diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
--- a/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
@@ -432,9 +432,22 @@
   for (unsigned I = 0, E = CI->getNumArgOperands(); I < E; ++I)
     ArgAttributes.push_back(InvokeAL.getParamAttributes(I));
 
+  AttrBuilder FnAttrs(InvokeAL.getFnAttributes());
+  if (FnAttrs.contains(Attribute::AllocSize)) {
+    // The allocsize attribute (if any) referes to parameters by index and needs
+    // to be adjusted.
+    unsigned SizeArg;
+    Optional<unsigned> NEltArg;
+    std::tie(SizeArg, NEltArg) = FnAttrs.getAllocSizeArgs();
+    SizeArg += 1;
+    if (NEltArg.hasValue())
+      NEltArg = NEltArg.getValue() + 1;
+    FnAttrs.addAllocSizeAttr(SizeArg, NEltArg);
+  }
+
   // Reconstruct the AttributesList based on the vector we constructed.
   AttributeList NewCallAL =
-      AttributeList::get(C, InvokeAL.getFnAttributes(),
+      AttributeList::get(C, AttributeSet::get(C, FnAttrs),
                          InvokeAL.getRetAttributes(), ArgAttributes);
   NewCall->setAttributes(NewCallAL);
 
diff --git a/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll b/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll
--- a/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll
+++ b/llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll
@@ -94,7 +94,7 @@
 }
 
 ; Test a case when a function call is within try-catch, after a setjmp
-define hidden void @exception_and_longjmp() #3 personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
+define hidden void @exception_and_longjmp() personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) {
 ; CHECK-LABEL: @exception_and_longjmp
 entry:
   %buf = alloca [1 x %struct.__jmp_buf_tag], align 16
@@ -203,6 +203,19 @@
   ret void
 }
 
+; Test that the allocsize attribute is being transformed properly
+declare void @allocator(i32, %struct.__jmp_buf_tag*) #3
+define hidden void @allocsize() {
+; CHECK-LABEL: @allocsize
+entry:
+  %buf = alloca [1 x %struct.__jmp_buf_tag], align 16
+  %arraydecay = getelementptr inbounds [1 x %struct.__jmp_buf_tag], [1 x %struct.__jmp_buf_tag]* %buf, i32 0, i32 0
+  %call = call i32 @setjmp(%struct.__jmp_buf_tag* %arraydecay) #0
+; CHECK: void @"__invoke_void_i32_%struct.__jmp_buf_tag*"([[ARGS:.*]]) #[[ALLOCSIZE_ATTR:[0-9]+]]
+  call void @allocator(i32 20, %struct.__jmp_buf_tag* %arraydecay) #3
+  ret void
+}
+
 declare void @foo()
 ; Function Attrs: returns_twice
 declare i32 @setjmp(%struct.__jmp_buf_tag*) #0
@@ -227,3 +240,5 @@
 attributes #0 = { returns_twice }
 attributes #1 = { noreturn }
 attributes #2 = { nounwind }
+attributes #3 = { allocsize(0) }
+; CHECK: attributes #[[ALLOCSIZE_ATTR]] = { allocsize(1) }