Index: lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
===================================================================
--- lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
+++ lib/Target/WebAssembly/WebAssemblyISelLowering.cpp
@@ -115,8 +115,12 @@
 
   // As a special case, these operators use the type to mean the type to
   // sign-extend from.
-  for (auto T : {MVT::i1, MVT::i8, MVT::i16, MVT::i32})
-    setOperationAction(ISD::SIGN_EXTEND_INREG, T, Expand);
+  setOperationAction(ISD::SIGN_EXTEND_INREG, MVT::i1, Expand);
+  if (!Subtarget->hasAtomics()) {
+    // The Atomics feature includes signext intructions.
+    for (auto T : {MVT::i8, MVT::i16, MVT::i32})
+      setOperationAction(ISD::SIGN_EXTEND_INREG, T, Expand);
+  }
 
   // Dynamic stack allocation: use the default expansion.
   setOperationAction(ISD::STACKSAVE, MVT::Other, Expand);
Index: lib/Target/WebAssembly/WebAssemblyInstrConv.td
===================================================================
--- lib/Target/WebAssembly/WebAssemblyInstrConv.td
+++ lib/Target/WebAssembly/WebAssemblyInstrConv.td
@@ -26,6 +26,24 @@
                          [(set I64:$dst, (zext I32:$src))],
                          "i64.extend_u/i32\t$dst, $src", 0xad>;
 
+let Predicates = [HasAtomics] in {
+def I32_EXTEND8_S_I32 : I<(outs I32:$dst), (ins I32:$src),
+                          [(set I32:$dst, (sext_inreg I32:$src, i8))],
+                          "i32.extend8_s\t$dst, $src", 0xc0>;
+def I32_EXTEND16_S_I32 : I<(outs I32:$dst), (ins I32:$src),
+                           [(set I32:$dst, (sext_inreg I32:$src, i16))],
+                           "i32.extend16_s\t$dst, $src", 0xc1>;
+def I64_EXTEND8_S_I64 : I<(outs I64:$dst), (ins I64:$src),
+                           [(set I64:$dst, (sext_inreg I64:$src, i8))],
+                           "i64.extend8_s\t$dst, $src", 0xc2>;
+def I64_EXTEND16_S_I64 : I<(outs I64:$dst), (ins I64:$src),
+                           [(set I64:$dst, (sext_inreg I64:$src, i16))],
+                           "i64.extend16_s\t$dst, $src", 0xc3>;
+def I64_EXTEND32_S_I64 : I<(outs I64:$dst), (ins I64:$src),
+                           [(set I64:$dst, (sext_inreg I64:$src, i32))],
+                           "i64.extend32_s\t$dst, $src", 0xc4>;
+} // Predicates = [HasAtomics]
+
 } // defs = [ARGUMENTS]
 
 // Expand a "don't care" extend into zero-extend (chosen over sign-extend
Index: test/CodeGen/WebAssembly/signext-inreg.ll
===================================================================
--- /dev/null
+++ test/CodeGen/WebAssembly/signext-inreg.ll
@@ -0,0 +1,71 @@
+; RUN: llc < %s -mattr=+atomics -asm-verbose=false -disable-wasm-fallthrough-return-opt -disable-wasm-explicit-locals | FileCheck %s
+; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -disable-wasm-explicit-locals | FileCheck %s --check-prefix=NOATOMIC
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown-wasm"
+
+; CHECK-LABEL: i32_extend8_s:
+; CHECK-NEXT: .param i32{{$}}
+; CHECK-NEXT: .result i32{{$}}
+; CHECK-NEXT: i32.extend8_s $push[[NUM:[0-9]+]]=, $0{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+
+; NOATOMIC-LABEL: i32_extend8_s
+; NOATOMIC-NOT: i32.extend8_s
+define i32 @i32_extend8_s(i8 %x) {
+  %a = sext i8 %x to i32
+  ret i32 %a
+}
+
+; CHECK-LABEL: i32_extend16_s:
+; CHECK-NEXT: .param i32{{$}}
+; CHECK-NEXT: .result i32{{$}}
+; CHECK-NEXT: i32.extend16_s $push[[NUM:[0-9]+]]=, $0{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+
+; NOATOMIC-LABEL: i32_extend16_s
+; NOATOMIC-NOT: i32.extend16_s
+define i32 @i32_extend16_s(i16 %x) {
+  %a = sext i16 %x to i32
+  ret i32 %a
+}
+
+; CHECK-LABEL: i64_extend8_s:
+; CHECK-NEXT: .param i32{{$}}
+; CHECK-NEXT: .result i64{{$}}
+; CHECK-NEXT: i64.extend_u/i32 $push[[NUM1:[0-9]+]]=, $0{{$}}
+; CHECK-NEXT: i64.extend8_s $push[[NUM2:[0-9]+]]=, $pop[[NUM1]]{{$}}
+; CHECK-NEXT: return $pop[[NUM2]]{{$}}
+
+; NOATOMIC-LABEL: i64_extend8_s
+; NOATOMIC-NOT: i64.extend8_s
+define i64 @i64_extend8_s(i8 %x) {
+  %a = sext i8 %x to i64
+  ret i64 %a
+}
+
+; CHECK-LABEL: i64_extend16_s:
+; CHECK-NEXT: .param i32{{$}}
+; CHECK-NEXT: .result i64{{$}}
+; CHECK-NEXT: i64.extend_u/i32 $push[[NUM1:[0-9]+]]=, $0{{$}}
+; CHECK-NEXT: i64.extend16_s $push[[NUM2:[0-9]+]]=, $pop[[NUM1]]{{$}}
+; CHECK-NEXT: return $pop[[NUM2]]{{$}}
+
+; NOATOMIC-LABEL: i64_extend16_s
+; NOATOMIC-NOT: i16.extend16_s
+define i64 @i64_extend16_s(i16 %x) {
+  %a = sext i16 %x to i64
+  ret i64 %a
+}
+
+; No SIGN_EXTEND_INREG is needed for 32->64 extension.
+; CHECK-LABEL: i64_extend32_s:
+; CHECK-NEXT: .param i32{{$}}
+; CHECK-NEXT: .result i64{{$}}
+; CHECK-NEXT: i64.extend_s/i32 $push[[NUM:[0-9]+]]=, $0{{$}}
+; CHECK-NEXT: return $pop[[NUM]]{{$}}
+define i64 @i64_extend32_s(i32 %x) {
+  %a = sext i32 %x to i64
+  ret i64 %a
+}
+