diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1698,8 +1698,11 @@
   // Emit target-specific gunk after the function body.
   emitFunctionBodyEnd();
 
-  if (needFuncLabelsForEHOrDebugInfo(*MF) ||
-      MAI->hasDotTypeDotSizeDirective()) {
+  // Even though wasm supports .type and .size in general, function symbols
+  // are automatically sized.
+  bool EmitFunctionSize = MAI->hasDotTypeDotSizeDirective() && !TT.isWasm();
+
+  if (needFuncLabelsForEHOrDebugInfo(*MF) || EmitFunctionSize) {
     // Create a symbol for the end of function.
     CurrentFnEnd = createTempSymbol("func_end");
     OutStreamer->emitLabel(CurrentFnEnd);
@@ -1707,7 +1710,7 @@
 
   // If the target wants a .size directive for the size of the function, emit
   // it.
-  if (MAI->hasDotTypeDotSizeDirective()) {
+  if (EmitFunctionSize) {
     // We can get the size as difference between the function label and the
     // temp label.
     const MCExpr *SizeExp = MCBinaryExpr::createSub(
diff --git a/llvm/lib/MC/MCParser/WasmAsmParser.cpp b/llvm/lib/MC/MCParser/WasmAsmParser.cpp
--- a/llvm/lib/MC/MCParser/WasmAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/WasmAsmParser.cpp
@@ -207,7 +207,7 @@
 
   // TODO: This function is almost the same as ELFAsmParser::ParseDirectiveSize
   // so maybe could be shared somehow.
-  bool parseDirectiveSize(StringRef, SMLoc) {
+  bool parseDirectiveSize(StringRef, SMLoc Loc) {
     StringRef Name;
     if (Parser->parseIdentifier(Name))
       return TokError("expected identifier in directive");
@@ -219,9 +219,14 @@
       return true;
     if (expect(AsmToken::EndOfStatement, "eol"))
       return true;
-    // This is done automatically by the assembler for functions currently,
-    // so this is only currently needed for data sections:
-    getStreamer().emitELFSize(Sym, Expr);
+    auto WasmSym = cast<MCSymbolWasm>(Sym);
+    if (WasmSym->isFunction()) {
+      // Ignore .size directives for function symbols.  They get their size
+      // set automatically based on their content.
+      Warning(Loc, ".size directive ignored for function symbols");
+    } else {
+      getStreamer().emitELFSize(Sym, Expr);
+    }
     return false;
   }
 
diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -72,7 +72,7 @@
 // A wasm function to be written into the function section.
 struct WasmFunction {
   uint32_t SigIndex;
-  const MCSymbolWasm *Sym;
+  MCSection *Section;
 };
 
 // A wasm global to be written into the global section.
@@ -1058,15 +1058,12 @@
   encodeULEB128(Functions.size(), W->OS);
 
   for (const WasmFunction &Func : Functions) {
-    auto &FuncSection = static_cast<MCSectionWasm &>(Func.Sym->getSection());
-
-    int64_t Size = 0;
-    if (!Func.Sym->getSize()->evaluateAsAbsolute(Size, Layout))
-      report_fatal_error(".size expression must be evaluatable");
+    auto *FuncSection = static_cast<MCSectionWasm *>(Func.Section);
 
+    int64_t Size = Layout.getSectionAddressSize(FuncSection);
     encodeULEB128(Size, W->OS);
-    FuncSection.setSectionOffset(W->OS.tell() - Section.ContentsOffset);
-    Asm.writeSectionData(W->OS, &FuncSection, Layout);
+    FuncSection->setSectionOffset(W->OS.tell() - Section.ContentsOffset);
+    Asm.writeSectionData(W->OS, FuncSection, Layout);
   }
 
   // Apply fixups.
@@ -1591,15 +1588,11 @@
             report_fatal_error(
                 "function sections must contain one function each");
 
-          if (WS.getSize() == nullptr)
-            report_fatal_error(
-                "function symbols must have a size set with .size");
-
           // A definition. Write out the function body.
           Index = NumFunctionImports + Functions.size();
           WasmFunction Func;
           Func.SigIndex = getFunctionType(WS);
-          Func.Sym = &WS;
+          Func.Section = &WS.getSection();
           assert(WasmIndices.count(&WS) == 0);
           WasmIndices[&WS] = Index;
           Functions.push_back(Func);
diff --git a/llvm/test/MC/WebAssembly/function-size-warning.s b/llvm/test/MC/WebAssembly/function-size-warning.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/function-size-warning.s
@@ -0,0 +1,15 @@
+# RUN: llvm-mc -triple=wasm32-unknown-unknown -filetype=obj %s -o %t.o 2>&1 | FileCheck %s
+# RUN: llvm-objdump -t %t.o
+
+foo:
+  .functype foo () -> ()
+  i32.const 1
+  drop
+  end_function
+
+# .size directives for functions are no longer required and will
+# be ignored but we continue to allow them to support legacy
+# assembly files.
+.size foo, 0
+
+# CHECK: warning: .size directive ignored for function symbols
diff --git a/llvm/test/MC/WebAssembly/stack-ptr-mclower.ll b/llvm/test/MC/WebAssembly/stack-ptr-mclower.ll
--- a/llvm/test/MC/WebAssembly/stack-ptr-mclower.ll
+++ b/llvm/test/MC/WebAssembly/stack-ptr-mclower.ll
@@ -28,8 +28,6 @@
 ; CHECK-NEXT: bar:
 ; CHECK-NEXT:         .functype       bar () -> ()
 ; CHECK-NEXT:         end_function
-; CHECK-NEXT: .Lfunc_end0:
-; CHECK-NEXT:         .size   bar, .Lfunc_end0-bar
 
 ; CHECK:              .section        .text.foo,"",@
 ; CHECK-NEXT:         .hidden foo
@@ -42,5 +40,3 @@
 ; CHECK-NEXT:         [[PTR]].sub
 ; CHECK-NEXT:         drop
 ; CHECK-NEXT:         end_function
-; CHECK-NEXT: .Lfunc_end1:
-; CHECK-NEXT:         .size   foo, .Lfunc_end1-foo