Failure to recognise commutable binop intrinsics #46241
Comments
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assigned to @rotateright |
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There are 2 parts to solving this:
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Weirdly, there is a Commutative property in Intrinsics.td but it seems to be getting completely ignored, as there is no corresponding attribute. Do we want to add one? It might be slightly odd in that this attribute wouldn't really be useful for anything but intrinsics. |
It's weirder still because we have things like this: So we should assume that for an intrinsic marked "Commutative" that we always mean only the 1st 2 args. I may be overlooking something, but I don't see any other place in the optimizer that would benefit from an attribute. Ie, we need to embed commute knowledge into transforms in instcombine anyway. If that's correct, then I think it's not worth the effort of creating a new attribute to map the property to. |
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Proposal: |
Extended Description
define <2 x i64> @smax(<2 x i64> %a, <2 x i64> %b) {
%x = call <2 x i64> @llvm.smax.v2i64(<2 x i64> %a, <2 x i64> %b)
%y = call <2 x i64> @llvm.smax.v2i64(<2 x i64> %b, <2 x i64> %a)
%o = or <2 x i64> %x, %y
ret <2 x i64> %o
}
declare <2 x i64> @llvm.smax.v2i64(<2 x i64>, <2 x i64>)
opt -O3
define <2 x i64> @smax(<2 x i64> %a, <2 x i64> %b) {
%x = tail call <2 x i64> @llvm.smax.v2i64(<2 x i64> %a, <2 x i64> %b)
%y = tail call <2 x i64> @llvm.smax.v2i64(<2 x i64> %b, <2 x i64> %a)
%o = or <2 x i64> %x, %y
ret <2 x i64> %o
}
but we should be able to recognise that %x and %y are equivalent:
define <2 x i64> @smax(<2 x i64> %a, <2 x i64> %b) {
%x = tail call <2 x i64> @llvm.smax.v2i64(<2 x i64> %a, <2 x i64> %b)
ret <2 x i64> %x
}
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