Re: [dpdk-dev] [PATCH] ixgbe: fix clang compile - remove truncation errors

[Neil Horman <nhorman@tuxdriver.com>]

On Mon, Dec 01, 2014 at 10:09:38AM +0100, Olivier MATZ wrote:
> Hi Bruce, Hi Neil,
> 
> On 11/30/2014 02:05 AM, Neil Horman wrote:
> > On Fri, Nov 28, 2014 at 03:31:00PM +0000, Bruce Richardson wrote:
> >> When compiling with clang, errors were being emitted due to truncation
> >> of values when assigning to the tx_offload_mask bit fields.
> >>
> >> dpdk.org/lib/librte_pmd_ixgbe/ixgbe_rxtx.c:404:27: fatal error: implicit truncation from 'int' to bitfield changes value from -1 to 127 [-Wbitfield-constant-conversion]
> >> 		    tx_offload_mask.l2_len = ~0;
> >>
> >> The fix proposed here is to define a static const value of the same type
> >> with all fields set to 1s, and use that instead of constants for assigning to.
> >>
> >> Other options would be to explicitily define the suitable constants that
> >> would not truncate for each individual field e.g. 0x7f for l2_len, 0x1FF
> >> for l3_len, etc., but this solution here has the advantage that it works
> >> without any changes to values if the field sizes are ever modified.
> >>
> >> Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> >> ---
> >>  lib/librte_pmd_ixgbe/ixgbe_rxtx.c | 29 +++++++++++++++--------------
> >>  1 file changed, 15 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> >> index 8559ef6..4f71194 100644
> >> --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> >> +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> >> @@ -367,6 +367,7 @@ ixgbe_set_xmit_ctx(struct igb_tx_queue* txq,
> >>  		volatile struct ixgbe_adv_tx_context_desc *ctx_txd,
> >>  		uint64_t ol_flags, union ixgbe_tx_offload tx_offload)
> >>  {
> >> +	static const union ixgbe_tx_offload offload_allones = { .data = ~0 };
> > Do you want to make this a static data structure?  If you make it a macro like
> > this:
> > #define ALLONES {.data = ~0}
> > Then you save the extra data space in the .data area (not that its that much),
> > and you can define it in a header file and use it in multiple c files (if you
> > need to)
> 
> I found that the following code works:
> 
> 	tx_offload_mask.l2_len |= ~0;
> 
> (note the '|=' instead of '=')
> 
How exactly does this work? does the or operator imply some level of type
promotion that the assignment doesn't to avoid the truncation?
Neil

> I would avoid to create a macro. What do you think?
> 
> Regards,
> Olivier
>