diff --git a/src/library.js b/src/library.js
index cd7d946c393fd..f3a9d1a91a236 100644
--- a/src/library.js
+++ b/src/library.js
@@ -951,7 +951,7 @@ LibraryManager.library = {
   llvm_ctlz_i8: function(x, isZeroUndef) {
     x = x | 0;
     isZeroUndef = isZeroUndef | 0;
-    return (Math_clz32(x) | 0) - 24 | 0;
+    return (Math_clz32(x & 0xff) | 0) - 24 | 0;
   },
 
   llvm_ctlz_i16__asm: true,
@@ -959,7 +959,7 @@ LibraryManager.library = {
   llvm_ctlz_i16: function(x, isZeroUndef) {
     x = x | 0;
     isZeroUndef = isZeroUndef | 0;
-    return (Math_clz32(x) | 0) - 16 | 0
+    return (Math_clz32(x & 0xffff) | 0) - 16 | 0
   },
 
   llvm_ctlz_i64__asm: true,
diff --git a/tests/cases/rust_ctlz_negative_i8.ll b/tests/cases/rust_ctlz_negative_i8.ll
new file mode 100644
index 0000000000000..08043359d2a61
--- /dev/null
+++ b/tests/cases/rust_ctlz_negative_i8.ll
@@ -0,0 +1,41 @@
+; ModuleID = 'foo.cgu-0.rs'
+source_filename = "foo.cgu-0.rs"
+target datalayout = "e-p:32:32-i64:64-v128:32:128-n32-S128"
+target triple = "asmjs-unknown-emscripten"
+
+@.str = private unnamed_addr constant [18 x i8] c"hello, world %d!\0A\00", align 1
+
+; Function Attrs: inlinehint uwtable
+define internal i32 @"_ZN4core3num20_$LT$impl$u20$u8$GT$13leading_zeros17h0d6a1b849828bff0E"(i8) unnamed_addr #0 {
+entry-block:
+  %tmp_ret = alloca i8
+  br label %start
+
+start:                                            ; preds = %entry-block
+  %1 = call i8 @llvm.ctlz.i8(i8 %0, i1 false)
+  store i8 %1, i8* %tmp_ret
+  %2 = load i8, i8* %tmp_ret
+  br label %bb1
+
+bb1:                                              ; preds = %start
+  %3 = zext i8 %2 to i32
+  ret i32 %3
+}
+
+define i32 @main() {
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval
+  %value = call i32 @"_ZN4core3num20_$LT$impl$u20$u8$GT$13leading_zeros17h0d6a1b849828bff0E"(i8 -1)
+  %call = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([18 x i8], [18 x i8]* @.str, i32 0, i32 0), i32 %value)
+  ret i32 0
+}
+
+; Function Attrs: nounwind readnone
+declare i8 @llvm.ctlz.i8(i8, i1) #2
+
+declare i32 @printf(i8*, ...)
+
+attributes #0 = { inlinehint uwtable }
+attributes #1 = { uwtable }
+attributes #2 = { nounwind readnone }
diff --git a/tests/cases/rust_ctlz_negative_i8.txt b/tests/cases/rust_ctlz_negative_i8.txt
new file mode 100644
index 0000000000000..dfba99f8be198
--- /dev/null
+++ b/tests/cases/rust_ctlz_negative_i8.txt
@@ -0,0 +1 @@
+hello, world 0!