diff --git a/include/swift/AST/IRGenOptions.h b/include/swift/AST/IRGenOptions.h
index 67ce1590cbc82..ea81d878c0ced 100644
--- a/include/swift/AST/IRGenOptions.h
+++ b/include/swift/AST/IRGenOptions.h
@@ -25,6 +25,7 @@
 #include "swift/Basic/OptimizationMode.h"
 #include "swift/Config.h"
 #include "clang/Basic/PointerAuthOptions.h"
+#include "llvm/IR/CallingConv.h"
 // FIXME: This include is just for llvm::SanitizerCoverageOptions. We should
 // split the header upstream so we don't include so much.
 #include "llvm/Transforms/Instrumentation.h"
@@ -477,6 +478,9 @@ class IRGenOptions {
   /// function instead of to trap instructions.
   std::string TrapFuncName = "";
 
+  /// The calling convention used to perform non-swift calls.
+  llvm::CallingConv::ID PlatformCCallingConvention;
+
   IRGenOptions()
       : DWARFVersion(2),
         OutputKind(IRGenOutputKind::LLVMAssemblyAfterOptimization),
@@ -517,7 +521,8 @@ class IRGenOptions {
         ColocateTypeDescriptors(true),
         UseRelativeProtocolWitnessTables(false), CmdArgs(),
         SanitizeCoverage(llvm::SanitizerCoverageOptions()),
-        TypeInfoFilter(TypeInfoDumpFilter::All) {
+        TypeInfoFilter(TypeInfoDumpFilter::All),
+        PlatformCCallingConvention(llvm::CallingConv::C) {
 #ifndef NDEBUG
     DisableRoundTripDebugTypes = false;
 #else
diff --git a/include/swift/Option/FrontendOptions.td b/include/swift/Option/FrontendOptions.td
index 8e8ed8d5b46bb..b88be4acad09a 100644
--- a/include/swift/Option/FrontendOptions.td
+++ b/include/swift/Option/FrontendOptions.td
@@ -1224,4 +1224,12 @@ def experimental_spi_only_imports :
 def enable_ossa_complete_lifetimes :
   Flag<["-"], "enable-ossa-complete-lifetimes">,
   HelpText<"Require linear OSSA lifetimes after SILGen">;
+
+def platform_c_calling_convention :
+    Separate<["-"], "experimental-platform-c-calling-convention">,
+    HelpText<"Which calling convention is used to perform non-swift calls. "
+             "Defaults to llvm's standard C calling convention.">;
+def platform_c_calling_convention_EQ :
+    Joined<["-"], "experimental-platform-c-calling-convention=">,
+    Alias<platform_c_calling_convention>;
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 7e4aa57a3c54b..c3182dffc347a 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -2807,6 +2807,16 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     return true;
   }
 
+  if (const Arg *A = Args.getLastArg(options::OPT_platform_c_calling_convention)) {
+    Opts.PlatformCCallingConvention =
+      llvm::StringSwitch<llvm::CallingConv::ID>(A->getValue())
+        .Case("c", llvm::CallingConv::C)
+        .Case("arm_apcs", llvm::CallingConv::ARM_APCS)
+        .Case("arm_aapcs", llvm::CallingConv::ARM_AAPCS)
+        .Case("arm_aapcs_vfp", llvm::CallingConv::ARM_AAPCS_VFP)
+        .Default(llvm::CallingConv::C);
+  }
+
   return false;
 }
 
diff --git a/lib/IRGen/GenCall.cpp b/lib/IRGen/GenCall.cpp
index 16fa1bc9e3a39..508fd5f0d1b4e 100644
--- a/lib/IRGen/GenCall.cpp
+++ b/lib/IRGen/GenCall.cpp
@@ -322,7 +322,7 @@ llvm::CallingConv::ID irgen::expandCallingConv(IRGenModule &IGM,
   case SILFunctionTypeRepresentation::ObjCMethod:
   case SILFunctionTypeRepresentation::CXXMethod:
   case SILFunctionTypeRepresentation::Block:
-    return llvm::CallingConv::C;
+    return IGM.getOptions().PlatformCCallingConvention;
 
   case SILFunctionTypeRepresentation::Method:
   case SILFunctionTypeRepresentation::WitnessMethod:
diff --git a/lib/IRGen/GenDecl.cpp b/lib/IRGen/GenDecl.cpp
index 711c5bd05e108..ed9c67dd2596a 100644
--- a/lib/IRGen/GenDecl.cpp
+++ b/lib/IRGen/GenDecl.cpp
@@ -3297,7 +3297,7 @@ llvm::Constant *swift::irgen::emitCXXConstructorThunkIfNeeded(
   llvm::Function *thunk = llvm::Function::Create(
       assumedFnType, llvm::Function::PrivateLinkage, name, &IGM.Module);
 
-  thunk->setCallingConv(llvm::CallingConv::C);
+  thunk->setCallingConv(IGM.getOptions().PlatformCCallingConvention);
 
   llvm::AttrBuilder attrBuilder(IGM.getLLVMContext());
   IGM.constructInitialFnAttributes(attrBuilder);
diff --git a/lib/IRGen/GenHeap.cpp b/lib/IRGen/GenHeap.cpp
index fd6b761dd9e2c..b9657b1955eab 100644
--- a/lib/IRGen/GenHeap.cpp
+++ b/lib/IRGen/GenHeap.cpp
@@ -1978,7 +1978,7 @@ emitHeapMetadataRefForUnknownHeapObject(IRGenFunction &IGF,
   auto metadata = IGF.Builder.CreateCall(
       IGF.IGM.getGetObjectClassFunctionPointer(), object);
   metadata->setName(object->getName() + ".Type");
-  metadata->setCallingConv(llvm::CallingConv::C);
+  metadata->setCallingConv(IGF.IGM.getOptions().PlatformCCallingConvention);
   metadata->setDoesNotThrow();
   metadata->addFnAttr(llvm::Attribute::ReadOnly);
   return metadata;
diff --git a/lib/IRGen/IRGenModule.cpp b/lib/IRGen/IRGenModule.cpp
index 970a555997210..1e01dfdcbb021 100644
--- a/lib/IRGen/IRGenModule.cpp
+++ b/lib/IRGen/IRGenModule.cpp
@@ -570,7 +570,7 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   InvariantNode = llvm::MDNode::get(getLLVMContext(), {});
   DereferenceableID = getLLVMContext().getMDKindID("dereferenceable");
   
-  C_CC = llvm::CallingConv::C;
+  C_CC = getOptions().PlatformCCallingConvention;
   // TODO: use "tinycc" on platforms that support it
   DefaultCC = SWIFT_DEFAULT_LLVM_CC;
   SwiftCC = llvm::CallingConv::Swift;