HotColdSplitting: Do not outline within noreturn functions

A function marked `noreturn` may contain unreachable terminators: these
should not be considered cold, as the function may be a trampoline.

rdar://58068594

2 files changed, 25 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/HotColdSplitting.cpp b/llvm/lib/Transforms/IPO/HotColdSplitting.cpp
index 2bd3df3add7..5e690714bfd 100644
--- a/llvm/lib/Transforms/IPO/HotColdSplitting.cpp
+++ b/llvm/lib/Transforms/IPO/HotColdSplitting.cpp
@@ -207,6 +207,11 @@ bool HotColdSplitting::shouldOutlineFrom(const Function &F) const {
   if (F.hasFnAttribute(Attribute::NoInline))
     return false;
 
+  // A function marked `noreturn` may contain unreachable terminators: these
+  // should not be considered cold, as the function may be a trampoline.
+  if (F.hasFnAttribute(Attribute::NoReturn))
+    return false;
+
   if (F.hasFnAttribute(Attribute::SanitizeAddress) ||
       F.hasFnAttribute(Attribute::SanitizeHWAddress) ||
       F.hasFnAttribute(Attribute::SanitizeThread) ||
diff --git a/llvm/test/Transforms/HotColdSplit/noreturn.ll b/llvm/test/Transforms/HotColdSplit/noreturn.ll
index ca0f58815bf..74f4cb18c7f 100644
--- a/llvm/test/Transforms/HotColdSplit/noreturn.ll
+++ b/llvm/test/Transforms/HotColdSplit/noreturn.ll
@@ -23,6 +23,24 @@ define void @foo(i32, %struct.__jmp_buf_tag*) {
   ret void
 }
 
+; Don't outline within a noreturn function.
+
+; CHECK: define {{.*}}@xpc_objc_main(i32 {{.*}}) [[XPC_OBJC_MAIN_ATTRS:#[0-9]+]]
+; CHECK-NOT: xpc_objc_main.cold.1
+define void @xpc_objc_main(i32) noreturn {
+  %2 = icmp eq i32 %0, 0
+  tail call void @_Z10sideeffectv()
+  br i1 %2, label %4, label %3
+
+; <label>:3:                                      ; preds = %2
+  call void @_Z10sideeffectv()
+  unreachable
+
+; <label>:4:                                      ; preds = %2
+  ; Crash with an error message, "not supposed to return".
+  unreachable
+}
+
 ; Do outline noreturn calls marked cold.
 
 ; CHECK-LABEL: define {{.*}}@bar(
@@ -62,6 +80,8 @@ define void @baz(i32, %struct.__jmp_buf_tag*) {
 ; CHECK-LABEL: define {{.*}}@bar.cold.1(
 ; CHECK: call {{.*}}@llvm.trap(
 
+; CHECK: attributes [[XPC_OBJC_MAIN_ATTRS]] = { noreturn }
+
 declare void @sink() cold
 
 declare void @llvm.trap() noreturn cold