[RFC][SelectionDAG] Not issue TRAP node if naked function

In [1], Nikita Popov suggested that during lowering 'unreachable' insn
should not generate extra code for naked functions, and this applies
to all architectures. Note that for naked functions, 'unreachable'
insn is necessary in IR since the basic block needs a terminator to end.

This patch checked whether a function is naked function or not.
If it is a naked function, 'unreachable' insn will not generate
ISD::TRAP.

I put the RFC in the subject as I am not sure how to find a test
which can validate the change in FastISel.cpp. Any advice is welcome.

  [1] llvm#131731

Author: Yonghong Song

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1862,6 +1862,9 @@ bool FastISel::selectOperator(const User *I, unsigned Opcode) {
           return true;
       }
 
+      if (cast<Instruction>(I)->getFunction()->hasFnAttribute(Attribute::Naked))
+        return true;
+
       return fastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
     }
     return true;

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3514,6 +3514,9 @@ void SelectionDAGBuilder::visitUnreachable(const UnreachableInst &I) {
       return;
   }
 
+  if (I.getFunction()->hasFnAttribute(Attribute::Naked))
+    return;
+
   DAG.setRoot(DAG.getNode(ISD::TRAP, getCurSDLoc(), MVT::Other, DAG.getRoot()));
 }
 

llvm/test/CodeGen/X86/naked-fn-with-unreachable-trap.ll

@@ -0,0 +1,11 @@
+; RUN: llc -o - %s -mtriple=x86_64-linux-gnu -trap-unreachable | FileCheck %s
+; RUN: llc -o - %s -mtriple=x86_64-linux-gnu -trap-unreachable -fast-isel | FileCheck %s
+
+define dso_local void @foo() #0 {
+entry:
+  tail call void asm sideeffect "movl 3,%eax", "~{dirflag},~{fpsr},~{flags}"()
+  unreachable
+}
+; CHECK-NOT: ud2
+
+attributes #0 = { naked }