[SYCL] Move deferred emit check to after "Can be emitted" checks. (#2102)

The previous implementation caused function declarations to be attempted
to be emitted if they were required to be emitted (such as in the case
of -femit-all-decls), which caused a crash.

This patch moves the check until after the 'can be emitted' checks for
function/var decls, and limits it to only functions that are
sycl-devices (or, in the case of implicit attributes, called from a
sycl-device function).

Author: Erich Keane

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2642,11 +2642,6 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
     }
   }
 
-  if (LangOpts.SYCLIsDevice && MustBeEmitted(Global)) {
-    addDeferredDeclToEmit(GD);
-    return;
-  }
-
   // Ignore declarations, they will be emitted on their first use.
   if (const auto *FD = dyn_cast<FunctionDecl>(Global)) {
     // Forward declarations are emitted lazily on first use.
@@ -2698,6 +2693,13 @@ void CodeGenModule::EmitGlobal(GlobalDecl GD) {
     }
   }
 
+  // clang::ParseAST ensures that we emit the SYCL devices at the end, so
+  // anything that is a device (or indirectly called) will be handled later.
+  if (LangOpts.SYCLIsDevice && MustBeEmitted(Global)) {
+    addDeferredDeclToEmit(GD);
+    return;
+  }
+
   // Defer code generation to first use when possible, e.g. if this is an inline
   // function. If the global must always be emitted, do it eagerly if possible
   // to benefit from cache locality.

clang/test/CodeGenSYCL/emit-all-decls-crash.cpp

@@ -0,0 +1,6 @@
+// RUN: %clang_cc1 -fsycl -fsycl-is-device -triple spir64-unknown-unknown-sycldevice -disable-llvm-passes -emit-llvm %s -femit-all-decls -o - | FileCheck %s
+
+// This should not crash and we should not emit this declaration, even though
+// we have 'emit-all-decls'.
+// CHECK-NOT: define
+void foo(void);