Add @naked UDA

Adding the LLVM `naked` function attribute and disabling LDC's
prologue/epilogue too.

gen/functions.cpp

@@ -827,6 +827,18 @@ void emitDMDStyleFunctionTrace(IRState &irs, FuncDeclaration *fd,
   }
 }
 
+// If the specified block is trivially unreachable, erases it and returns true.
+// This is a common case because it happens when 'return' is the last statement
+// in a function.
+bool eraseDummyAfterReturnBB(llvm::BasicBlock *bb) {
+  if (pred_begin(bb) == pred_end(bb) &&
+      bb != &bb->getParent()->getEntryBlock()) {
+    bb->eraseFromParent();
+    return true;
+  }
+  return false;
+}
+
 } // anonymous namespace
 
 void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
@@ -1034,6 +1046,9 @@ void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
       llvm::BasicBlock::Create(gIR->context(), "", func);
 
   gIR->scopes.push_back(IRScope(beginbb));
+  SCOPE_EXIT {
+    gIR->scopes.pop_back();
+  };
 
 // Set the FastMath options for this function scope.
 #if LDC_LLVM_VER >= 308
@@ -1042,6 +1057,18 @@ void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
   gIR->scopes.back().builder.SetFastMathFlags(irFunc->FMF);
 #endif
 
+  // @naked: emit body and return, no prologue/epilogue
+  if (func->hasFnAttribute(llvm::Attribute::Naked)) {
+    Statement_toIR(fd->fbody, gIR);
+    const bool wasDummy = eraseDummyAfterReturnBB(gIR->scopebb());
+    if (!wasDummy && !gIR->scopereturned()) {
+      // this is what clang does to prevent LLVM complaining about
+      // non-terminated function
+      gIR->ir->CreateUnreachable();
+    }
+    return;
+  }
+
   // create alloca point
   // this gets erased when the function is complete, so alignment etc does not
   // matter at all
@@ -1163,14 +1190,8 @@ void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
     funcGen.scopes.popCleanups(0);
   }
 
-  llvm::BasicBlock *bb = gIR->scopebb();
-  if (pred_begin(bb) == pred_end(bb) &&
-      bb != &bb->getParent()->getEntryBlock()) {
-    // This block is trivially unreachable, so just delete it.
-    // (This is a common case because it happens when 'return'
-    // is the last statement in a function)
-    bb->eraseFromParent();
-  } else if (!gIR->scopereturned()) {
+  const bool wasDummy = eraseDummyAfterReturnBB(gIR->scopebb());
+  if (!wasDummy && !gIR->scopereturned()) {
     // llvm requires all basic blocks to end with a TerminatorInst but DMD does
     // not put a return statement in automatically, so we do it here.
 
@@ -1201,8 +1222,6 @@ void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
     allocaPoint = nullptr;
   }
 
-  gIR->scopes.pop_back();
-
   if (gIR->dcomputetarget && hasKernelAttr(fd)) {
     auto fn = gIR->module.getFunction(fd->mangleString);
     gIR->dcomputetarget->addKernelMetadata(fd, fn);

gen/uda.cpp

@@ -222,10 +222,16 @@ void applyAttrLLVMAttr(StructLiteralExp *sle, llvm::Function *func) {
     const auto kind = llvm::getAttrKindFromName(key);
     if (kind != llvm::Attribute::None) {
       func->addFnAttr(kind);
-      return;
+    }
+#else
+    // no getAttrKindFromName(); just detect `naked` for now
+    if (key == "naked") {
+      func->addFnAttr(llvm::Attribute::Naked);
     }
 #endif
+    else {
       func->addFnAttr(key);
+    }
   } else {
     func->addFnAttr(key, value);
   }

runtime/druntime

@@ -1 +1 @@
-Subproject commit 6378c56b2f114866f2c2efe3b78c086965e1729d
+Subproject commit 4cc2de28e47beb24f95830371ca8b349042a4beb

tests/codegen/attr_naked.d

@@ -0,0 +1,31 @@
+// REQUIRES: target_X86
+// RUN: %ldc -mtriple=x86_64-pc-linux-gnu -O -output-s -of=%t.s %s && FileCheck %s < %t.s
+
+import ldc.attributes;
+import ldc.llvmasm;
+
+// Without -O, LLVM (6) dumps the 2 params to stack for Linux x64 (but doesn't for Win64).
+// Clang is no different though.
+
+// CHECK: withParams:
+extern(C) int withParams(int param1, void* param2) @naked
+{
+    // CHECK-NEXT: .cfi_startproc
+    // CHECK-NEXT: #APP
+    // CHECK-NEXT: xorl %eax, %eax
+    // CHECK-NEXT: #NO_APP
+    // CHECK-NEXT: retq
+    return __asm!int("xor %eax, %eax", "={eax}");
+}
+
+// CHECK: _D10attr_naked8noReturnFZv:
+void noReturn() @naked
+{
+    // CHECK-NEXT: .cfi_startproc
+    // CHECK-NEXT: #APP
+    // CHECK-NEXT: jmpq *%rax
+    // CHECK-NEXT: #NO_APP
+    __asm("jmp *%rax", "");
+    // CHECK-NOT: retq
+    // CHECK:     .cfi_endproc
+}