Roger Sayle <ro...@nextmovesoftware.com> 于2023年12月24日周日 16:51写道： > > > > What's exceedingly weird is T_N_T_M_P (DImode, SImode) isn't actually a > > truncation! The output precision is first, the input precision is second. > > The docs > > explicitly state the output precision should be smaller than the input > > precision > > (which makes sense for truncation). > > > > That's where I'd start with trying to untangle this mess. > > Thanks (both) for correcting my misunderstanding. > At the very least might I suggest that we introduce a new > TRULY_NOOP_EXTENSION_MODES_P target hook that MIPS > can use for this purpose? It'd help reduce confusion, and keep > the documentation/function naming correct. >
Yes. It is good for me. T_N_T_M_P is a really confusion naming. > When Richard Sandiford "hookized" truly_noop_truncation in 2017 > https://gcc.gnu.org/legacy-ml/gcc-patches/2017-09/msg00836.html > he mentions the inprec/outprec confusion [deciding not to add a > gcc_assert outprec < inprec here, which might be a good idea]. > > The next question is whether this is just > TRULY_NOOP_SIGN_EXTENSION_MODES_P > or whether there are any targets that usefully ensure some modes > are zero-extended forms of others. TRULY_NOOP_ZERO_EXTENSION... > I guess ARM64 is the one TRULY_NOOP_ZERO_EXTENSION? > My vote is that a DINS instruction that updates the most significant > bit of an SImode value should then expand or define_insn_and_split > with an explicit following sign-extension operation. To avoid this being > eliminated by the RTL optimizers/combine the DINS should return a > DImode result, with the following extension truncating it to canonical Is it this one? https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626137.html > SImode form. This preserves the required backend invariant (and > doesn't require tweaking machine-independent code in combine). > SImode DINS instructions that don't/can't affect the MSB, can be a > single SImode instruction. > Yes. As most of MIPS microarchitecture, INS may have slight better performance than DINS. While, I am worrying that: will some body do something like INS <SI_REG1>,<DI_REG2>,24,8 In this case, if <DI_REG2> is not sign-extended, the result will be UNPREDICTABLE. For this, now, I prefer to use DINS and append a SLL. I tried to write a C code that can produce this case, but not yet success. > Cheers, > Roger > -- > >
