[SelectionDAG] Allow constant folding for implicitly truncating BUILD_VECTOR nodes

[guyblank]

Summary

In FoldConstantArithmetic, handle BUILD_VECTOR nodes that do implicit truncation on the elements.

This is similar to what is done in FoldConstantVectorArithmetic.

[RKSimon]

LGTM. The duplication between this and FoldConstantVectorArithmetic suggests that a merging is overdue - add a TODO comment?

[guyblank]

Sure, I'll add a TODO, thanks.

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
   BuildVectorSDNode *BV1 = dyn_cast<BuildVectorSDNode>(Cst1);
   BuildVectorSDNode *BV2 = dyn_cast<BuildVectorSDNode>(Cst2);
   if (!BV1 || !BV2)
     return SDValue();
 
   assert(BV1->getNumOperands() == BV2->getNumOperands() && "Out of sync!");
 
   EVT SVT = VT.getScalarType();
+  EVT LegalSVT = SVT;
+  if (NewNodesMustHaveLegalTypes && LegalSVT.isInteger()) {
+    LegalSVT = TLI->getTypeToTransformTo(*getContext(), LegalSVT);
+    if (LegalSVT.bitsLT(SVT))
+      return SDValue();
+  }
   SmallVector<SDValue, 4> Outputs;
   for (unsigned I = 0, E = BV1->getNumOperands(); I != E; ++I) {
     SDValue V1 = BV1->getOperand(I);
     SDValue V2 = BV2->getOperand(I);
 
-    // Avoid BUILD_VECTOR nodes that perform implicit truncation.
-    // FIXME: This is valid and could be handled by truncation.
+    if (SVT.isInteger()) {
+        if (V1->getValueType(0).bitsGT(SVT))
+          V1 = getNode(ISD::TRUNCATE, DL, SVT, V1);
+        if (V2->getValueType(0).bitsGT(SVT))
+          V2 = getNode(ISD::TRUNCATE, DL, SVT, V2);
+    }
+
     if (V1->getValueType(0) != SVT || V2->getValueType(0) != SVT)
       return SDValue();
 
     // Fold one vector element.
     SDValue ScalarResult = getNode(Opcode, DL, SVT, V1, V2);
+    if (LegalSVT != SVT)
+      ScalarResult = getNode(ISD::SIGN_EXTEND, DL, LegalSVT, ScalarResult);
 
     // Scalar folding only succeeded if the result is a constant or UNDEF.
     if (!ScalarResult.isUndef() && ScalarResult.getOpcode() != ISD::Constant &&
         ScalarResult.getOpcode() != ISD::ConstantFP)
       return SDValue();
     Outputs.push_back(ScalarResult);
   }
 

test/CodeGen/X86/avx512-intrinsics.ll

 ; CHECK-LABEL: test_kxnor:
 ; CHECK:       ## BB#0:
 ; CHECK-NEXT:    kmovw %esi, %k0
 ; CHECK-NEXT:    kmovw %edi, %k1
 ; CHECK-NEXT:    movw $8, %ax
 ; CHECK-NEXT:    kmovw %eax, %k2
 ; CHECK-NEXT:    kxorw %k0, %k1, %k0
 ; CHECK-NEXT:    kxorw %k0, %k2, %k0
-; CHECK-NEXT:    kxnorw %k0, %k0, %k1
-; CHECK-NEXT:    kxnorw %k1, %k0, %k0
 ; CHECK-NEXT:    kmovw %k0, %eax
 ; CHECK-NEXT:    ## kill: %AX<def> %AX<kill> %EAX<kill>
 ; CHECK-NEXT:    retq
   %t1 = call i16 @llvm.x86.avx512.kxnor.w(i16 %a0, i16 8)
   %t2 = call i16 @llvm.x86.avx512.kxnor.w(i16 %t1, i16 %a1)
   ret i16 %t2
 }