diff --git a/js/src/jit-test/tests/wasm/basic-integer.js b/js/src/jit-test/tests/wasm/basic-integer.js
index 815195ee8992..ab9929ad42b9 100644
--- a/js/src/jit-test/tests/wasm/basic-integer.js
+++ b/js/src/jit-test/tests/wasm/basic-integer.js
@@ -139,6 +139,11 @@ testComparison32('gt_u', 40, 40, 0);
 testComparison32('ge_s', 40, 40, 1);
 testComparison32('ge_u', 40, 40, 1);
 
+// Test MTest's GVN branch inversion.
+var testTrunc = wasmEvalText(`(module (func (param f32) (result i32) (if (i32.eqz (i32.trunc_s/f32 (get_local 0))) (i32.const 0) (i32.const 1))) (export "" 0))`);
+assertEq(testTrunc(0), 0);
+assertEq(testTrunc(13.37), 1);
+
 if (hasI64()) {
 
     setJitCompilerOption('wasm.test-mode', 1);
@@ -253,6 +258,11 @@ if (hasI64()) {
     testUnary('i64', 'popcnt', -1, 64);
     testUnary('i64', 'popcnt', 0, 0);
 
+    // Test MTest's GVN branch inversion.
+    var testTrunc = wasmEvalText(`(module (func (param f32) (result i32) (if (i64.eqz (i64.trunc_s/f32 (get_local 0))) (i32.const 0) (i32.const 1))) (export "" 0))`);
+    assertEq(testTrunc(0), 0);
+    assertEq(testTrunc(13.37), 1);
+
     testI64Eqz(40, 0);
     testI64Eqz(0, 1);
 
diff --git a/js/src/jit/Lowering.cpp b/js/src/jit/Lowering.cpp
index c7758901d994..4ef619626d36 100644
--- a/js/src/jit/Lowering.cpp
+++ b/js/src/jit/Lowering.cpp
@@ -936,6 +936,9 @@ LIRGenerator::visitTest(MTest* test)
       case MIRType::Boolean:
         add(new(alloc()) LTestIAndBranch(useRegister(opd), ifTrue, ifFalse));
         break;
+      case MIRType::Int64:
+        add(new(alloc()) LTestI64AndBranch(useInt64Register(opd), ifTrue, ifFalse));
+        break;
       default:
         MOZ_CRASH("Bad type");
     }
diff --git a/js/src/jit/MIR.cpp b/js/src/jit/MIR.cpp
index a5fdc14af51a..395b5963ef7b 100644
--- a/js/src/jit/MIR.cpp
+++ b/js/src/jit/MIR.cpp
@@ -4568,10 +4568,8 @@ MNot::foldsTo(TempAllocator& alloc)
     if (MConstant* inputConst = input()->maybeConstantValue()) {
         bool b;
         if (inputConst->valueToBoolean(&b)) {
-            if (type() == MIRType::Int32)
+            if (type() == MIRType::Int32 || type() == MIRType::Int64)
                 return MConstant::New(alloc, Int32Value(!b));
-            if (type() == MIRType::Int64)
-                return MConstant::NewInt64(alloc, int64_t(!b));
             return MConstant::New(alloc, BooleanValue(!b));
         }
     }
diff --git a/js/src/jit/shared/LIR-shared.h b/js/src/jit/shared/LIR-shared.h
index 41301ca556ea..6eb350d469e6 100644
--- a/js/src/jit/shared/LIR-shared.h
+++ b/js/src/jit/shared/LIR-shared.h
@@ -2258,6 +2258,27 @@ class LTestIAndBranch : public LControlInstructionHelper<2, 1, 0>
     }
 };
 
+// Takes in an int64 input and tests it for truthiness.
+class LTestI64AndBranch : public LControlInstructionHelper<2, INT64_PIECES, 0>
+{
+  public:
+    LIR_HEADER(TestI64AndBranch)
+
+    LTestI64AndBranch(const LInt64Allocation& in, MBasicBlock* ifTrue, MBasicBlock* ifFalse)
+    {
+        setInt64Operand(0, in);
+        setSuccessor(0, ifTrue);
+        setSuccessor(1, ifFalse);
+    }
+
+    MBasicBlock* ifTrue() const {
+        return getSuccessor(0);
+    }
+    MBasicBlock* ifFalse() const {
+        return getSuccessor(1);
+    }
+};
+
 // Takes in either an integer or boolean input and tests it for truthiness.
 class LTestDAndBranch : public LControlInstructionHelper<2, 1, 0>
 {
diff --git a/js/src/jit/shared/LOpcodes-shared.h b/js/src/jit/shared/LOpcodes-shared.h
index 5eb400d43949..b3a37507921b 100644
--- a/js/src/jit/shared/LOpcodes-shared.h
+++ b/js/src/jit/shared/LOpcodes-shared.h
@@ -116,6 +116,7 @@
     _(Throw)                        \
     _(Phi)                          \
     _(TestIAndBranch)               \
+    _(TestI64AndBranch)             \
     _(TestDAndBranch)               \
     _(TestFAndBranch)               \
     _(TestVAndBranch)               \
diff --git a/js/src/jit/x64/CodeGenerator-x64.cpp b/js/src/jit/x64/CodeGenerator-x64.cpp
index 9b1bc8edb228..1418756a69e4 100644
--- a/js/src/jit/x64/CodeGenerator-x64.cpp
+++ b/js/src/jit/x64/CodeGenerator-x64.cpp
@@ -1391,3 +1391,11 @@ CodeGeneratorX64::visitPopcntI64(LPopcntI64* lir)
 
     masm.popcnt64(input, output, temp);
 }
+
+void
+CodeGeneratorX64::visitTestI64AndBranch(LTestI64AndBranch* lir)
+{
+    Register input = ToRegister(lir->input());
+    masm.testq(input, input);
+    emitBranch(Assembler::NonZero, lir->ifTrue(), lir->ifFalse());
+}
diff --git a/js/src/jit/x64/CodeGenerator-x64.h b/js/src/jit/x64/CodeGenerator-x64.h
index c6c93cdbf447..1fa0447c9be1 100644
--- a/js/src/jit/x64/CodeGenerator-x64.h
+++ b/js/src/jit/x64/CodeGenerator-x64.h
@@ -80,6 +80,7 @@ class CodeGeneratorX64 : public CodeGeneratorX86Shared
     void visitAsmJSUInt32ToFloat32(LAsmJSUInt32ToFloat32* lir);
     void visitAsmReinterpretFromI64(LAsmReinterpretFromI64* lir);
     void visitAsmReinterpretToI64(LAsmReinterpretToI64* lir);
+    void visitTestI64AndBranch(LTestI64AndBranch* lir);
 
     void visitWasmTruncateToInt32(LWasmTruncateToInt32* lir);
 };
diff --git a/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp b/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
index 677183f640f5..0ff3fbe3b475 100644
--- a/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
+++ b/js/src/jit/x86-shared/CodeGenerator-x86-shared.cpp
@@ -81,10 +81,8 @@ CodeGeneratorX86Shared::visitFloat32(LFloat32* ins)
 void
 CodeGeneratorX86Shared::visitTestIAndBranch(LTestIAndBranch* test)
 {
-    const LAllocation* opd = test->input();
-
-    // Test the operand
-    masm.test32(ToRegister(opd), ToRegister(opd));
+    Register input = ToRegister(test->input());
+    masm.test32(input, input);
     emitBranch(Assembler::NonZero, test->ifTrue(), test->ifFalse());
 }