gcc miscompiles csum_tcpudp_magic() on ARMv5

[mbizon@freebox.fr (Maxime Bizon)]

Hello,

I tried using csum_tcpudp_magic() like this:

csum_tcpudp_magic(src, dst, ntohs(udph->len), IPPROTO_UDP, csum);

instead of the more common:

len = ntohs(udhp->len);
[...]
csum_tcpudp_magic(src, dst, len, IPPROTO_UDP, csum);

the first one gives a bad checksum while the second one is ok.


I've tracked down the problem to csum_tcpudp_nofold(), which uses inline
asm and an unsigned short for len.

If the len value is say 0x3412, and I pass ntohs(len), then the assigned
register for len contains 0x00341234 instead of the expected 0x1234.

The ntohs() expand to a dumb swab16 on my arch, and gcc does the swap
but does not clear the high nibble, I guess it expects the assembly code
to only use the low 16 bits.

Is there a missing constraint or gcc is doing something wrong here ?

-- 
Maxime