https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751
--- Comment #8 from rguenther at suse dot de <rguenther at suse dot de> --- On Thu, 20 Jul 2023, juzhe.zhong at rivai dot ai wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751 > > --- Comment #6 from JuzheZhong <juzhe.zhong at rivai dot ai> --- > (In reply to rguent...@suse.de from comment #5) > > On Thu, 20 Jul 2023, kito at gcc dot gnu.org wrote: > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751 > > > > > > --- Comment #4 from Kito Cheng <kito at gcc dot gnu.org> --- > > > > OK, so TA is either merge or all-ones. > > > > > > Yes, your understand is correct, just few more detail is that can be > > > mixing > > > with either merge or all-ones. > > > > > > e.g. > > > > > > An 4 x i32 vector with mask 1 0 1 0 > > > > > > Op = | a | b | c | d | > > > Mask = | 1 | 0 | 1 | 0 | > > > > > > the result could be: > > > | a | b | c | d | > > > | a | all-1 | c | d | > > > | a | all-1 | c | all-1 | > > > | a | all-1 | c | d | > > > > > > > > > > Not sure how you can use MA at the moment since you specify an existing > > > > operand in your target hook. As far as > > > > I can see there's no value the target hook can provide that matches any > > > of the implementation semantics? > > > > > > That's the key point - we don't know how to return an undefined value > > > there, we > > > have intrinsic can generate undefined value, but it seems impossible to > > > generate that within the hook. > > > > Well, neither *A nor *U can be specified currently. As said for 'merge' > > we would need another operand. And since 'unspecified' is either merge > > or all-ones we can't express that either. It's not really 'undefined' > > either. > > > > Note this also means the proposal to define a .MASK_LOAD as zeroing > > masked elements is not going to work for RISC-V, instead we'd need > > an explicit 'else' value there as well. > > > > In fact we could follow .MASK_LOAD for .COND_* and simply omit > > the 'else' operand for the case of 'unspecified', no? GIMPLE would > > be fine omitting it, not sure whether there's precedent for > > optabs with optional operands? > > For RVV auto-vectorization, we define COND_LEN_* has else value in the > arguments. But the else value is not always the real value we need to > care about, this is the code from vectorizable_operation: > > if (reduc_idx >= 0) > { > /* Perform the operation on active elements only and take > inactive elements from the reduction chain input. */ > gcc_assert (!vop2); > vops.quick_push (reduc_idx == 1 ? vop1 : vop0); > } > else > { > auto else_value = targetm.preferred_else_value > (cond_fn, vectype, vops.length () - 1, &vops[1]); > vops.quick_push (else_value); > } > > > You can see for reduction operations, the else value is the real value we > need to depend on, we should use "TU" (Undisturbed or merge value) in RVV. > Meaning the inactive elements should remain the "old" value that's why we > use "TU". Sure. For the above case that's obviously correct. > However, for single binary operations for example, division, we just only > need to forbid the division operations of the inactive elements in the > hardware, we don't care the value of the inactive elements value. so in > this case, we want to use "TA". In this case, we want the else value be > a meaningless placeholder in Gimple IR (similar to "undef" or "poison" in > LLVM). > > Such meaningless placeholder in the argument of Gimple IR, can be beneficail > for RVV for 2 following reasons: > 1. allow us use "TA". > 2. Doesn't consume a register. > > I am not sure whether we can represent such placeholder in Gimple IR. As said, just drop the 'else' operand and assign 'unspecified' to its semantics? Like we do for .LEN_MASK_LOAD where there isn't any 'else' value and I presume you'll use 'TA' as well there?
