IR/Verifier: Do not allow kernel to kernel calls.

Allowing a kernel to call another kernel is invalid. This extends
the Verifier to recognize this fact. The calling convention is
retrieved from the module because it may not be labeling the callsite.

Author: jofrn

llvm/lib/IR/Verifier.cpp

@@ -3636,6 +3636,18 @@ void Verifier::visitCallBase(CallBase &Call) {
   Check(isCallableCC(Call.getCallingConv()),
         "calling convention does not permit calls", Call);
 
+  // Find the actual CC of the callee from the Module.
+  CallingConv::ID CalleeCC = Call.getParent()->getParent()->getParent()
+      ->getFunction(Call.getCalledFunction()->getName())->getCallingConv();
+  // Verify that a kernel does not call another kernel.
+  if (CalleeCC == CallingConv::AMDGPU_KERNEL ||
+      CalleeCC == CallingConv::SPIR_KERNEL) {
+    CallingConv::ID CallerCC = Call.getParent()->getParent()->getCallingConv();
+    Check(CallerCC != CallingConv::AMDGPU_KERNEL &&
+          CallerCC != CallingConv::SPIR_KERNEL,
+          "a kernel may not call a kernel", Call.getParent()->getParent());
+  }
+
   // Disallow passing/returning values with alignment higher than we can
   // represent.
   // FIXME: Consider making DataLayout cap the alignment, so this isn't

llvm/test/Verifier/AMDGPU/kernel-recursivecall.ll

@@ -0,0 +1,9 @@
+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+define amdgpu_kernel void @kernel(ptr addrspace(1) %out, i32 %n) {
+entry:
+; CHECK: a kernel may not call a kernel
+; CHECK-NEXT: ptr @kernel
+  call void @kernel(ptr addrspace(1) %out, i32 %n)
+  ret void
+}