[GlobalISel] Match G_SHUFFLE_VECTORs representing sub-vector extracts #224

konstantinschwarz · 2024-10-25T19:03:54Z

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khallouh

The change looks good. Just a few nits. No comments on the tests yet because of the draft status. I have 2 questions:

Is the goal to match only half vector extract patterns or Sub-Vector extracts in general? (quarter extracts being also relevant for AIE's API)
How about the missing canonicalization combine for shufflevector in GlobalISel? I think this is needed if we want to support extracts from the second vector input with the current combine implementation in this PR.

khallouh · 2024-10-28T16:24:11Z

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

+  const LLT Src1Ty = MRI.getType(Src1Reg);
+
+  if (!DstTy.isVector() || !Src1Ty.isVector())
+    return false;


Is this needed? G_SHUFFLE_VECTOR only operates on vectors.

Actually, G_SHUFFLE_VECTOR can have scalar inputs. The opcode is a bit weird.

https://github.com/Xilinx/llvm-aie/blob/aie-public/llvm/test/CodeGen/AArch64/GlobalISel/legalize-shuffle-vector.mir#L119

I think that this misses the following edge case:

%0:_(<2 x s32>) = COPY $x0 %1:_(<2 x s32>) = UNDEF %2:_(s32) = G_SHUFFLE_VECTOR %0, %1, shufflemask(0)

In this case, you could unmerge the first array into the destination. Since the target is not a vector, this would skip it. How useful this is? Who knows.

I think that case could even be matched to a G_EXTRACT_VECTOR_ELT. It might not be very useful for AIE's API since element extracts would be implemented using extractelement instead of shufflevector.

Yeah, it is a bit obscure and the overhead it would cause is minimal.

khallouh · 2024-10-28T16:39:00Z

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

+  assert(MI.getOpcode() == TargetOpcode::G_SHUFFLE_VECTOR);
+  const Register DstReg = MI.getOperand(0).getReg();
+  const Register Src1Reg = MI.getOperand(1).getReg();
+  ArrayRef<int> Mask = MI.getOperand(3).getShuffleMask();


Nit: ShuffleMask?

ValentijnvdBeek

Hi Konstantin, thanks for taking my design and generally unjunior/internifying. This looks very clean, great job. I am not able to run it a.t.m., but I will look at that whenever the PR is complete.

There are two small worries:

This check is really expensive, this replaces the basic design but makes it hard to add the optimizations that might be needed to merge upstream.

Having separate optimizer causes it to be missed by the G_EXTRACT->G_INSERT->G_BUILD sequences that might be valid representations. Upstream doesn't, for sensible reasons, canonize those into shufflevectors so you need to make sure that this optimizer is on those sequences as well.

See MR linked in: #41 (comment)

ValentijnvdBeek · 2024-10-28T21:27:33Z

llvm/include/llvm/Target/GlobalISel/Combine.td

+def combine_shuffle_to_extract_vector : GICombineRule<
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_SHUFFLE_VECTOR):$root,
+        [{ return Helper.matchShuffleToExtractSubvector(*${root}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;


Avoid using wip_match_opcode, upstream really doesn't like this since it causes imprecise patterns and slows down the compilation. In this case, it should be very little difference compared to just matching SHUFFLE_VECTOR directly.

https://llvm.org/docs/GlobalISel/MIRPatterns.html#gallery
See MR linked in: #41 (comment)

ValentijnvdBeek · 2024-10-28T21:34:17Z

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

+  const LLT Src1Ty = MRI.getType(Src1Reg);
+
+  if (!DstTy.isVector() || !Src1Ty.isVector())
+    return false;


Actually, G_SHUFFLE_VECTOR can have scalar inputs. The opcode is a bit weird.

https://github.com/Xilinx/llvm-aie/blob/aie-public/llvm/test/CodeGen/AArch64/GlobalISel/legalize-shuffle-vector.mir#L119

ValentijnvdBeek · 2024-10-28T21:45:41Z

llvm/include/llvm/Target/GlobalISel/Combine.td

    fsub_to_fneg, commute_constant_to_rhs, match_ands, match_ors,
    combine_concat_vector, double_icmp_zero_and_or_combine, match_addos,
-    combine_shuffle_concat]>;
+    combine_shuffle_concat, combine_shuffle_to_extract_vector]>;


Note: this is a bit of an expensive combiner to run since it iterates through a decent chunk of elements twice. Previously, this was an explicitly opt in combiner that a target enabled by calling a specific function. This will now run it on all targets, this is probably worth it. Also, upstream will probably be running those checks now anyways, so it doesn't matter as much.

llvm-aie/llvm/lib/Target/AIE/AIE2PreLegalizerCombiner.cpp

Line 172 in 5a53fd6

return Helper.tryCombineShuffleVector(MI);

See MR linked in: #41 (comment)

What upstream checks are you referring to? I think the current tryCombineShuffleVector only matches merge-like patterns, but we could definitely move our combine_shuffle_to_extract_vector combine there, would that make it less expensive?

No, checking elements to the shuffle vector is just an O(n²) operations unless you use a hashmap. At the moment, this is done in tryCombineShuffleVector which a backend can enable. This is now run always in all backends. I don't know exactly why someone made the decision to have that combiner be run explicitly rather than using tablegen like the others

ValentijnvdBeek · 2024-10-28T21:53:38Z

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

+  const LLT Src1Ty = MRI.getType(Src1Reg);
+
+  if (!DstTy.isVector() || !Src1Ty.isVector())
+    return false;


I think that this misses the following edge case:

%0:_(<2 x s32>) = COPY $x0 %1:_(<2 x s32>) = UNDEF %2:_(s32) = G_SHUFFLE_VECTOR %0, %1, shufflemask(0)

In this case, you could unmerge the first array into the destination. Since the target is not a vector, this would skip it. How useful this is? Who knows.

ValentijnvdBeek · 2024-10-28T21:55:04Z

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

+  auto CheckExtractMask = [=](unsigned Start, unsigned NumElems) -> bool {
+    auto ExtractMask = createSequentialMask(Start, NumElems, 0);
+
+    for (unsigned I = 0; I < NumDstElems; I++) {
+      if (Mask[I] == -1)
+        continue;
+
+      if (Mask[I] != ExtractMask[I])
+        return false;
+    }
+
+    return true;
+  };


I like this is a lot, this is great.

andcarminati · 2024-11-05T16:58:14Z

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

+  if (NumDstElems * 2 != NumSrc1Elems)
+    return false;
+
+  auto CheckExtractMask = [=](unsigned Start, unsigned NumElems) -> bool {


nit: we can use by-reference capture and automatic return type deduction.

konstantinschwarz · 2025-02-06T00:09:28Z

We added a lot of helper functions for matching sub-vector extraction operations recently. Those will be extended to cover the combine in this PR.

[GlobalISel] Match G_SHUFFLE_VECTORs representing sub-vector extracts

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konstantinschwarz mentioned this pull request Oct 25, 2024

[LLVM] Optimize G_SHUFFLE_VECTOR into more efficient generic opcodes #41

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khallouh reviewed Oct 28, 2024

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ValentijnvdBeek reviewed Oct 28, 2024

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andcarminati reviewed Nov 5, 2024

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konstantinschwarz closed this Feb 6, 2025

konstantinschwarz deleted the kschwarz.shufflevector.to.extract.subvector branch February 6, 2025 00:09

mgehre-amd pushed a commit that referenced this pull request Aug 21, 2025

[FXML-4642] Lower extf, truncf to EmitC (#224)

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[GlobalISel] Match G_SHUFFLE_VECTORs representing sub-vector extracts #224

[GlobalISel] Match G_SHUFFLE_VECTORs representing sub-vector extracts #224

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konstantinschwarz commented Oct 25, 2024

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konstantinschwarz commented Feb 6, 2025

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