[PATCH] D104156: [DAGCombine][X86][ARM] EXTRACT_SUBVECTOR(VECTOR_SHUFFLE(?,?,Mask)) -> VECTOR_SHUFFLE(EXTRACT_SUBVECTOR(?, ?), EXTRACT_SUBVECTOR(?, ?), Mask')

Wed Nov 3 05:33:56 PDT 2021

lebedev.ri added inline comments.

================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:20673-20675
+  // Did we discover that the shuffle does not actually depend on operands?
+  if (DemandedSubvectors.empty())
+    return DAG.getUNDEF(NarrowVT);
----------------
> I suppose it depends if these UNDEF paths are actually active?

If i comment-out this block, then `"Should end up with either one or two ops"` assertion fires at least once:
```
$ ninja check-llvm-codegen
[ 99% 261/262][ 30% 0:00:21 + 0:00:47] Running lit suite /repositories/llvm-project/llvm/test/CodeGen
FAIL: LLVM :: CodeGen/X86/sad.ll (1064 of 18987)
******************** TEST 'LLVM :: CodeGen/X86/sad.ll' FAILED ********************
Script:
--
: 'RUN: at line 2';   /builddirs/llvm-project/build-Clang13/bin/llc < /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll -mtriple=x86_64-unknown-unknown -mattr=+sse2 | /builddirs/llvm-project/build-Clang13/bin/FileCheck /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll --check-prefixes=SSE2
: 'RUN: at line 3';   /builddirs/llvm-project/build-Clang13/bin/llc < /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll -mtriple=x86_64-unknown-unknown -mattr=+avx  | /builddirs/llvm-project/build-Clang13/bin/FileCheck /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll --check-prefixes=AVX,AVX1
: 'RUN: at line 4';   /builddirs/llvm-project/build-Clang13/bin/llc < /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll -mtriple=x86_64-unknown-unknown -mattr=+avx2 | /builddirs/llvm-project/build-Clang13/bin/FileCheck /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll --check-prefixes=AVX,AVX2
: 'RUN: at line 5';   /builddirs/llvm-project/build-Clang13/bin/llc < /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll -mtriple=x86_64-unknown-unknown -mattr=+avx512f | /builddirs/llvm-project/build-Clang13/bin/FileCheck /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll --check-prefixes=AVX,AVX512,AVX512F
: 'RUN: at line 6';   /builddirs/llvm-project/build-Clang13/bin/llc < /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll -mtriple=x86_64-unknown-unknown -mattr=+avx512bw | /builddirs/llvm-project/build-Clang13/bin/FileCheck /repositories/llvm-project/llvm/test/CodeGen/X86/sad.ll --check-prefixes=AVX,AVX512,AVX512BW
--
Exit Code: 1

Command Output (stderr):
--
llc: /repositories/llvm-project/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:20710: llvm::SDValue foldExtractSubvectorFromShuffleVector(llvm::SDNode *, llvm::SelectionDAG &, const llvm::TargetLowering &, bool): Assertion `(NewOps.size() == 1 || NewOps.size() == 2) && "Should end up with either one or two ops"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0.      Program arguments: /builddirs/llvm-project/build-Clang13/bin/llc -mtriple=x86_64-unknown-unknown -mattr=+avx
1.      Running pass 'Function Pass Manager' on module '<stdin>'.
2.      Running pass 'X86 DAG->DAG Instruction Selection' on function '@sad_unroll_nonzero_initial'
```
So i would say it is active.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104156/new/

https://reviews.llvm.org/D104156