Index: lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
===================================================================
--- lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
+++ lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp
@@ -348,8 +348,7 @@
     // FIXME: there is probably a cleaner way to do this.
     auto IsLoadStore = InstName.startswith("load") ||
                        InstName.startswith("store") ||
-                       InstName.startswith("atomic_load") ||
-                       InstName.startswith("atomic_store");
+                       InstName.startswith("atomic");
     if (IsLoadStore) {
       // Parse load/store operands of the form: offset align
       auto &Offset = Lexer.getTok();
Index: lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
===================================================================
--- lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
+++ lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
@@ -337,7 +337,7 @@
               (ins P2Align:$p2align, offset32_op:$off, I32:$addr, rc:$val),
               (outs), (ins P2Align:$p2align, offset32_op:$off), [],
               !strconcat(Name, "\t$dst, ${off}(${addr})${p2align}, $val"),
-              !strconcat(Name, "\t${off}, ${p2align}"), Opcode>;
+              !strconcat(Name, "\t${off}${p2align}"), Opcode>;
 }
 
 defm ATOMIC_RMW_ADD_I32 : WebAssemblyBinRMW<I32, "i32.atomic.rmw.add", 0xfe1e>;
@@ -668,7 +668,7 @@
                    rc:$new),
               (outs), (ins P2Align:$p2align, offset32_op:$off), [],
               !strconcat(Name, "\t$dst, ${off}(${addr})${p2align}, $exp, $new"),
-              !strconcat(Name, "\t${off}, ${p2align}"), Opcode>;
+              !strconcat(Name, "\t${off}${p2align}"), Opcode>;
 }
 
 defm ATOMIC_RMW_CMPXCHG_I32 :
@@ -901,20 +901,20 @@
     (ins P2Align:$p2align, offset32_op:$off, I32:$addr, I32:$count),
     (outs), (ins P2Align:$p2align, offset32_op:$off), [],
     "atomic.notify \t$dst, ${off}(${addr})${p2align}, $count",
-    "atomic.notify \t${off}, ${p2align}", 0xfe00>;
+    "atomic.notify \t${off}${p2align}", 0xfe00>;
 let mayLoad = 1 in {
 defm ATOMIC_WAIT_I32 :
   I<(outs I32:$dst),
     (ins P2Align:$p2align, offset32_op:$off, I32:$addr, I32:$exp, I64:$timeout),
     (outs), (ins P2Align:$p2align, offset32_op:$off), [],
     "i32.atomic.wait \t$dst, ${off}(${addr})${p2align}, $exp, $timeout",
-    "i32.atomic.wait \t${off}, ${p2align}", 0xfe01>;
+    "i32.atomic.wait \t${off}${p2align}", 0xfe01>;
 defm ATOMIC_WAIT_I64 :
   I<(outs I32:$dst),
     (ins P2Align:$p2align, offset32_op:$off, I32:$addr, I64:$exp, I64:$timeout),
     (outs), (ins P2Align:$p2align, offset32_op:$off), [],
     "i64.atomic.wait \t$dst, ${off}(${addr})${p2align}, $exp, $timeout",
-    "i64.atomic.wait \t${off}, ${p2align}", 0xfe02>;
+    "i64.atomic.wait \t${off}${p2align}", 0xfe02>;
 } // mayLoad = 1
 } // hasSideEffects = 1
 
Index: test/MC/WebAssembly/atomics-encodings.s
===================================================================
--- /dev/null
+++ test/MC/WebAssembly/atomics-encodings.s
@@ -0,0 +1,149 @@
+# RUN: llvm-mc -show-encoding -triple=wasm32-unkown-unknown -mattr=+atomics < %s | FileCheck %s
+
+main:
+  .functype main () -> ()
+
+  # FIXME This doesn't work because of PR40728. Enable this once it's fixed.
+  # C HECK:  atomic.notify 0:p2align=0 # encoding: [0xfe,0x00,0x00,0x00]
+  # atomic.notify 0
+  # CHECK:  i32.atomic.wait 0:p2align=0 # encoding: [0xfe,0x01,0x00,0x00]
+  i32.atomic.wait 0
+  # CHECK:  i64.atomic.wait 0:p2align=0 # encoding: [0xfe,0x02,0x00,0x00]
+  i64.atomic.wait 0
+
+  # CHECK: i32.atomic.load 0:p2align=0 # encoding: [0xfe,0x10,0x00,0x00]
+  i32.atomic.load 0
+  # CHECK: i64.atomic.load 4:p2align=0 # encoding: [0xfe,0x11,0x00,0x04]
+  i64.atomic.load 4
+  # CHECK:  i32.atomic.load8_u 48 # encoding: [0xfe,0x12,0x00,0x30]
+  i32.atomic.load8_u 48
+  # CHECK:  i32.atomic.load16_u 0:p2align=0 # encoding: [0xfe,0x13,0x00,0x00]
+  i32.atomic.load16_u 0
+  # CHECK:  i64.atomic.load8_u 0 # encoding: [0xfe,0x14,0x00,0x00]
+  i64.atomic.load8_u 0
+  # CHECK:  i64.atomic.load16_u 0:p2align=0 # encoding: [0xfe,0x15,0x00,0x00]
+  i64.atomic.load16_u 0
+  # CHECK:  i64.atomic.load32_u 0:p2align=0 # encoding: [0xfe,0x16,0x00,0x00]
+  i64.atomic.load32_u 0
+
+  # CHECK:  i32.atomic.store 0:p2align=0 # encoding: [0xfe,0x17,0x00,0x00]
+  i32.atomic.store 0
+  # CHECK:  i64.atomic.store 8:p2align=0 # encoding: [0xfe,0x18,0x00,0x08]
+  i64.atomic.store 8
+  # CHECK:  i32.atomic.store8 0 # encoding: [0xfe,0x19,0x00,0x00]
+  i32.atomic.store8 0
+  # CHECK:  i32.atomic.store16 0:p2align=0 # encoding: [0xfe,0x1a,0x00,0x00]
+  i32.atomic.store16 0
+  # CHECK:  i64.atomic.store8 16 # encoding: [0xfe,0x1b,0x00,0x10]
+  i64.atomic.store8 16
+  # CHECK:  i64.atomic.store16 0:p2align=0 # encoding: [0xfe,0x1c,0x00,0x00]
+  i64.atomic.store16 0
+  # CHECK:  i64.atomic.store32 0:p2align=0 # encoding: [0xfe,0x1d,0x00,0x00]
+  i64.atomic.store32 0
+
+  # CHECK:  i32.atomic.rmw.add 0:p2align=0 # encoding: [0xfe,0x1e,0x00,0x00]
+  i32.atomic.rmw.add 0
+  # CHECK:  i64.atomic.rmw.add 0:p2align=0 # encoding: [0xfe,0x1f,0x00,0x00]
+  i64.atomic.rmw.add 0
+  # CHECK:  i32.atomic.rmw8.add_u 0 # encoding: [0xfe,0x20,0x00,0x00]
+  i32.atomic.rmw8.add_u 0
+  # CHECK:  i32.atomic.rmw16.add_u 0:p2align=0 # encoding: [0xfe,0x21,0x00,0x00]
+  i32.atomic.rmw16.add_u 0
+  # CHECK:  i64.atomic.rmw8.add_u 0 # encoding: [0xfe,0x22,0x00,0x00]
+  i64.atomic.rmw8.add_u 0
+  # CHECK:  i64.atomic.rmw16.add_u 0:p2align=0 # encoding: [0xfe,0x23,0x00,0x00]
+  i64.atomic.rmw16.add_u 0
+  # CHECK:  i64.atomic.rmw32.add_u 16:p2align=0 # encoding: [0xfe,0x24,0x00,0x10]
+  i64.atomic.rmw32.add_u 16
+
+  # CHECK:  i32.atomic.rmw.sub 0:p2align=0 # encoding: [0xfe,0x25,0x00,0x00]
+  i32.atomic.rmw.sub 0
+  # CHECK:  i64.atomic.rmw.sub 0:p2align=0 # encoding: [0xfe,0x26,0x00,0x00]
+  i64.atomic.rmw.sub 0
+  # CHECK:  i32.atomic.rmw8.sub_u 0 # encoding: [0xfe,0x27,0x00,0x00]
+  i32.atomic.rmw8.sub_u 0
+  # CHECK:  i32.atomic.rmw16.sub_u 0:p2align=0 # encoding: [0xfe,0x28,0x00,0x00]
+  i32.atomic.rmw16.sub_u 0
+  # CHECK:  i64.atomic.rmw8.sub_u 8 # encoding: [0xfe,0x29,0x00,0x08]
+  i64.atomic.rmw8.sub_u 8
+  # CHECK:  i64.atomic.rmw16.sub_u 0:p2align=0 # encoding: [0xfe,0x2a,0x00,0x00]
+  i64.atomic.rmw16.sub_u 0
+  # CHECK:  i64.atomic.rmw32.sub_u 0:p2align=0 # encoding: [0xfe,0x2b,0x00,0x00]
+  i64.atomic.rmw32.sub_u 0
+
+  # CHECK:  i32.atomic.rmw.and 0:p2align=0 # encoding: [0xfe,0x2c,0x00,0x00]
+  i32.atomic.rmw.and 0
+  # CHECK:  i64.atomic.rmw.and 0:p2align=0 # encoding: [0xfe,0x2d,0x00,0x00]
+  i64.atomic.rmw.and 0
+  # CHECK:  i32.atomic.rmw8.and_u 0 # encoding: [0xfe,0x2e,0x00,0x00]
+  i32.atomic.rmw8.and_u 0
+  # CHECK:  i32.atomic.rmw16.and_u 0:p2align=0 # encoding: [0xfe,0x2f,0x00,0x00]
+  i32.atomic.rmw16.and_u 0
+  # CHECK:  i64.atomic.rmw8.and_u 96 # encoding: [0xfe,0x30,0x00,0x60]
+  i64.atomic.rmw8.and_u 96
+  # CHECK:  i64.atomic.rmw16.and_u 0:p2align=0 # encoding: [0xfe,0x31,0x00,0x00]
+  i64.atomic.rmw16.and_u 0
+  # CHECK:  i64.atomic.rmw32.and_u 0:p2align=0 # encoding: [0xfe,0x32,0x00,0x00]
+  i64.atomic.rmw32.and_u 0
+
+  # CHECK:  i32.atomic.rmw.or 0:p2align=0 # encoding: [0xfe,0x33,0x00,0x00]
+  i32.atomic.rmw.or 0
+  # CHECK:  i64.atomic.rmw.or 0:p2align=0 # encoding: [0xfe,0x34,0x00,0x00]
+  i64.atomic.rmw.or 0
+  # CHECK:  i32.atomic.rmw8.or_u 0 # encoding: [0xfe,0x35,0x00,0x00]
+  i32.atomic.rmw8.or_u 0
+  # CHECK:  i32.atomic.rmw16.or_u 0:p2align=0 # encoding: [0xfe,0x36,0x00,0x00]
+  i32.atomic.rmw16.or_u 0
+  # CHECK:  i64.atomic.rmw8.or_u 0 # encoding: [0xfe,0x37,0x00,0x00]
+  i64.atomic.rmw8.or_u 0
+  # CHECK:  i64.atomic.rmw16.or_u 48:p2align=0 # encoding: [0xfe,0x38,0x00,0x30]
+  i64.atomic.rmw16.or_u 48
+  # CHECK:  i64.atomic.rmw32.or_u 0:p2align=0 # encoding: [0xfe,0x39,0x00,0x00]
+  i64.atomic.rmw32.or_u 0
+
+  # CHECK:  i32.atomic.rmw.xor 0:p2align=0 # encoding: [0xfe,0x3a,0x00,0x00]
+  i32.atomic.rmw.xor 0
+  # CHECK:  i64.atomic.rmw.xor 0:p2align=0 # encoding: [0xfe,0x3b,0x00,0x00]
+  i64.atomic.rmw.xor 0
+  # CHECK:  i32.atomic.rmw8.xor_u 4 # encoding: [0xfe,0x3c,0x00,0x04]
+  i32.atomic.rmw8.xor_u 4
+  # CHECK:  i32.atomic.rmw16.xor_u 0:p2align=0 # encoding: [0xfe,0x3d,0x00,0x00]
+  i32.atomic.rmw16.xor_u 0
+  # CHECK:  i64.atomic.rmw8.xor_u 0 # encoding: [0xfe,0x3e,0x00,0x00]
+  i64.atomic.rmw8.xor_u 0
+  # CHECK:  i64.atomic.rmw16.xor_u 0:p2align=0 # encoding: [0xfe,0x3f,0x00,0x00]
+  i64.atomic.rmw16.xor_u 0
+  # CHECK:  i64.atomic.rmw32.xor_u 0:p2align=0 # encoding: [0xfe,0x40,0x00,0x00]
+  i64.atomic.rmw32.xor_u 0
+
+  # CHECK:  i32.atomic.rmw.xchg 0:p2align=0 # encoding: [0xfe,0x41,0x00,0x00]
+  i32.atomic.rmw.xchg 0
+  # CHECK:  i64.atomic.rmw.xchg 0:p2align=0 # encoding: [0xfe,0x42,0x00,0x00]
+  i64.atomic.rmw.xchg 0
+  # CHECK:  i32.atomic.rmw8.xchg_u 0 # encoding: [0xfe,0x43,0x00,0x00]
+  i32.atomic.rmw8.xchg_u 0
+  # CHECK:  i32.atomic.rmw16.xchg_u 0:p2align=0 # encoding: [0xfe,0x44,0x00,0x00]
+  i32.atomic.rmw16.xchg_u 0
+  # CHECK:  i64.atomic.rmw8.xchg_u 0 # encoding: [0xfe,0x45,0x00,0x00]
+  i64.atomic.rmw8.xchg_u 0
+  # CHECK:  i64.atomic.rmw16.xchg_u 8:p2align=0 # encoding: [0xfe,0x46,0x00,0x08]
+  i64.atomic.rmw16.xchg_u 8
+  # CHECK:  i64.atomic.rmw32.xchg_u 0:p2align=0 # encoding: [0xfe,0x47,0x00,0x00]
+  i64.atomic.rmw32.xchg_u 0
+
+  # CHECK:  i32.atomic.rmw.cmpxchg 32:p2align=0 # encoding: [0xfe,0x48,0x00,0x20]
+  i32.atomic.rmw.cmpxchg 32
+  # CHECK:  i64.atomic.rmw.cmpxchg 0:p2align=0 # encoding: [0xfe,0x49,0x00,0x00]
+  i64.atomic.rmw.cmpxchg 0
+  # CHECK:  i32.atomic.rmw8.cmpxchg_u 0 # encoding: [0xfe,0x4a,0x00,0x00]
+  i32.atomic.rmw8.cmpxchg_u 0
+  # CHECK:  i32.atomic.rmw16.cmpxchg_u 0:p2align=0 # encoding: [0xfe,0x4b,0x00,0x00]
+  i32.atomic.rmw16.cmpxchg_u 0
+  # CHECK:  i64.atomic.rmw8.cmpxchg_u 16 # encoding: [0xfe,0x4c,0x00,0x10]
+  i64.atomic.rmw8.cmpxchg_u 16
+  # CHECK:  i64.atomic.rmw16.cmpxchg_u 0:p2align=0 # encoding: [0xfe,0x4d,0x00,0x00]
+  i64.atomic.rmw16.cmpxchg_u 0
+  # CHECK:  i64.atomic.rmw32.cmpxchg_u 0:p2align=0 # encoding: [0xfe,0x4e,0x00,0x00]
+  i64.atomic.rmw32.cmpxchg_u 0
+
+  end_function