X86: Utilize ZeroableElements for canWidenShuffleElements

[RKSimon]

Summary

canWidenShuffleElements can do a better job if given a mask with ZeroableElements info. Apparently, ZeroableElements was being only used to identify AllZero candidates, but possibly we could plug it into more shuffle matchers.

[RKSimon]

lib/Target/X86/X86ISelLowering.cpp

14401 ↗

(On Diff #129794)

ISD::isBuildVectorAllZeros allows undef elements (as long as it isn't all undefs) - should we test for a splat build vector of zero and check that we have no undefs at all?

[zvi]

lib/Target/X86/X86ISelLowering.cpp

14401 ↗

(On Diff #129794)

What would be the benefit of that? I mean, undefs can be zeroed if profitable.

[zvi]

Rebase + ping

[RKSimon]

lib/Target/X86/X86ISelLowering.cpp

14571 ↗

(On Diff #132777)

Are there any cases where the zero entry came from V1 that could cause a problem?

14595 ↗

(On Diff #132777)

APInt Undefs = APInt::getAllOnesValue(NewNumElts);

14597 ↗

(On Diff #132777)

APInt::getNullValue(NewEltVT.getSizeInBits())

14600 ↗

(On Diff #132777)

i > NewNumElts can't ever be true? And anyway, we expect the zero to have come from V2.

[zvi]

• Rebase
• Simplify the creation of the shuffle by only modifying the widened mask to take all zeros from the all-zero operand.

[zvi]

lib/Target/X86/X86ISelLowering.cpp

14571 ↗

(On Diff #132777)

Sorry i didn't understand the question.

14595 ↗

(On Diff #132777)

Thanks, this code was removed from latest revision

14597 ↗

(On Diff #132777)

Thanks, this code was removed from latest revision

14600 ↗

(On Diff #132777)

whoops. I fixed this in the latest revision

[RKSimon]

@zvi Do you mind if I commandeer this and get it completed?

[RKSimon]

Commandeering as @zvi is busy with other things

[RKSimon]

Rebased and updated to fix potential issue if whole of V2 isn't zeros

[andreadb]

In D42044#1159826, @RKSimon wrote:

Rebased and updated to fix potential issue if whole of V2 isn't zeros

Looks good to me. Thanks Simon!

llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

static SDValue lowerVectorShuffle(SDValue Op, const X86Subtarget &Subtarget,
 
   // We actually see shuffles that are entirely re-arrangements of a set of
   // zero inputs. This mostly happens while decomposing complex shuffles into
   // simple ones. Directly lower these as a buildvector of zeros.
   APInt Zeroable = computeZeroableShuffleElements(Mask, V1, V2);
   if (Zeroable.isAllOnesValue())
     return getZeroVector(VT, Subtarget, DAG, DL);
 
+  bool V2IsZero = !V2IsUndef && ISD::isBuildVectorAllZeros(V2.getNode());
+
+  // Create an alternative mask with info about zeroable elements.
+  // Here we do not set undef elements as zeroable.
+  SmallVector<int, 64> ZeroableMask(Mask.begin(), Mask.end());
+  if (V2IsZero) {
+    assert(!Zeroable.isNullValue() && "V2's non-undef elements are used?!");
+    for (int i = 0; i != NumElements; ++i)
+      if (Mask[i] != SM_SentinelUndef && Zeroable[i])
+        ZeroableMask[i] = SM_SentinelZero;
+  }
+
   // Try to collapse shuffles into using a vector type with fewer elements but
   // wider element types. We cap this to not form integers or floating point
   // elements wider than 64 bits, but it might be interesting to form i128
   // integers to handle flipping the low and high halves of AVX 256-bit vectors.
   SmallVector<int, 16> WidenedMask;
   if (VT.getScalarSizeInBits() < 64 && !Is1BitVector &&
-      canWidenShuffleElements(Mask, WidenedMask)) {
+      canWidenShuffleElements(ZeroableMask, WidenedMask)) {
     MVT NewEltVT = VT.isFloatingPoint()
                        ? MVT::getFloatingPointVT(VT.getScalarSizeInBits() * 2)
                        : MVT::getIntegerVT(VT.getScalarSizeInBits() * 2);
-    MVT NewVT = MVT::getVectorVT(NewEltVT, VT.getVectorNumElements() / 2);
+    int NewNumElts = NumElements / 2;
+    MVT NewVT = MVT::getVectorVT(NewEltVT, NewNumElts);
     // Make sure that the new vector type is legal. For example, v2f64 isn't
     // legal on SSE1.
     if (DAG.getTargetLoweringInfo().isTypeLegal(NewVT)) {
+      if (V2IsZero) {
+        // Modify the new Mask to take all zeros from the all-zero vector.
+        // Choose indices that are blend-friendly.
+        bool UsedZeroVector = false;
+        assert(find(WidenedMask, SM_SentinelZero) != WidenedMask.end() &&
+               "V2's non-undef elements are used?!");
+        for (int i = 0; i != NewNumElts; ++i)
+          if (WidenedMask[i] == SM_SentinelZero) {
+            WidenedMask[i] = i + NewNumElts;
+            UsedZeroVector = true;
+          }
+        // Ensure all elements of V2 are zero - isBuildVectorAllZeros permits
+        // some elements to be undef.
+        if (UsedZeroVector)
+          V2 = getZeroVector(NewVT, Subtarget, DAG, DL);
+      }
       V1 = DAG.getBitcast(NewVT, V1);
       V2 = DAG.getBitcast(NewVT, V2);
       return DAG.getBitcast(
           VT, DAG.getVectorShuffle(NewVT, DL, V1, V2, WidenedMask));
     }
   }
 
   // Commute the shuffle if it will improve canonicalization.

llvm/trunk/test/CodeGen/X86/avx-cast.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=x86_64-apple-darwin -mattr=+avx  | FileCheck %s --check-prefix=AVX --check-prefix=AVX1
 ; RUN: llc < %s -mtriple=x86_64-apple-darwin -mattr=+avx2 | FileCheck %s --check-prefix=AVX --check-prefix=AVX2
 
 ; Prefer a blend instruction to a vinsert128 instruction because blends
 ; are simpler (no lane changes) and therefore will have equal or better
 ; performance.
 
 define <8 x float> @castA(<4 x float> %m) nounwind uwtable readnone ssp {
 ; AVX-LABEL: castA:
 ; AVX:       ## %bb.0:
-; AVX-NEXT:    ## kill: def $xmm0 killed $xmm0 def $ymm0
-; AVX-NEXT:    vxorps %xmm1, %xmm1, %xmm1
-; AVX-NEXT:    vblendps {{.*#+}} ymm0 = ymm0[0,1,2,3],ymm1[4,5,6,7]
+; AVX-NEXT:    vmovaps %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %shuffle.i = shufflevector <4 x float> %m, <4 x float> zeroinitializer, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 4, i32 4, i32 4>
   ret <8 x float> %shuffle.i
 }
 
 define <4 x double> @castB(<2 x double> %m) nounwind uwtable readnone ssp {
 ; AVX-LABEL: castB:
 ; AVX:       ## %bb.0:

llvm/trunk/test/CodeGen/X86/vector-shuffle-256-v32.ll

; AVX512VL-NEXT:    retq
   %tmp4 = shufflevector <16 x i8> %tmp3, <16 x i8> undef, <32 x i32> <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1>
   ret <32 x i8> %tmp4
 }
 
 define <32 x i8> @zeroable_src_to_zext(<32 x i8> %a0) {
 ; AVX1-LABEL: zeroable_src_to_zext:
 ; AVX1:       # %bb.0:
 ; AVX1-NEXT:    vpmovzxwq {{.*#+}} xmm1 = xmm0[0],zero,zero,zero,xmm0[1],zero,zero,zero
-; AVX1-NEXT:    vpshufb {{.*#+}} xmm0 = xmm0[4,5],zero,zero,zero,zero,zero,zero,xmm0[6,7],zero,zero,zero,zero,zero,zero
+; AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[1,1,2,3]
+; AVX1-NEXT:    vpmovzxwq {{.*#+}} xmm0 = xmm0[0],zero,zero,zero,xmm0[1],zero,zero,zero
 ; AVX1-NEXT:    vinsertf128 $1, %xmm0, %ymm1, %ymm0
 ; AVX1-NEXT:    retq
 ;
-; AVX2-LABEL: zeroable_src_to_zext:
-; AVX2:       # %bb.0:
-; AVX2-NEXT:    vpmovzxwq {{.*#+}} ymm0 = xmm0[0],zero,zero,zero,xmm0[1],zero,zero,zero,xmm0[2],zero,zero,zero,xmm0[3],zero,zero,zero
-; AVX2-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX2-NEXT:    vpblendw {{.*#+}} ymm0 = ymm0[0],ymm1[1,2,3],ymm0[4],ymm1[5,6,7],ymm0[8],ymm1[9,10,11],ymm0[12],ymm1[13,14,15]
-; AVX2-NEXT:    retq
-;
-; AVX512VLBW-LABEL: zeroable_src_to_zext:
-; AVX512VLBW:       # %bb.0:
-; AVX512VLBW-NEXT:    vpmovzxwq {{.*#+}} ymm0 = xmm0[0],zero,zero,zero,xmm0[1],zero,zero,zero,xmm0[2],zero,zero,zero,xmm0[3],zero,zero,zero
-; AVX512VLBW-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX512VLBW-NEXT:    vpblendw {{.*#+}} ymm0 = ymm0[0],ymm1[1,2,3],ymm0[4],ymm1[5,6,7],ymm0[8],ymm1[9,10,11],ymm0[12],ymm1[13,14,15]
-; AVX512VLBW-NEXT:    retq
-;
-; AVX512VLVBMI-LABEL: zeroable_src_to_zext:
-; AVX512VLVBMI:       # %bb.0:
-; AVX512VLVBMI-NEXT:    vmovdqa {{.*#+}} ymm2 = [32,33,0,0,0,0,0,0,34,35,0,0,0,0,0,0,36,37,16,16,16,16,16,16,38,39,16,16,16,16,16,16]
-; AVX512VLVBMI-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX512VLVBMI-NEXT:    vpermt2b %ymm0, %ymm2, %ymm1
-; AVX512VLVBMI-NEXT:    vmovdqa %ymm1, %ymm0
-; AVX512VLVBMI-NEXT:    retq
+; AVX2OR512VL-LABEL: zeroable_src_to_zext:
+; AVX2OR512VL:       # %bb.0:
+; AVX2OR512VL-NEXT:    vpmovzxwq {{.*#+}} ymm0 = xmm0[0],zero,zero,zero,xmm0[1],zero,zero,zero,xmm0[2],zero,zero,zero,xmm0[3],zero,zero,zero
+; AVX2OR512VL-NEXT:    retq
   %1 = shufflevector <32 x i8> %a0, <32 x i8> undef, <32 x i32> <i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 0, i32 1, i32 2, i32 3, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 4, i32 5, i32 6, i32 7, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef, i32 undef>
   %2 = shufflevector <32 x i8> %1, <32 x i8> <i8 0, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 0, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef, i8 undef>, <32 x i32> <i32 8, i32 9, i32 32, i32 32, i32 32, i32 32, i32 32, i32 32, i32 10, i32 11, i32 32, i32 32, i32 32, i32 32, i32 32, i32 32, i32 20, i32 21, i32 48, i32 48, i32 48, i32 48, i32 48, i32 48, i32 22, i32 23, i32 48, i32 48, i32 48, i32 48, i32 48, i32 48>
   ret <32 x i8> %2
 }