[HotColdSplit] Don't outline musttail calls.

llvm/lib/Transforms/Utils/CodeExtractor.cpp

@@ -170,6 +170,14 @@ static bool isBlockValidForExtraction(const BasicBlock &BB,
     }
 
     if (const CallInst *CI = dyn_cast<CallInst>(I)) {
+      // musttail calls have several restrictions, generally enforcing matching
+      // calling conventions between the caller parent and musttail callee.
+      // We can't usually honor them, because the extracted function has a
+      // different signature altogether, taking inputs/outputs and returning
+      // a control-flow identifier rather than the actual return value.
+      if (CI->isMustTailCall())
+        return false;
+
       if (const Function *F = CI->getCalledFunction()) {
         auto IID = F->getIntrinsicID();
         if (IID == Intrinsic::vastart) {

llvm/test/Transforms/HotColdSplit/musttail.ll

@@ -0,0 +1,53 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=hotcoldsplit -hotcoldsplit-threshold=0 -S < %s | FileCheck %s
+
+; musttail calls can't be outlined, because they have several restrictions
+; enforcing matching calling conventions between the parent and callee.
+
+define void @test_musttail(i1 %b) {
+; CHECK-LABEL: define void @test_musttail(
+; CHECK-SAME: i1 [[B:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br i1 [[B]], label %[[IF_THEN:.*]], label %[[IF_ELSE:.*]]
+; CHECK:       [[IF_THEN]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[IF_ELSE]]:
+; CHECK-NEXT:    musttail call void @target(i1 [[B]])
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %b, label %if.then, label %if.else
+
+if.then:
+  ret void
+
+if.else:
+  musttail call void @target(i1 %b)
+  ret void
+}
+
+define void @test_tail(i1 %b) {
+; CHECK-LABEL: define void @test_tail(
+; CHECK-SAME: i1 [[B:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br i1 [[B]], label %[[IF_THEN:.*]], label %[[CODEREPL:.*]]
+; CHECK:       [[IF_THEN]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[CODEREPL]]:
+; CHECK-NEXT:    call void @test_tail.cold.1(i1 [[B]]) #[[ATTR2:[0-9]+]]
+; CHECK-NEXT:    br label %[[IF_ELSE_RET:.*]]
+; CHECK:       [[IF_ELSE_RET]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %b, label %if.then, label %if.else
+
+if.then:
+  ret void
+
+if.else:
+  tail call void @target(i1 %b)
+  ret void
+}
+
+declare void @target() cold