On Fri, Sep 22, 2017 at 7:11 AM, Y Song <ys114...@gmail.com> wrote: > On Fri, Sep 22, 2017 at 6:46 AM, Edward Cree <ec...@solarflare.com> wrote: >> On 22/09/17 00:11, Y Song wrote: >>> On Thu, Sep 21, 2017 at 12:58 PM, Edward Cree <ec...@solarflare.com> wrote: >>>> On 21/09/17 20:44, Alexei Starovoitov wrote: >>>>> On Thu, Sep 21, 2017 at 09:29:33PM +0200, Daniel Borkmann wrote: >>>>>> More intuitive, but agree on the from_be/le. Maybe we should >>>>>> just drop the "to_" prefix altogether, and leave the rest as is since >>>>>> it's not surrounded by braces, it's also not a cast but rather an op. >>>> That works for me. >>>>> 'be16 r4' is ambiguous regarding upper bits. >>>>> >>>>> what about my earlier suggestion: >>>>> r4 = (be16) (u16) r4 >>>>> r4 = (le64) (u64) r4 >>>>> >>>>> It will be pretty clear what instruction is doing (that upper bits become >>>>> zero). >>>> Trouble with that is that's very *not* what C will do with those casts >>>> and it doesn't really capture the bidirectional/symmetry thing. The >>>> closest I could see with that is something like `r4 = (be16/u16) r4`, >>>> but that's quite an ugly mongrel. >>>> I think Daniel's idea of `be16`, `le32` etc one-arg opcodes is the >>>> cleanest and clearest. Should it be >>>> r4 = be16 r4 >>>> or just >>>> be16 r4 >>>> ? Personally I incline towards the latter, but admit it doesn't really >>>> match the syntax of other opcodes. >>> I did some quick prototyping in llvm to make sure we have a syntax >>> llvm is happy. Apparently, llvm does not like the syntax >>> r4 = be16 r4 or r4 = (be16) (u16) r4. >>> >>> In llvm:utils/TableGen/AsmMatcherEmitter.cpp: >>> >>> // Verify that any operand is only mentioned once. >> Wait, how do you deal with (totally legal) r4 += r4? >> Or r4 = *(r4 +0)? >> Even jumps can have src_reg == dst_reg, though it doesn't seem useful. > > We are talking about dag node here. The above "r4", although using the same > register, will be different dag nodes. So it will be okay. > > The "r4 = be16 r4" tries to use the *same* dag node as both source and > destination > in the asm output which is prohibited.
With second thought, we may allow "r4 = be16 r4" by using different dag nodes. (I need to do experiment for this.) But we do have constraints that the two "r4" must be the same register. "r5 = be16 r4" is not allowed. So from that perspective, referencing "r4" only once is a good idea and less confusing.
