[PATCH] D24254: [SelectionDAG] Simplify extract_subvector( insert_subvector ( Vec, In, Idx ), Idx ) -> In

[Simon Pilgrim via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: mkuper, eli.friedman, andreadb, spatel.
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If we are extracting a subvector that has just been inserted then we should just use the original inserted subvector.

This has come up in certain several x86 shuffle lowering cases where we are crossing 128-bit lanes. 

Repository:
  rL LLVM

https://reviews.llvm.org/D24254

Files:
  lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  test/CodeGen/X86/vector-shuffle-256-v16.ll

Index: test/CodeGen/X86/vector-shuffle-256-v16.ll
===================================================================
--- test/CodeGen/X86/vector-shuffle-256-v16.ll
+++ test/CodeGen/X86/vector-shuffle-256-v16.ll
@@ -3500,8 +3500,7 @@
 ; AVX1-LABEL: insert_dup_mem_v16i16_i32:
 ; AVX1:       # BB#0:
 ; AVX1-NEXT:    vmovd {{.*#+}} xmm0 = mem[0],zero,zero,zero
-; AVX1-NEXT:    vpshuflw {{.*#+}} xmm0 = xmm0[0,0,0,0,4,5,6,7]
-; AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,0,1,1]
+; AVX1-NEXT:    vpshufb {{.*#+}} xmm0 = xmm0[0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1]
 ; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm0, %ymm0
 ; AVX1-NEXT:    retq
 ;
@@ -3521,8 +3520,7 @@
 ; AVX1:       # BB#0:
 ; AVX1-NEXT:    movswl (%rdi), %eax
 ; AVX1-NEXT:    vmovd %eax, %xmm0
-; AVX1-NEXT:    vpshuflw {{.*#+}} xmm0 = xmm0[0,0,0,0,4,5,6,7]
-; AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,0,1,1]
+; AVX1-NEXT:    vpshufb {{.*#+}} xmm0 = xmm0[0,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1]
 ; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm0, %ymm0
 ; AVX1-NEXT:    retq
 ;
@@ -3544,8 +3542,7 @@
 ; AVX1-LABEL: insert_dup_elt1_mem_v16i16_i32:
 ; AVX1:       # BB#0:
 ; AVX1-NEXT:    vmovd {{.*#+}} xmm0 = mem[0],zero,zero,zero
-; AVX1-NEXT:    vpshuflw {{.*#+}} xmm0 = xmm0[1,1,1,1,4,5,6,7]
-; AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,0,1,1]
+; AVX1-NEXT:    vpshufb {{.*#+}} xmm0 = xmm0[2,3,2,3,2,3,2,3,2,3,2,3,2,3,2,3]
 ; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm0, %ymm0
 ; AVX1-NEXT:    retq
 ;
@@ -3564,9 +3561,7 @@
 ; AVX1-LABEL: insert_dup_elt3_mem_v16i16_i32:
 ; AVX1:       # BB#0:
 ; AVX1-NEXT:    vmovd {{.*#+}} xmm0 = mem[0],zero,zero,zero
-; AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[1,0,1,1]
-; AVX1-NEXT:    vpshuflw {{.*#+}} xmm0 = xmm0[3,3,3,3,4,5,6,7]
-; AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,0,1,1]
+; AVX1-NEXT:    vpshufb {{.*#+}} xmm0 = xmm0[2,3,2,3,2,3,2,3,2,3,2,3,2,3,2,3]
 ; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm0, %ymm0
 ; AVX1-NEXT:    retq
 ;
Index: lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3750,6 +3750,12 @@
       // Trivial extraction.
       if (VT.getSimpleVT() == N1.getSimpleValueType())
         return N1;
+
+      // EXTRACT_SUBVECTOR of INSERT_SUBVECTOR is often created
+      // during shuffle legalization.
+      if (N1.getOpcode() == ISD::INSERT_SUBVECTOR && N2 == N1.getOperand(2) &&
+          VT == N1.getOperand(1).getValueType())
+          return N1.getOperand(1);
     }
     break;
   }


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