[Coroutines] Add missing llvm.dbg.declare's to cover for more allocas

Tracking local variables across suspend points is still somewhat incomplete.
Consider this coroutine snippet:

```
resumable foo() {
  int x[10] = {};
  int a = 3;
  co_await std::experimental::suspend_always();
  a++;
  x[0] = 1;
  a += 2;
  x[1] = 2;
  a += 3;
  x[2] = 3;
}
```

Can't manage to print `a` or `x` if they turn out to be allocas during
CoroSplit (which happens if you build this code with `-O0` prior to this
commit):

```
* thread #1, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x0000000100003729 main-noprint`foo() at main-noprint.cpp:43:5
   40     co_await std::experimental::suspend_always();
   41     a++;
   42     x[0] = 1;
-> 43     a += 2;
   44     x[1] = 2;
   45     a += 3;
   46     x[2] = 3;
(lldb) p x
error: <user expression 21>:1:1: use of undeclared identifier 'x'
x
^
```

The generated IR contains a `llvm.dbg.declare` for `x` in it's initialization
basic block. After CoroSplit, the `llvm.dbg.declare` might not dominate all of
`x` uses and we lose debugging quality.

Add `llvm.dbg.value`s to all relevant basic blocks such that if later
transformations break the dominance the reliable debug info is already in
place. For instance, this BB:

```
await.ready:
  ...
  %arrayidx = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 0, !dbg !760
  ...
  %arrayidx19 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 1, !dbg !763
  ...
  %arrayidx21 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 2, !dbg !766
```

becomes:

```
await.ready:
  ...
  call void @llvm.dbg.value(metadata [10 x i32]* %x.reload.addr, metadata !751, metadata !DIExpression()), !dbg !753
  ...
  %arrayidx = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 0, !dbg !760
  ...
  %arrayidx19 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 1, !dbg !763
  ...
  %arrayidx21 = getelementptr inbounds [10 x i32], [10 x i32]* %x.reload.addr, i64 0, i64 2, !dbg !766
```

Differential Revision: https://reviews.llvm.org/D90772

Author: bcardosolopes

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -1191,19 +1191,51 @@ static Instruction *insertSpills(const FrameDataInfo &FrameData,
       continue;
     auto *G = GetFramePointer(Alloca);
     G->setName(Alloca->getName() + Twine(".reload.addr"));
+
+    SmallPtrSet<BasicBlock *, 4> SeenDbgBBs;
     TinyPtrVector<DbgDeclareInst *> DIs = FindDbgDeclareUses(Alloca);
-    if (!DIs.empty())
-      DIBuilder(*Alloca->getModule(),
-                /*AllowUnresolved*/ false)
-          .insertDeclare(G, DIs.front()->getVariable(),
-                         DIs.front()->getExpression(),
-                         DIs.front()->getDebugLoc(), DIs.front());
+    DIBuilder DIB(*Alloca->getModule(), /*AllowUnresolved*/ false);
+    Instruction *FirstDbgDecl = nullptr;
+
+    if (!DIs.empty()) {
+      FirstDbgDecl = DIB.insertDeclare(G, DIs.front()->getVariable(),
+                                       DIs.front()->getExpression(),
+                                       DIs.front()->getDebugLoc(), DIs.front());
+      SeenDbgBBs.insert(DIs.front()->getParent());
+    }
     for (auto *DI : FindDbgDeclareUses(Alloca))
       DI->eraseFromParent();
     replaceDbgUsesWithUndef(Alloca);
 
-    for (Instruction *I : UsersToUpdate)
+    for (Instruction *I : UsersToUpdate) {
       I->replaceUsesOfWith(Alloca, G);
+
+      // After cloning, transformations might not guarantee that all uses
+      // of this alloca are dominated by the already existing dbg.declare's,
+      // compromising the debug quality. Instead of writing another
+      // transformation to patch each clone, go ahead and early populate
+      // basic blocks that use such allocas with more debug info.
+      if (SeenDbgBBs.count(I->getParent()))
+        continue;
+
+      // If there isn't a prior dbg.declare for this alloca, it probably
+      // means the state hasn't changed prior to one of the relevant suspend
+      // point for this frame access.
+      if (!FirstDbgDecl)
+        continue;
+
+      // These instructions are all dominated by the alloca, insert the
+      // dbg.value in the beginning of the BB to enhance debugging
+      // experience and allow values to be inspected as early as possible.
+      // Prefer dbg.value over dbg.declare since it better sets expectations
+      // that control flow can be later changed by other passes.
+      auto *DI = cast<DbgDeclareInst>(FirstDbgDecl);
+      BasicBlock *CurrentBlock = I->getParent();
+      DIB.insertDbgValueIntrinsic(G, DI->getVariable(), DI->getExpression(),
+                                  DI->getDebugLoc(),
+                                  &*CurrentBlock->getFirstInsertionPt());
+      SeenDbgBBs.insert(CurrentBlock);
+    }
   }
   Builder.SetInsertPoint(FramePtr->getNextNode());
   for (const auto &A : FrameData.Allocas) {

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

@@ -7,15 +7,19 @@
 ;   void foo() {
 ;     int i = 0;
 ;     ++i;
+;     int x = {};
 ;     print(i);  // Prints '1'
 ;
 ;     co_await suspend_always();
 ;
 ;     int j = 0;
+;     x[0] = 1;
+;     x[1] = 2;
 ;     ++i;
 ;     print(i);  // Prints '2'
 ;     ++j;
 ;     print(j);  // Prints '1'
+;     print(x);  // Print '1'
 ;   }
 ;
 ; The CHECKs verify that dbg.declare intrinsics are created for the coroutine
@@ -28,35 +32,47 @@
 ; CHECK:       entry:
 ; CHECK:         %j = alloca i32, align 4
 ; CHECK:         [[IGEP:%.+]] = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32 0, i32 4
+; CHECK:         [[XGEP:%.+]] = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32 0, i32 6
 ; CHECK:       init.ready:
 ; CHECK:         call void @llvm.dbg.declare(metadata i32* [[IGEP]], metadata ![[IVAR:[0-9]+]], metadata !DIExpression()), !dbg ![[IDBGLOC:[0-9]+]]
+; CHECK:         call void @llvm.dbg.declare(metadata [10 x i32]* [[XGEP]], metadata ![[XVAR:[0-9]+]], metadata !DIExpression()), !dbg ![[IDBGLOC]]
 ; CHECK:       await.ready:
+; CHECK:         call void @llvm.dbg.value(metadata [10 x i32]* [[XGEP]], metadata ![[XVAR]], metadata !DIExpression()), !dbg ![[IDBGLOC]]
+; CHECK:         call void @llvm.dbg.value(metadata i32* [[IGEP]], metadata ![[IVAR]], metadata !DIExpression()), !dbg ![[IDBGLOC]]
 ; CHECK:         call void @llvm.dbg.declare(metadata i32* %j, metadata ![[JVAR:[0-9]+]], metadata !DIExpression()), !dbg ![[JDBGLOC:[0-9]+]]
 ;
 ; CHECK-LABEL: define internal fastcc void @f.resume({{.*}}) {
 ; CHECK:       entry.resume:
 ; CHECK:         %j = alloca i32, align 4
 ; CHECK:         [[IGEP_RESUME:%.+]] = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32 0, i32 4
+; CHECK:         [[XGEP_RESUME:%.+]] = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32 0, i32 6
 ; CHECK:       init.ready:
 ; CHECK:         call void @llvm.dbg.declare(metadata i32* [[IGEP_RESUME]], metadata ![[IVAR_RESUME:[0-9]+]], metadata !DIExpression()), !dbg ![[IDBGLOC_RESUME:[0-9]+]]
+; CHECK:         call void @llvm.dbg.declare(metadata [10 x i32]* [[XGEP_RESUME]], metadata ![[XVAR_RESUME:[0-9]+]], metadata !DIExpression()), !dbg ![[IDBGLOC_RESUME]]
 ; CHECK:       await.ready:
+; CHECK:         call void @llvm.dbg.value(metadata [10 x i32]* [[XGEP_RESUME]], metadata ![[XVAR_RESUME]], metadata !DIExpression()), !dbg ![[IDBGLOC_RESUME]]
+; CHECK:         call void @llvm.dbg.value(metadata i32* [[IGEP_RESUME]], metadata ![[IVAR_RESUME]], metadata !DIExpression()), !dbg ![[IDBGLOC_RESUME]]
 ; CHECK:         call void @llvm.dbg.declare(metadata i32* %j, metadata ![[JVAR_RESUME:[0-9]+]], metadata !DIExpression()), !dbg ![[JDBGLOC_RESUME:[0-9]+]]
 ;
 ; CHECK: ![[IVAR]] = !DILocalVariable(name: "i"
 ; CHECK: ![[SCOPE:[0-9]+]] = distinct !DILexicalBlock(scope: !8, file: !1, line: 23, column: 12)
 ; CHECK: ![[IDBGLOC]] = !DILocation(line: 24, column: 7, scope: ![[SCOPE]])
+; CHECK: ![[XVAR]] = !DILocalVariable(name: "x"
 ; CHECK: ![[JVAR]] = !DILocalVariable(name: "j"
 ; CHECK: ![[JDBGLOC]] = !DILocation(line: 32, column: 7, scope: ![[SCOPE]])
+
 ; CHECK: ![[IVAR_RESUME]] = !DILocalVariable(name: "i"
 ; CHECK: ![[RESUME_SCOPE:[0-9]+]] = distinct !DILexicalBlock(scope: !8, file: !1, line: 23, column: 12)
 ; CHECK: ![[IDBGLOC_RESUME]] = !DILocation(line: 24, column: 7, scope: ![[RESUME_SCOPE]])
+; CHECK: ![[XVAR_RESUME]] = !DILocalVariable(name: "x"
 ; CHECK: ![[JVAR_RESUME]] = !DILocalVariable(name: "j"
 ; CHECK: ![[JDBGLOC_RESUME]] = !DILocation(line: 32, column: 7, scope: ![[RESUME_SCOPE]])
 define void @f() {
 entry:
   %__promise = alloca i8, align 8
   %i = alloca i32, align 4
   %j = alloca i32, align 4
+  %x = alloca [10 x i32], align 16
   %id = call token @llvm.coro.id(i32 16, i8* %__promise, i8* null, i8* null)
   %alloc = call i1 @llvm.coro.alloc(token %id)
   br i1 %alloc, label %coro.alloc, label %coro.init
@@ -91,6 +107,9 @@ init.ready:                                       ; preds = %init.suspend, %coro
   %i.init.ready.load = load i32, i32* %i, align 4
   %i.init.ready.inc = add nsw i32 %i.init.ready.load, 1
   store i32 %i.init.ready.inc, i32* %i, align 4
+  call void @llvm.dbg.declare(metadata [10 x i32]* %x, metadata !14, metadata !DIExpression()), !dbg !11
+  %memset = bitcast [10 x i32]* %x to i8*, !dbg !11
+  call void @llvm.memset.p0i8.i64(i8* align 16 %memset, i8 0, i64 40, i1 false), !dbg !11
   %i.init.ready.reload = load i32, i32* %i, align 4
   call void @print(i32 %i.init.ready.reload)
   %ready.again = call zeroext i1 @await_ready()
@@ -113,6 +132,10 @@ await.ready:                                      ; preds = %await.suspend, %ini
   call void @await_resume()
   call void @llvm.dbg.declare(metadata i32* %j, metadata !12, metadata !DIExpression()), !dbg !13
   store i32 0, i32* %j, align 4
+  %arrayidx0 = getelementptr inbounds [10 x i32], [10 x i32]* %x, i64 0, i64 0, !dbg !18
+  store i32 1, i32* %arrayidx0, align 16, !dbg !19
+  %arrayidx1 = getelementptr inbounds [10 x i32], [10 x i32]* %x, i64 0, i64 1, !dbg !20
+  store i32 2, i32* %arrayidx1, align 4, !dbg !21
   %i.await.ready.load = load i32, i32* %i, align 4
   %i.await.ready.inc = add nsw i32 %i.await.ready.load, 1
   store i32 %i.await.ready.inc, i32* %i, align 4
@@ -195,6 +218,8 @@ declare i8* @from_address(i8*)
 declare void @return_void()
 declare void @final_suspend()
 
+declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg)
+
 !llvm.dbg.cu = !{!0}
 !llvm.linker.options = !{}
 !llvm.module.flags = !{!3, !4}
@@ -214,3 +239,12 @@ declare void @final_suspend()
 !11 = !DILocation(line: 24, column: 7, scope: !7)
 !12 = !DILocalVariable(name: "j", scope: !7, file: !1, line: 32, type: !10)
 !13 = !DILocation(line: 32, column: 7, scope: !7)
+!14 = !DILocalVariable(name: "x", scope: !22, file: !1, line: 34, type: !15)
+!15 = !DICompositeType(tag: DW_TAG_array_type, baseType: !10, size: 320, elements: !16)
+!16 = !{!17}
+!17 = !DISubrange(count: 10)
+!18 = !DILocation(line: 42, column: 3, scope: !7)
+!19 = !DILocation(line: 42, column: 8, scope: !7)
+!20 = !DILocation(line: 43, column: 3, scope: !7)
+!21 = !DILocation(line: 43, column: 8, scope: !7)
+!22 = distinct !DILexicalBlock(scope: !8, file: !1, line: 23, column: 12)