Index: lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -5570,9 +5570,7 @@
   EVT EltVT = VecVT.getVectorElementType();
 
   // Calculate the element offset and add it to the pointer.
-  unsigned EltSize = EltVT.getSizeInBits() / 8; // FIXME: should be ABI size.
-  assert(EltSize * 8 == EltVT.getSizeInBits() &&
-         "Converting bits to bytes lost precision");
+  unsigned EltSize = EltVT.getStoreSize();
 
   Index = clampDynamicVectorIndex(DAG, Index, VecVT, dl);
 
Index: test/CodeGen/WebAssembly/bug42304.ll
===================================================================
--- /dev/null
+++ test/CodeGen/WebAssembly/bug42304.ll
@@ -0,0 +1,55 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=wasm32-unknown-unknown < %s | FileCheck %s
+
+define void @dynamic_vector_extract_nonpow2() {
+; CHECK-LABEL: dynamic_vector_extract_nonpow2:
+; CHECK:         .functype dynamic_vector_extract_nonpow2 () -> ()
+; CHECK-NEXT:    .local i32, i64
+; CHECK-NEXT:  # %bb.0: # %bb
+; CHECK-NEXT:    global.get __stack_pointer
+; CHECK-NEXT:    i32.const 16
+; CHECK-NEXT:    i32.sub
+; CHECK-NEXT:    local.tee 0
+; CHECK-NEXT:    i64.const 0
+; CHECK-NEXT:    i64.store8 10
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    i64.const 0
+; CHECK-NEXT:    i64.store16 8
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    i64.const 0
+; CHECK-NEXT:    i64.store 0
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    i32.load16_u 0
+; CHECK-NEXT:    i32.const 1
+; CHECK-NEXT:    i32.and
+; CHECK-NEXT:    i32.const 6
+; CHECK-NEXT:    i32.mul
+; CHECK-NEXT:    i32.or
+; CHECK-NEXT:    local.tee 0
+; CHECK-NEXT:    i64.load32_u 0
+; CHECK-NEXT:    local.tee 1
+; CHECK-NEXT:    i64.store32 0
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    local.get 1
+; CHECK-NEXT:    local.get 0
+; CHECK-NEXT:    i32.const 4
+; CHECK-NEXT:    i32.add
+; CHECK-NEXT:    i64.load16_u 0
+; CHECK-NEXT:    i64.const 32
+; CHECK-NEXT:    i64.shl
+; CHECK-NEXT:    i64.or
+; CHECK-NEXT:    i64.const 4398046511103
+; CHECK-NEXT:    i64.and
+; CHECK-NEXT:    i64.const 32
+; CHECK-NEXT:    i64.shr_u
+; CHECK-NEXT:    i64.store16 0
+; CHECK-NEXT:    # fallthrough-return-void
+; CHECK-NEXT:    end_function
+bb:
+  %L = load i16, i16* undef
+  %E = extractelement <2 x i42> zeroinitializer, i16 %L
+  store i42 %E, i42* undef
+  ret void
+}