Messages in this thread | | | Date | Tue, 24 May 2022 09:38:27 -0700 | Subject | Re: [PATCH] MIPS: Rewrite `csum_tcpudp_nofold' in plain C | From | Florian Fainelli <> |
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On 5/22/22 13:48, 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@orcam.me.uk> > --- > 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.
How about QEMU is not that a viable option for testing big/little endian configurations? -- Florian
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