Index: lib/Basic/Targets/OSTargets.h
===================================================================
--- lib/Basic/Targets/OSTargets.h
+++ lib/Basic/Targets/OSTargets.h
@@ -771,6 +771,8 @@
     // Follow g++ convention and predefine _GNU_SOURCE for C++.
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
+    // Indicate that we have __float128.
+    Builder.defineMacro("__FLOAT128__");
   }
 
 public:
@@ -779,6 +781,7 @@
       : OSTargetInfo<Target>(Triple, Opts) {
     this->MCountName = "__mcount";
     this->TheCXXABI.set(TargetCXXABI::WebAssembly);
+    this->HasFloat128 = true;
   }
 };
 
Index: test/Preprocessor/init.c
===================================================================
--- test/Preprocessor/init.c
+++ test/Preprocessor/init.c
@@ -9159,6 +9159,7 @@
 // WEBASSEMBLY-NEXT:#define __DECIMAL_DIG__ __LDBL_DECIMAL_DIG__
 // WEBASSEMBLY-NOT:#define __ELF__
 // WEBASSEMBLY-NEXT:#define __FINITE_MATH_ONLY__ 0
+// WEBASSEMBLY-NEXT:#define __FLOAT128__ 1
 // WEBASSEMBLY-NEXT:#define __FLT16_DECIMAL_DIG__ 5
 // WEBASSEMBLY-NEXT:#define __FLT16_DENORM_MIN__ 5.9604644775390625e-8F16
 // WEBASSEMBLY-NEXT:#define __FLT16_DIG__ 3