[WebAssembly] Fix ISel crash in SIGN_EXTEND_INREG lowering

[tlively]

Summary

The code previously assumed that the index of a vector extract was
constant, but this was not always true. This patch fixes the problem
by bailing out of the lowering if the index is nonconstant and also
replaces static_casts in the lowering function with casts because
the latter contain type-checking asserts that would make similar
issues easier to find and debug.

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

WebAssemblyTargetLowering::LowerSIGN_EXTEND_INREG(SDValue Op,
   assert(!Subtarget->hasSignExt() && Subtarget->hasSIMD128());
   if (Op.getOperand(0).getOpcode() != ISD::EXTRACT_VECTOR_ELT)
     return SDValue();
 
   const SDValue &Extract = Op.getOperand(0);
   MVT VecT = Extract.getOperand(0).getSimpleValueType();
   if (VecT.getVectorElementType().getSizeInBits() > 32)
     return SDValue();
-  MVT ExtractedLaneT = static_cast<VTSDNode *>(Op.getOperand(1).getNode())
-                           ->getVT()
-                           .getSimpleVT();
+  MVT ExtractedLaneT =
+      cast<VTSDNode>(Op.getOperand(1).getNode())->getVT().getSimpleVT();
   MVT ExtractedVecT =
       MVT::getVectorVT(ExtractedLaneT, 128 / ExtractedLaneT.getSizeInBits());
   if (ExtractedVecT == VecT)
     return Op;
 
   // Bitcast vector to appropriate type to ensure ISel pattern coverage
-  const SDValue &Index = Extract.getOperand(1);
-  unsigned IndexVal =
-      static_cast<ConstantSDNode *>(Index.getNode())->getZExtValue();
+  const SDNode *Index = Extract.getOperand(1).getNode();
+  if (!isa<ConstantSDNode>(Index))
+    return SDValue();
+  unsigned IndexVal = cast<ConstantSDNode>(Index)->getZExtValue();
   unsigned Scale =
       ExtractedVecT.getVectorNumElements() / VecT.getVectorNumElements();
   assert(Scale > 1);
   SDValue NewIndex =
-      DAG.getConstant(IndexVal * Scale, DL, Index.getValueType());
+      DAG.getConstant(IndexVal * Scale, DL, Index->getValueType(0));
   SDValue NewExtract = DAG.getNode(
       ISD::EXTRACT_VECTOR_ELT, DL, Extract.getValueType(),
       DAG.getBitcast(ExtractedVecT, Extract.getOperand(0)), NewIndex);
   return DAG.getNode(ISD::SIGN_EXTEND_INREG, DL, Op.getValueType(), NewExtract,
                      Op.getOperand(1));
 }
 
 SDValue WebAssemblyTargetLowering::LowerBUILD_VECTOR(SDValue Op,

llvm/test/CodeGen/WebAssembly/simd-nonconst-sext.ll

+; RUN: llc < %s -asm-verbose=false -verify-machineinstrs -mattr=+simd128 | FileCheck %s
+
+; A regression test for a bug in the lowering of SIGN_EXTEND_INREG
+; with SIMD and without sign-ext where ISel would crash if the index
+; of the vector extract was not a constant.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32"
+
+; CHECK-LABEL: foo:
+; CHECK-NEXT: .functype foo () -> (f32)
+; CHECK: i32x4.load16x4_u
+; CHECK: f32.convert_i32_s
+define float @foo() {
+  %1 = load <4 x i16>, <4 x i16>* undef, align 8
+  %2 = load i32, i32* undef, align 4
+  %vecext = extractelement <4 x i16> %1, i32 %2
+  %conv = sitofp i16 %vecext to float
+  ret float %conv
+}