DAGCombine: Let truncates negate extension through extract-subvector

[zvi]

Summary

Fold cases such as:
(v8i8 truncate (v8i32 extract_subvector (v16i32 sext (v16i8 V), Idx)))
->
(v8i8 extract_subvector (v16i8 V), Idx)

This can be generalized to cases where the truncate and extend do not
fully cancel each other out, but it may require querying the target
about profitability.

[RKSimon]

LGTM - possibly add some explicit zext test cases as well?

[zvi]

Rebase after adding the missing zext cases

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if ((N0.getOpcode() == ISD::ADDE || N0.getOpcode() == ISD::ADDCARRY) &&
       (!LegalOperations || TLI.isOperationLegal(N0.getOpcode(), VT))) {
     SDLoc SL(N);
     auto X = DAG.getNode(ISD::TRUNCATE, SL, VT, N0.getOperand(0));
     auto Y = DAG.getNode(ISD::TRUNCATE, SL, VT, N0.getOperand(1));
     auto VTs = DAG.getVTList(VT, N0->getValueType(1));
     return DAG.getNode(N0.getOpcode(), SL, VTs, X, Y, N0.getOperand(2));
   }
 
+  // fold (truncate (extract_subvector(ext x))) ->
+  //      (extract_subvector x)
+  // TODO: This can be generalized to cover cases where the truncate and extract
+  // do not fully cancel each other out.
+  if (!LegalTypes && N0.getOpcode() == ISD::EXTRACT_SUBVECTOR) {
+    SDValue N00 = N0.getOperand(0);
+    if (N00.getOpcode() == ISD::SIGN_EXTEND ||
+        N00.getOpcode() == ISD::ZERO_EXTEND ||
+        N00.getOpcode() == ISD::ANY_EXTEND) {
+      if (N00.getOperand(0)->getValueType(0).getVectorElementType() ==
+          VT.getVectorElementType())
+        return DAG.getNode(ISD::EXTRACT_SUBVECTOR, SDLoc(N0->getOperand(0)), VT,
+                           N00.getOperand(0), N0.getOperand(1));
+    }
+  }
+
   if (SDValue NewVSel = matchVSelectOpSizesWithSetCC(N))
     return NewVSel;
 
   return SDValue();
 }
 
 static SDNode *getBuildPairElt(SDNode *N, unsigned i) {
   SDValue Elt = N->getOperand(i);

test/CodeGen/X86/trunc-subvector.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+sse2 | FileCheck %s --check-prefix=SSE2
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx2 | FileCheck %s --check-prefixes=AVX,AVX2
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512f | FileCheck %s --check-prefixes=AVX,AVX512,AVX512F
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512bw | FileCheck %s --check-prefixes=AVX,AVX512,AVX512BW
 
 define <4 x i32> @test1(<8 x i32> %v) {
 ; SSE2-LABEL: test1:
 ; SSE2:       # %bb.0:
-; SSE2-NEXT:    pshufd {{.*#+}} xmm1 = xmm0[2,3,0,1]
-; SSE2-NEXT:    punpcklqdq {{.*#+}} xmm0 = xmm0[0],xmm1[0]
 ; SSE2-NEXT:    retq
 ;
-; AVX2-LABEL: test1:
-; AVX2:       # %bb.0:
-; AVX2-NEXT:    vpmovsxdq %xmm0, %ymm0
-; AVX2-NEXT:    vpshufd {{.*#+}} ymm0 = ymm0[0,2,2,3,4,6,6,7]
-; AVX2-NEXT:    vpermq {{.*#+}} ymm0 = ymm0[0,2,2,3]
-; AVX2-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX2-NEXT:    vzeroupper
-; AVX2-NEXT:    retq
-;
-; AVX512-LABEL: test1:
-; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovsxdq %ymm0, %zmm0
-; AVX512-NEXT:    vpmovqd %zmm0, %ymm0
-; AVX512-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX512-NEXT:    vzeroupper
-; AVX512-NEXT:    retq
+; AVX-LABEL: test1:
+; AVX:       # %bb.0:
+; AVX-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
+; AVX-NEXT:    vzeroupper
+; AVX-NEXT:    retq
   %x = sext <8 x i32> %v to <8 x i64>
   %s = shufflevector <8 x i64> %x, <8 x i64> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
   %t = trunc <4 x i64> %s to <4 x i32>
   ret <4 x i32> %t
 }
 
 define <4 x i32> @test2(<8 x i32> %v) {
 ; SSE2-LABEL: test2:
 ; SSE2:       # %bb.0:
-; SSE2-NEXT:    pshufd {{.*#+}} xmm0 = xmm1[2,3,0,1]
-; SSE2-NEXT:    punpcklqdq {{.*#+}} xmm1 = xmm1[0],xmm0[0]
-; SSE2-NEXT:    movdqa %xmm1, %xmm0
+; SSE2-NEXT:    movaps %xmm1, %xmm0
 ; SSE2-NEXT:    retq
 ;
-; AVX2-LABEL: test2:
-; AVX2:       # %bb.0:
-; AVX2-NEXT:    vextracti128 $1, %ymm0, %xmm0
-; AVX2-NEXT:    vpmovsxdq %xmm0, %ymm0
-; AVX2-NEXT:    vpshufd {{.*#+}} ymm0 = ymm0[0,2,2,3,4,6,6,7]
-; AVX2-NEXT:    vpermq {{.*#+}} ymm0 = ymm0[0,2,2,3]
-; AVX2-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX2-NEXT:    vzeroupper
-; AVX2-NEXT:    retq
-;
-; AVX512-LABEL: test2:
-; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovsxdq %ymm0, %zmm0
-; AVX512-NEXT:    vextracti64x4 $1, %zmm0, %ymm0
-; AVX512-NEXT:    vpmovqd %zmm0, %ymm0
-; AVX512-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX512-NEXT:    vzeroupper
-; AVX512-NEXT:    retq
+; AVX-LABEL: test2:
+; AVX:       # %bb.0:
+; AVX-NEXT:    vextractf128 $1, %ymm0, %xmm0
+; AVX-NEXT:    vzeroupper
+; AVX-NEXT:    retq
   %x = sext <8 x i32> %v to <8 x i64>
   %s = shufflevector <8 x i64> %x, <8 x i64> undef, <4 x i32> <i32 4, i32 5, i32 6, i32 7>
   %t = trunc <4 x i64> %s to <4 x i32>
   ret <4 x i32> %t
 }
 
 define <2 x i32> @test3(<8 x i32> %v) {
 ; SSE2-LABEL: test3:
; AVX512-NEXT:    retq
   ret <2 x i32> %t
 }
 
 define <4 x i32> @test6(<8 x i32> %v) {
 ; SSE2-LABEL: test6:
 ; SSE2:       # %bb.0:
 ; SSE2-NEXT:    retq
 ;
-; AVX2-LABEL: test6:
-; AVX2:       # %bb.0:
-; AVX2-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX2-NEXT:    vzeroupper
-; AVX2-NEXT:    retq
-;
-; AVX512-LABEL: test6:
-; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovzxdq {{.*#+}} zmm0 = ymm0[0],zero,ymm0[1],zero,ymm0[2],zero,ymm0[3],zero,ymm0[4],zero,ymm0[5],zero,ymm0[6],zero,ymm0[7],zero
-; AVX512-NEXT:    vpmovqd %zmm0, %ymm0
-; AVX512-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX512-NEXT:    vzeroupper
-; AVX512-NEXT:    retq
+; AVX-LABEL: test6:
+; AVX:       # %bb.0:
+; AVX-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
+; AVX-NEXT:    vzeroupper
+; AVX-NEXT:    retq
   %x = zext <8 x i32> %v to <8 x i64>
   %s = shufflevector <8 x i64> %x, <8 x i64> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
   %t = trunc <4 x i64> %s to <4 x i32>
   ret <4 x i32> %t
 }
 
 define <4 x i32> @test7(<8 x i32> %v) {
 ; SSE2-LABEL: test7:
 ; SSE2:       # %bb.0:
 ; SSE2-NEXT:    movaps %xmm1, %xmm0
 ; SSE2-NEXT:    retq
 ;
-; AVX2-LABEL: test7:
-; AVX2:       # %bb.0:
-; AVX2-NEXT:    vextractf128 $1, %ymm0, %xmm0
-; AVX2-NEXT:    vzeroupper
-; AVX2-NEXT:    retq
-;
-; AVX512-LABEL: test7:
-; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpmovzxdq {{.*#+}} zmm0 = ymm0[0],zero,ymm0[1],zero,ymm0[2],zero,ymm0[3],zero,ymm0[4],zero,ymm0[5],zero,ymm0[6],zero,ymm0[7],zero
-; AVX512-NEXT:    vextracti64x4 $1, %zmm0, %ymm0
-; AVX512-NEXT:    vpmovqd %zmm0, %ymm0
-; AVX512-NEXT:    # kill: def %xmm0 killed %xmm0 killed %ymm0
-; AVX512-NEXT:    vzeroupper
-; AVX512-NEXT:    retq
+; AVX-LABEL: test7:
+; AVX:       # %bb.0:
+; AVX-NEXT:    vextractf128 $1, %ymm0, %xmm0
+; AVX-NEXT:    vzeroupper
+; AVX-NEXT:    retq
   %x = zext <8 x i32> %v to <8 x i64>
   %s = shufflevector <8 x i64> %x, <8 x i64> undef, <4 x i32> <i32 4, i32 5, i32 6, i32 7>
   %t = trunc <4 x i64> %s to <4 x i32>
   ret <4 x i32> %t
 }
 
 define <2 x i32> @test8(<8 x i32> %v) {
 ; SSE2-LABEL: test8: