IRGen: correctly cast C-function pointers in statically initialized globals.

This fixes a IRGen crash, introduced with swiftlang#35780

rdar://74358259

Author: eeckstein

lib/IRGen/GenConstant.cpp

@@ -249,6 +249,8 @@ llvm::Constant *irgen::emitConstantValue(IRGenModule &IGM, SILValue operand) {
       fnPtr = IGM.getConstantSignedPointer(fnPtr, authInfo.getKey(), nullptr,
         constantDiscriminator);
     }
+    llvm::Type *ty = IGM.getTypeInfo(FRI->getType()).getStorageType();
+    fnPtr = llvm::ConstantExpr::getBitCast(fnPtr, ty);
     return fnPtr;
   } else {
     llvm_unreachable("Unsupported SILInstruction in static initializer!");

test/SILOptimizer/global-c-functionptr.swift

@@ -0,0 +1,32 @@
+// Testcase for an IRGen crash with function pointers in static globals and multi-threaded compilation.
+
+// First test: check if the compilation succeeds and the code is correct.
+
+// RUN: %empty-directory(%t) 
+// RUN: %target-build-swift -O -module-name=Test %s -o %t/a.out
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s -check-prefix=CHECK-OUTPUT
+
+// Second test (bonus): check if the optimization is done: statically initialize a global with a C-function pointer
+
+// RUN: %target-build-swift -O -module-name=Test %s -emit-sil | %FileCheck %s -check-prefix=CHECK-SIL
+
+// REQUIRES: executable_test
+
+
+internal func cFn(_ i: Int) -> Int {
+  return i + 1
+}
+
+public struct S {
+  // CHECK-SIL-LABEL: sil_global @$s4Test1SV6cFnPtryS2iXCvpZ : $@convention(c) (Int) -> Int = {
+  // CHECK-SIL:         %initval = function_ref @$s4Test3cFnyS2iFTo : $@convention(c) (Int) -> Int
+  static public var cFnPtr: @convention(c) (Int) -> Int = cFn
+}
+
+func testit() {
+  // CHECK-OUTPUT: 28
+  print(S.cFnPtr(27))
+}
+
+testit()