on 2021/6/2 下午3:43, Richard Biener wrote: > On Wed, Jun 2, 2021 at 9:28 AM Kewen.Lin <li...@linux.ibm.com> wrote: >> >> Hi Richi, >> >> on 2021/6/2 下午3:04, Richard Biener wrote: >>> On Wed, Jun 2, 2021 at 7:05 AM Kewen Lin <li...@linux.ibm.com> wrote: >>>> >>>> As Segher suggested, this patch is to emit the error message >>>> if the split condition of define_insn_and_split is empty while >>>> the insn condition isn't. >>> >>> I wonder whether it would be a good idea to automagically make >>> the split condition "&& 1" via gensupport? >>> >> >> Thanks for the comment! Do you happen to have some similar examples? > > Not sure, the docs say > > @var{insn-pattern}, @var{condition}, @var{output-template}, and > @var{insn-attributes} are used as in @code{define_insn}. > ... > The @var{split-condition} is also used as in > @code{define_split}, with the additional behavior that if the condition starts > with @samp{&&}, the condition used for the split will be the constructed as a > logical ``and'' of the split condition with the insn condition. > > so one can indeed read this as "" meaning 'true' w/o considering the > define_insn condition.
Yes, the "" in split condition does mean 'true' (always). > But then we say > > The @code{define_insn_and_split} construction provides exactly the same > functionality as two separate @code{define_insn} and @code{define_split} > patterns. It exists for compactness, and as a maintenance tool to prevent > having to ensure the two patterns' templates match. > > But then when I split a define_insn_and_split with a "" split condition > they are not functionally identical? Without this patch, they are indeed functionally identical. It's like the writer want to have one define_insn to match under some condition, but want to have one define_split to match always. > Also "" as split condition _does_ > seem valid, just maybe unintended? Yes, it's valid without this patch. That's why I asked whether there is some good reason to keep it be [1]. In Segher's opinion, there is no good reason, he pointed out "A reader does not expect a define_insn_and_split to split any other insns." > How would one create a > functionally equivalent example? "|| 1" will likely not work. > I think "|| 1" works just like "" if people want the define_split to split all the time, even with this patch. > Note I'm not familiar with all the details here but the documentation > does seem ambiguous at best, not supporting to error on empty > split-conditions at least. > Yes, the current patch will stop the "" condition which was accepted before. Thanks for bringing this up! We have to update the documentation if people reach a consensus. [1] https://gcc.gnu.org/pipermail/gcc-patches/2021-March/567014.html BR, Kewen