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llvm/lib/Target/RISCV/RISCVInstrInfoVSDPatterns.td | ||
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256 | Should we VMERGE_VIM imm 0 when the true operand is (splat_vector fpimm0)? |
llvm/lib/Target/RISCV/RISCVInstrInfoVSDPatterns.td | ||
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256 | Yep good idea, I've done that now. |
llvm/lib/Target/RISCV/RISCVInstrInfoVSDPatterns.td | ||
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191 | This is a heads up of an issue we may find by using VMV0 here: this register class is a singleton and regalloc may fail in some cases because virtual registers of VMV0 and further constrained classes built during codegen (such as VR_and_VMV0) only have one physical register v0. I will try to create a smaller reproducer. For the time being, an example of what I'm seeing: %1047:vr_and_vmv0 = PseudoVMOR_MM_MF8 %1042:vr_and_vmv0, %1046:vr_and_vmv0, $noreg, -1, implicit $vl, implicit $vtype, It does not seem possible to have a possible for a register class that is the intersection of VR and VMV0 (as it will only contain v0). Perhaps we can teach the passes that constraint RCs (MachineCSE comes to mind but perhaps there are others) to refrain to do so when the restricted class would be a singleton. I wonder if in the whole hierarchy of register classes, built using tablegen, we should avoid creating singletons in the first place (though there might be cases where this is useful?). Thoughts? |
llvm/lib/Target/RISCV/RISCVInstrInfoVSDPatterns.td | ||
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191 |
It does not seem possible to have a valid allocation for a |
llvm/lib/Target/RISCV/RISCVInstrInfoVSDPatterns.td | ||
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191 | Thanks for bringing this up. I was wondering if/when this would become a problem. I don't have any thoughts right now, but I wanted to add that I don't think this is strictly a problem with singletons, since you could have an intersection of only two registers but need three to satisfy an instruction. It just so happens that V0 is going to be the main sticking point for RVV. I bring this up in the context of teaching the passes to solve this, as I don't think that'll be possible in general. |
This is a heads up of an issue we may find by using VMV0 here: this register class is a singleton and regalloc may fail in some cases because virtual registers of VMV0 and further constrained classes built during codegen (such as VR_and_VMV0) only have one physical register v0.
I will try to create a smaller reproducer. For the time being, an example of what I'm seeing:
It does not seem possible to have a possible for a register class that is the intersection of VR and VMV0 (as it will only contain v0).
Perhaps we can teach the passes that constraint RCs (MachineCSE comes to mind but perhaps there are others) to refrain to do so when the restricted class would be a singleton.
I wonder if in the whole hierarchy of register classes, built using tablegen, we should avoid creating singletons in the first place (though there might be cases where this is useful?).
Thoughts?