CoroSplit: Replace ad-hoc implementation of reachability with API from CFG.h

The current ad-hoc implementation used to determine whether a basic
block is unreachable doesn't work correctly in the general case (for
example it won't detect successors of unreachable blocks as
unreachable). This patch replaces it with the correct API that uses a
DominatorTree to answer the question correctly and quickly.

rdar://77181156

Differential Revision: https://reviews.llvm.org/D102963

Author: adrian-prantl

llvm/lib/Transforms/Coroutines/CoroSplit.cpp

@@ -26,6 +26,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/CallGraph.h"
 #include "llvm/Analysis/CallGraphSCCPass.h"
 #include "llvm/Analysis/LazyCallGraph.h"
@@ -37,6 +38,7 @@
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/IR/GlobalVariable.h"
@@ -659,8 +661,10 @@ static void salvageCoroDebugInfo(llvm::Function &NewF) {
 
   // Remove all salvaged dbg.declare intrinsics that became
   // either unreachable or stale due to the CoroSplit transformation.
+  DominatorTree DomTree(NewF);
+  SmallDenseMap<BasicBlock *, bool, 8> UnreachableCache;
   auto IsUnreachableBlock = [&](BasicBlock *BB) {
-    return BB->hasNPredecessors(0) && BB != &NewF.getEntryBlock();
+    return !isPotentiallyReachable(&NewF.getEntryBlock(), BB, &DomTree);
   };
   for (DbgDeclareInst *DDI : Worklist) {
     if (IsUnreachableBlock(DDI->getParent()))

llvm/test/Transforms/Coroutines/coro-debug.ll

@@ -20,6 +20,7 @@ entry:
   store i8* %2, i8** %coro_hdl, align 8, !dbg !16
   %3 = call i8 @llvm.coro.suspend(token none, i1 false), !dbg !17
   %conv = sext i8 %3 to i32, !dbg !17
+  %late_local = alloca i32, align 4
   call void @coro.devirt.trigger(i8* null)
   switch i32 %conv, label %sw.default [
     i32 0, label %sw.bb
@@ -57,6 +58,7 @@ coro_Cleanup:                                     ; preds = %sw.epilog, %sw.bb1
 coro_Suspend:                                     ; preds = %coro_Cleanup, %sw.default
   %7 = call i1 @llvm.coro.end(i8* null, i1 false) #7, !dbg !24
   %8 = load i8*, i8** %coro_hdl, align 8, !dbg !24
+  store i32 0, i32* %late_local, !dbg !24
   ret i8* %8, !dbg !24
 }
 
@@ -147,6 +149,8 @@ attributes #7 = { noduplicate }
 ; CHECK: store %f.Frame* {{.*}}, %f.Frame** %[[DBG_PTR]]
 ; CHECK-NOT: alloca %struct.test*
 ; CHECK: call void @llvm.dbg.declare(metadata i32 0, metadata ![[RESUME_CONST:[0-9]+]], metadata !DIExpression())
+; Note that keeping the undef value here could be acceptable, too.
+; CHECK-NOT: call void @llvm.dbg.declare(metadata i32* undef, metadata !{{[0-9]+}}, metadata !DIExpression())
 ; CHECK: call void @coro.devirt.trigger(i8* null)
 ; CHECK: define internal fastcc void @f.destroy(%f.Frame* noalias nonnull align 8 dereferenceable(32) %FramePtr) #0 !dbg ![[DESTROY:[0-9]+]]
 ; CHECK: define internal fastcc void @f.cleanup(%f.Frame* noalias nonnull align 8 dereferenceable(32) %FramePtr) #0 !dbg ![[CLEANUP:[0-9]+]]