[ObjC] Override TailCallKind when lowering objc intrinsics

The tail-call-kind-ness is known by the ObjCARC analysis and can be
enforced while lowering the intrinsics to calls.

This allows us to get the requested tail calls at -O0 without trying to
preserve the attributes throughout passes that change code even at -O0
,like the Always Inliner, where the ObjCOpt pass doesn't run.

Differential Revision: https://reviews.llvm.org/D69980

Author: francisvm

llvm/lib/CodeGen/PreISelIntrinsicLowering.cpp

@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/CodeGen/PreISelIntrinsicLowering.h"
+#include "llvm/Analysis/ObjCARCInstKind.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Intrinsics.h"
@@ -56,6 +57,17 @@ static bool lowerLoadRelative(Function &F) {
   return Changed;
 }
 
+// ObjCARC has knowledge about whether an obj-c runtime function needs to be
+// always tail-called or never tail-called.
+static CallInst::TailCallKind getOverridingTailCallKind(const Function &F) {
+  objcarc::ARCInstKind Kind = objcarc::GetFunctionClass(&F);
+  if (objcarc::IsAlwaysTail(Kind))
+    return CallInst::TCK_Tail;
+  else if (objcarc::IsNeverTail(Kind))
+    return CallInst::TCK_NoTail;
+  return CallInst::TCK_None;
+}
+
 static bool lowerObjCCall(Function &F, const char *NewFn,
                           bool setNonLazyBind = false) {
   if (F.use_empty())
@@ -75,6 +87,8 @@ static bool lowerObjCCall(Function &F, const char *NewFn,
     }
   }
 
+  CallInst::TailCallKind OverridingTCK = getOverridingTailCallKind(F);
+
   for (auto I = F.use_begin(), E = F.use_end(); I != E;) {
     auto *CI = cast<CallInst>(I->getUser());
     assert(CI->getCalledFunction() && "Cannot lower an indirect call!");
@@ -84,7 +98,17 @@ static bool lowerObjCCall(Function &F, const char *NewFn,
     SmallVector<Value *, 8> Args(CI->arg_begin(), CI->arg_end());
     CallInst *NewCI = Builder.CreateCall(FCache, Args);
     NewCI->setName(CI->getName());
-    NewCI->setTailCallKind(CI->getTailCallKind());
+
+    // Try to set the most appropriate TailCallKind based on both the current
+    // attributes and the ones that we could get from ObjCARC's special
+    // knowledge of the runtime functions.
+    //
+    // std::max respects both requirements of notail and tail here:
+    // * notail on either the call or from ObjCARC becomes notail
+    // * tail on either side is stronger than none, but not notail
+    CallInst::TailCallKind TCK = CI->getTailCallKind();
+    NewCI->setTailCallKind(std::max(TCK, OverridingTCK));
+
     if (!CI->use_empty())
       CI->replaceAllUsesWith(NewCI);
     CI->eraseFromParent();

llvm/test/Transforms/PreISelIntrinsicLowering/objc-arc.ll

@@ -7,7 +7,7 @@
 define i8* @test_objc_autorelease(i8* %arg0) {
 ; CHECK-LABEL: test_objc_autorelease
 ; CHECK-NEXT: entry
-; CHECK-NEXT: %0 = call i8* @objc_autorelease(i8* %arg0)
+; CHECK-NEXT: %0 = notail call i8* @objc_autorelease(i8* %arg0)
 ; CHECK-NEXT: ret i8* %0
 entry:
   %0 = call i8* @llvm.objc.autorelease(i8* %arg0)
@@ -37,7 +37,7 @@ entry:
 define i8* @test_objc_autoreleaseReturnValue(i8* %arg0) {
 ; CHECK-LABEL: test_objc_autoreleaseReturnValue
 ; CHECK-NEXT: entry
-; CHECK-NEXT: %0 = call i8* @objc_autoreleaseReturnValue(i8* %arg0)
+; CHECK-NEXT: %0 = tail call i8* @objc_autoreleaseReturnValue(i8* %arg0)
 ; CHECK-NEXT: ret i8* %0
 entry:
   %0 = call i8* @llvm.objc.autoreleaseReturnValue(i8* %arg0)
@@ -117,7 +117,7 @@ entry:
 define i8* @test_objc_retain(i8* %arg0) {
 ; CHECK-LABEL: test_objc_retain
 ; CHECK-NEXT: entry
-; CHECK-NEXT: %0 = call i8* @objc_retain(i8* %arg0)
+; CHECK-NEXT: %0 = tail call i8* @objc_retain(i8* %arg0)
 ; CHECK-NEXT: ret i8* %0
 entry:
   %0 = call i8* @llvm.objc.retain(i8* %arg0)
@@ -147,7 +147,7 @@ entry:
 define i8* @test_objc_retainAutoreleasedReturnValue(i8* %arg0) {
 ; CHECK-LABEL: test_objc_retainAutoreleasedReturnValue
 ; CHECK-NEXT: entry
-; CHECK-NEXT: %0 = call i8* @objc_retainAutoreleasedReturnValue(i8* %arg0)
+; CHECK-NEXT: %0 = tail call i8* @objc_retainAutoreleasedReturnValue(i8* %arg0)
 ; CHECK-NEXT: ret i8* %0
 entry:
   %0 = call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %arg0)
@@ -187,7 +187,7 @@ entry:
 define i8* @test_objc_unsafeClaimAutoreleasedReturnValue(i8* %arg0) {
 ; CHECK-LABEL: test_objc_unsafeClaimAutoreleasedReturnValue
 ; CHECK-NEXT: entry
-; CHECK-NEXT: %0 = call i8* @objc_unsafeClaimAutoreleasedReturnValue(i8* %arg0)
+; CHECK-NEXT: %0 = tail call i8* @objc_unsafeClaimAutoreleasedReturnValue(i8* %arg0)
 ; CHECK-NEXT: ret i8* %0
 entry:
   %0 = call i8* @llvm.objc.unsafeClaimAutoreleasedReturnValue(i8* %arg0)