On Mon, Dec 01, 2014 at 02:36:46PM +0000, Bruce Richardson wrote: > On Mon, Dec 01, 2014 at 09:25:44AM -0500, Neil Horman wrote: > > On Mon, Dec 01, 2014 at 11:24:58AM +0000, Bruce Richardson wrote: > > > On Mon, Dec 01, 2014 at 06:18:17AM -0500, Neil Horman wrote: > > > > 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 at 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 > > > > > > > > > > For any aithmetic, and presumably logical, operation on two values, any > > > values > > > smaller than "int" are promoted to type int before the operation takes > > > place. I > > > believe the exact rules for this are in the C specs e.g. C99. > > > > > Yes, but I would have thought that assignment was included in the list of > > logical operations for that promotion to occur. The above change seems to > > suggest it isn't, which is why I'm asking. C99 specifies |= explicitly as a > > type of assignment operator (see 6.5.16 here: > > http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf > > ) > > > > So I would presume that using = should work exactly the same as |= in terms > > of > > type promotion. We also don't get this warning on gcc, which concerns me > > that > > we might just be papering over a compiler problem here. > > > > Looking at the error, its complaining that we're truncating an int value to > > a > > bitfield (which we are), and that the resultant value is 127 rather that -1 > > (which it would be if we actually intended to treat it as an integer > > > > Which I think is where the problem lies. That is to say we've typed the > > bitfields in ixgbe_tx_offload as uint64_t. I'm guessing gcc just promotes > > ~0 to > > an unsigned int during the assignment, and supresses the warning (unless you > > turn on -pedantic or some such), but clang does not. The correct solution I > > think here is to either: > > > > 1) modify the bitfield types so that they are signed integers, thereby > > maintaining the resultant value of -1 after the assignment > > > > or > > > > 2) Modify the ~0 to be ~0UL, so that the clang compiler sees that the > > resultant > > value will be MAXLONG after the assignment > > > > I'd think operation 2 would be the better choice > > Neil > > > I'm not a compiler expert, but looking at it a bit more what I think is > happening is that we are simply changing the assignment from a constant one to > a computed one instead. With the constant assignment, the compiler can check > that > the assignment doesn't overflow, while with the computed value, it has no > choice > to accept the truncation since any computation is going to take place with > variables > of at least size "int" and there is no way to typecast the resulting value to > a bit field. > > As for papering-over compiler niggles, possibly so, but this solution is > shorter and > less impactful than the other solutions which are less workaround-like - i.e. > those > that assign values of exactly the right size using either magic numbers or a > special-value copy of the structure. > > Also, in terms of the two options you propose, I tried the second and it > still gives > errors, so the signed-ness or unsigned-ness is not the problem the compiler > has, its > the truncation. > > CC ixgbe_rxtx.o > /usr/home/bruce/dpdk.org/lib/librte_pmd_ixgbe/ixgbe_rxtx.c:384:28: fatal > error: implicit truncation from 'unsigned long' to bitfield changes value > from 18446744073709551615 to 65535 > [-Wbitfield-constant-conversion] > tx_offload_mask.vlan_tci = ~0UL; > > > /Bruce > You're right, it does, and looking at it now, I wonder if the warning really just needs to be removed. Gcc has no such warning in it currently (though I expect -pedantic would say something here). Regardless, looking at the clang docs I can't find any documentation about the bitfield-constant-conversion warning, and it seems to exist only to tell us that we're truncating an integer to an integer of a smaller size (which will clearly be the case anytime we are assigning a constant to a bitfield). Instead of avoiding the warning by doing any sort of code trickery, why not just remove the warning?
Neil