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

[Neil Horman <nhorman@tuxdriver.com>]

On Wed, Dec 17, 2014 at 08:40:17AM -0800, Ravi Kerur wrote:
> On Tue, Dec 16, 2014 at 1:40 PM, Neil Horman <nhorman@tuxdriver.com> wrote:
> >
> > On Tue, Dec 16, 2014 at 08:46:51AM -0800, Ravi Kerur wrote:
> > > 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.
> > >
> > Pretty sure isn't sure.  Theres no point in code churn if theres no obvious
> > advantage.  Some perf tests to deomonstrate the advantage here would be
> > great.
> >
> 
> <rk> I have used this before with the intent to avoid branching and it was
> part of other changes I did for performance improvement in our code.
> 

Then it should be pretty easy to provide the perf data demonstrating the
advantage in this code.

> >
> > > >         })
> > > > >
> > > > >  /**
> > > > > @@ -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.
> > >
> >
> > This is your changelog:
> > =============================================================
> > 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
> > =============================================================
> >
> > Nowhere does your changelog indicate that you are fixing compliation
> > errors.
> > That would in and of itself be far more serious that making micro
> > optimizations.
> > If you want to fix build breaks, great, please do, but send a patch that
> > clearly
> > indicates what the break is and how your fixing it. Don't just toss it in
> > with
> > whatever other work you happen to be doing.
> >
> 
> <rk> Main reason was to replace int with explicit sized int, it happened to
> fix compiler errors as well. I will make sure comments cover everything
> next time. Anyways I will drop this patch and just include fix for
> power_of_2.
Please separate the compiler warning fixes from the performance enhancing fixes.
They shouldn't be mashed together.

Neil