diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -449,6 +449,20 @@
     } else if (Opts.LongDoubleSize == 128) {
       LongDoubleWidth = LongDoubleAlign = 128;
       LongDoubleFormat = &llvm::APFloat::IEEEquad();
+    } else if (Opts.LongDoubleSize == 80) {
+      LongDoubleFormat = &llvm::APFloat::x87DoubleExtended();
+      if (getTriple().isWindowsMSVCEnvironment()) {
+        LongDoubleWidth = 128;
+        LongDoubleAlign = 128;
+      } else { // Linux
+        if (getTriple().getArch() == llvm::Triple::x86) {
+          LongDoubleWidth = 96;
+          LongDoubleAlign = 32;
+        } else {
+          LongDoubleWidth = 128;
+          LongDoubleAlign = 128;
+        }
+      }
     }
   }
 
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -3456,6 +3456,8 @@
     GenerateArg(Args, OPT_mlong_double_128, SA);
   else if (Opts.LongDoubleSize == 64)
     GenerateArg(Args, OPT_mlong_double_64, SA);
+  else if (Opts.LongDoubleSize == 80)
+    GenerateArg(Args, OPT_mlong_double_80, SA);
 
   // Not generating '-mrtd', it's just an alias for '-fdefault-calling-conv='.
 
@@ -3838,9 +3840,16 @@
   Opts.NoBuiltin = Args.hasArg(OPT_fno_builtin) || Opts.Freestanding;
   if (!Opts.NoBuiltin)
     getAllNoBuiltinFuncValues(Args, Opts.NoBuiltinFuncs);
-  Opts.LongDoubleSize = Args.hasArg(OPT_mlong_double_128)
-                            ? 128
-                            : Args.hasArg(OPT_mlong_double_64) ? 64 : 0;
+  if (Arg *A = Args.getLastArg(options::OPT_LongDouble_Group)) {
+    if (A->getOption().matches(options::OPT_mlong_double_64))
+      Opts.LongDoubleSize = 64;
+    else if (A->getOption().matches(options::OPT_mlong_double_80))
+      Opts.LongDoubleSize = 80;
+    else if (A->getOption().matches(options::OPT_mlong_double_128))
+      Opts.LongDoubleSize = 128;
+    else
+      Opts.LongDoubleSize = 0;
+  }
   if (Opts.FastRelaxedMath)
     Opts.setDefaultFPContractMode(LangOptions::FPM_Fast);
   llvm::sort(Opts.ModuleFeatures);
diff --git a/clang/test/CodeGen/X86/long-double-config-size.c b/clang/test/CodeGen/X86/long-double-config-size.c
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/X86/long-double-config-size.c
@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -triple x86_64-windows-msvc %s -emit-llvm -mlong-double-64 -o - | FileCheck %s --check-prefix=SIZE64
+// RUN: %clang_cc1 -triple i386-windows-msvc %s -emit-llvm -mlong-double-80 -o - | FileCheck %s --check-prefix=SIZE80
+// RUN: %clang_cc1 -triple x86_64-windows-msvc %s -emit-llvm -mlong-double-80 -o - | FileCheck %s --check-prefix=SIZE80
+// RUN: %clang_cc1 -triple x86_64-windows-msvc %s -emit-llvm -mlong-double-128 -o - | FileCheck %s --check-prefix=SIZE128
+// RUN: %clang_cc1 -triple x86_64-windows-msvc %s -emit-llvm -o - | FileCheck %s --check-prefix=SIZE64
+
+long double global;
+// SIZE64: @global = dso_local global double 0
+// SIZE80: @global = dso_local global x86_fp80 0xK{{0+}}, align 16
+// SIZE128: @global = dso_local global fp128 0
+
+long double func(long double param) {
+  // SIZE64: define dso_local double @func(double noundef %param)
+  // SIZE80: define dso_local x86_fp80 @func(x86_fp80 noundef %param)
+  // SIZE128: define dso_local fp128  @func(fp128 noundef %param)
+  long double local = param;
+  // SIZE64: alloca double
+  // SIZE80: alloca x86_fp80, align 16
+  // SIZE128: alloca fp128
+  local = param;
+  return local + param;
+}