On Sun, May 22, 2022 at 09:48:14PM +0100, Maciej W. Rozycki wrote:
> Recent commit 198688edbf77 ("MIPS: Fix inline asm input/output type
> mismatch in checksum.h used with Clang") introduced a code size and
> performance regression with 64-bit code emitted for `csum_tcpudp_nofold'
> by GCC, caused by a redundant truncation operation produced due to a
> data type change made to the variable associated with the inline
> assembly's output operand.
>
> The intent previously expressed here with operands and constraints for
> optimal code was to have the output operand share a register with one
> inputs, both of a different integer type each. This is perfectly valid
> with the MIPS psABI where a register can hold integer data of different
> types and the assembly code used here makes data stored in the output
> register match the data type used with the output operand, however it
> has turned out impossible to express this arrangement in source code
> such as to satisfy LLVM, apparently due to the compiler's internal
> limitations.
>
> There is nothing peculiar about the inline assembly `csum_tcpudp_nofold'
> includes however, though it does choose assembly instructions carefully.
>
> Rewrite this piece of assembly in plain C then, using corresponding C
> language operations, making GCC produce the same assembly instructions,
> possibly shuffled, in the general case and sometimes actually fewer of
> them where an input is constant, because the compiler does not have to
> reload it to a register (operand constraints could be adjusted for that,
> but the plain C approach is cleaner anyway).
>
> Example code size changes are as follows, for a 32-bit configuration:
>
> text data bss total filename
> 5920480 1347236 126592 7394308 vmlinux-old
> 5920480 1347236 126592 7394308 vmlinux-now
> 5919728 1347236 126592 7393556 vmlinux-c
>
> and for a 64-bit configuration:
>
> text data bss total filename
> 6024112 1790828 225728 8040668 vmlinux-old
> 6024128 1790828 225728 8040684 vmlinux-now
> 6023760 1790828 225728 8040316 vmlinux-c
>
> respectively, where "old" is with the commit referred reverted, "now" is
> with no change, and "c" is with this change applied.
>
> Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
> ---
> Hi,
>
> I have visually inspected code produced and verified this change to boot
> with TCP networking performing just fine, both with a 32-bit and a 64-bit
> configuration. Sadly with the little endianness only, because in the
> course of this verification I have discovered the core card of my Malta
> board bit the dust a few days ago, apparently in a permanent manner, and I
> have no other big-endian MIPS system available here to try.
>
> The only difference between the two endiannesses is the left-shift
> operation on (proto + len) however, which doesn't happen for big-endian
> configurations, so the little endianness should in principle provide
> enough coverage.
>
> Also I'm leaving it to LLVM folks to verify, however this is plain C, so
> it is expected to just work.
>
> Please apply.
>
> Maciej
> ---
> arch/mips/include/asm/checksum.h | 71 ++++++++++++++++++---------------------
applied to mips-next.
Thomas.
--
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea. [ RFC1925, 2.3 ]
