT = 5;
> > >         const uint32_t IXGBE_MTA_BIT_MASK = (0x1 << IXGBE_MTA_BIT_SHIFT)
> > -
> > > 1;
> > >         uint32_t reg_val;
> > > -       int i;
> > > +       int16_t i;
> > ditto
> >
> > <rk> Same as above.
> 
> > >
> > >         /* only so many hash values supported */
> > >         nb_entries = RTE_MIN(nb_entries, IXGBE_MAX_VF_MC_ENTRIES);
> > > --
> > > 1.9.1
> > >
> >