[cfe-dev] Clang rejects inline assembly that has tied operands with different types

[Reid Kleckner]

I think Akira Hatanaka might have made this change recently.

On Wed, Feb 4, 2015 at 9:41 AM, Daniel Sanders <Daniel.Sanders at imgtec.com>
wrote:

>  Hi All,
>
>
>
> Does anyone remember why clang rejects inline assembly where an input and
> output are tied together but are differently sized types? The code that
> rejects it (Sema::ActOnGCCAsmStmt()) is looking like it's intentional but
> it's not clear why it does so.
>
> My current theory is that it's to avoid dealing with cases where a
> register has to be tied to its own subregister or something along those
> lines.
>
>
>
> Here's a trimmed version of the original failing case from the Linux
> Kernel:
>
> static __inline__ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
>
>                                           const struct in6_addr *daddr,
>
>                                           __u32 len, unsigned short proto,
>
>                                           __wsum sum)
>
> {
>
>         __asm__(
>
>         /* snip */
>
>         "       addu    %0, %5          # proto (long in network byte
> order)\n"
>
>         "       sltu    $1, %0, %5      \n"
>
>         /* snip */
>
>         "       lw      %1, 0(%2)       # four words source address\n"
>
>         /* snip */
>
>         : "=r" (sum), "=r" (proto)
>
>         : "r" (saddr), "r" (daddr),
>
>           "0" (htonl(len)), "1" (htonl(proto)), "r" (sum));
>
>
>
>         return csum_fold(sum);
>
> }
>
>
>
> Clang objects to the 'htonl(proto)' on the grounds that the type doesn't
> match 'proto':
>
> arch/mips/include/asm/checksum.h:285:27: error: unsupported inline asm:
> input with type '__be32' (aka 'unsigned int') matching output with type
> 'unsigned short'
>
>           "0" (htonl(len)), "1" (htonl(proto)), "r" (sum));
>
>                                  ^~~~~~~~~~~~
>
>
>
> *Daniel Sanders*
>
> Leading Software Design Engineer, MIPS Processor IP
>
> Imagination Technologies Limited
>
> www.imgtec.com
>
>
>
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>
>
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