[MachineBlockPlacement] Don't make blocks "uneditable"

Summary:
This fixes an issue with MachineBlockPlacement due to a badly timed call
to `analyzeBranch` with `AllowModify` set to true.  The timeline is as
follows:

 1. `MachineBlockPlacement::maybeTailDuplicateBlock` calls
    `TailDup.shouldTailDuplicate` on its argument, which in turn calls
    `analyzeBranch` with `AllowModify` set to true.

 2. This `analyzeBranch` call edits the terminator sequence of the block
    based on the physical layout of the machine function, turning an
    unanalyzable non-fallthrough block to a unanalyzable fallthrough
    block.  Normally MBP bails out of rearranging such blocks, but this
    block was unanalyzable non-fallthrough (and thus rearrangeable) the
    first time MBP looked at it, and so it goes ahead and decides where
    it should be placed in the function.

 3. When placing this block MBP fails to analyze and thus update the
    block in keeping with the new physical layout.

Concretely, before (1) we have something like:

```
LBL0:
  < unknown terminator op that may branch to LBL1 >
  jmp LBL1

LBL1:
  ... A

LBL2:
  ... B
```

In (2), analyze branch simplifies this to

```
LBL0:
  < unknown terminator op that may branch to LBL2 >
  ;; jmp LBL1 <- redundant jump removed

LBL1:
  ... A

LBL2:
  ... B
```

In (3), MachineBlockPlacement goes ahead with its plan of putting LBL2
after the first block since that is profitable.

```
LBL0:
  < unknown terminator op that may branch to LBL2 >
  ;; jmp LBL1 <- redundant jump

LBL2:
  ... B

LBL1:
  ... A
```

and the program now has incorrect behavior (we no longer fall-through
from `LBL0` to `LBL1`) because MBP can no longer edit LBL0.

There are several possible solutions, but I went with removing the teeth
off of the `analyzeBranch` calls in TailDuplicator.  That makes thinking
about the result of these calls easier, and breaks nothing in the lit
test suite.

I've also added some bookkeeping to the MachineBlockPlacement pass and
used that to write an assert that would have caught this.

Reviewers: chandlerc, gberry, MatzeB, iteratee

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D27783

llvm-svn: 289764

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -324,6 +324,14 @@ class MachineBlockPlacement : public MachineFunctionPass {
   /// between basic blocks.
   DenseMap<MachineBasicBlock *, BlockChain *> BlockToChain;
 
+#ifndef NDEBUG
+  /// The set of basic blocks that have terminators that cannot be fully
+  /// analyzed.  These basic blocks cannot be re-ordered safely by
+  /// MachineBlockPlacement, and we must preserve physical layout of these
+  /// blocks and their successors through the pass.
+  SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits;
+#endif
+
   /// Decrease the UnscheduledPredecessors count for all blocks in chain, and
   /// if the count goes to 0, add them to the appropriate work list.
   void markChainSuccessors(BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
@@ -1589,6 +1597,7 @@ void MachineBlockPlacement::buildCFGChains() {
                    << getBlockName(BB) << " -> " << getBlockName(NextBB)
                    << "\n");
       Chain->merge(NextBB, nullptr);
+      BlocksWithUnanalyzableExits.insert(&*BB);
       FI = NextFI;
       BB = NextBB;
     }
@@ -1661,6 +1670,19 @@ void MachineBlockPlacement::buildCFGChains() {
     Cond.clear();
     MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
 
+#ifndef NDEBUG
+    if (!BlocksWithUnanalyzableExits.count(PrevBB)) {
+      // Given the exact block placement we chose, we may actually not _need_ to
+      // be able to edit PrevBB's terminator sequence, but not being _able_ to
+      // do that at this point is a bug.
+      assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) ||
+              !PrevBB->canFallThrough()) &&
+             "Unexpected block with un-analyzable fallthrough!");
+      Cond.clear();
+      TBB = FBB = nullptr;
+    }
+#endif
+
     // The "PrevBB" is not yet updated to reflect current code layout, so,
     //   o. it may fall-through to a block without explicit "goto" instruction
     //      before layout, and no longer fall-through it after layout; or

llvm/lib/CodeGen/TailDuplicator.cpp

@@ -550,8 +550,8 @@ bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
   // blocks with unanalyzable fallthrough get layed out contiguously.
   MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
   SmallVector<MachineOperand, 4> PredCond;
-  if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond, true)
-      && TailBB.canFallThrough())
+  if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond) &&
+      TailBB.canFallThrough())
     return false;
 
   // If the target has hardware branch prediction that can handle indirect
@@ -660,7 +660,7 @@ bool TailDuplicator::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
 
     MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
     SmallVector<MachineOperand, 4> PredCond;
-    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
       return false;
 
     if (!PredCond.empty())
@@ -687,7 +687,7 @@ bool TailDuplicator::duplicateSimpleBB(
 
     MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
     SmallVector<MachineOperand, 4> PredCond;
-    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+    if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
       continue;
 
     Changed = true;
@@ -750,7 +750,7 @@ bool TailDuplicator::canTailDuplicate(MachineBasicBlock *TailBB,
 
   MachineBasicBlock *PredTBB, *PredFBB;
   SmallVector<MachineOperand, 4> PredCond;
-  if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
+  if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
     return false;
   if (!PredCond.empty())
     return false;
@@ -833,7 +833,7 @@ bool TailDuplicator::tailDuplicate(bool IsSimple, MachineBasicBlock *TailBB,
     // Simplify
     MachineBasicBlock *PredTBB, *PredFBB;
     SmallVector<MachineOperand, 4> PredCond;
-    TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true);
+    TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond);
 
     NumTailDupAdded += TailBB->size() - 1; // subtract one for removed branch
 
@@ -861,7 +861,7 @@ bool TailDuplicator::tailDuplicate(bool IsSimple, MachineBasicBlock *TailBB,
   if (PrevBB->succ_size() == 1 &&
       // Layout preds are not always CFG preds. Check.
       *PrevBB->succ_begin() == TailBB &&
-      !TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
+      !TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond) &&
       PriorCond.empty() &&
       (!PriorTBB || PriorTBB == TailBB) &&
       TailBB->pred_size() == 1 &&

llvm/test/CodeGen/X86/block-placement.mir

@@ -0,0 +1,87 @@
+# RUN: llc -O3 -run-pass=block-placement -o - %s | FileCheck %s
+
+--- |
+  ; ModuleID = 'test.ll'
+  source_filename = "test.ll"
+  target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+  
+  declare void @stub(i32*)
+  
+  define i32 @f(i32* %ptr, i1 %cond) {
+  entry:
+    br i1 %cond, label %left, label %right
+  
+  left:                                             ; preds = %entry
+    %is_null = icmp eq i32* %ptr, null
+    br i1 %is_null, label %null, label %not_null, !prof !0, !make.implicit !1
+  
+  not_null:                                         ; preds = %left
+    %val = load i32, i32* %ptr
+    ret i32 %val
+  
+  null:                                             ; preds = %left
+    call void @stub(i32* %ptr)
+    unreachable
+  
+  right:                                            ; preds = %entry
+    call void @stub(i32* null)
+    unreachable
+  }
+  
+  ; Function Attrs: nounwind
+  declare void @llvm.stackprotector(i8*, i8**) #0
+  
+  attributes #0 = { nounwind }
+  
+  !0 = !{!"branch_weights", i32 1048575, i32 1}
+  !1 = !{}
+
+...
+---
+# CHECK: name:            f
+name:            f
+alignment:       4
+tracksRegLiveness: true
+liveins:         
+  - { reg: '%rdi' }
+  - { reg: '%esi' }
+
+# CHECK: %eax = FAULTING_LOAD_OP %bb.3.null, 1684, killed %rdi, 1, _, 0, _ :: (load 4 from %ir.ptr)
+# CHECK-NEXT: JMP_1 %bb.2.not_null
+# CHECK: bb.3.null:
+# CHECK:  bb.4.right:
+# CHECK:  bb.2.not_null:
+
+body:             |
+  bb.0.entry:
+    successors: %bb.1.left(0x7ffff800), %bb.3.right(0x00000800)
+    liveins: %esi, %rdi
+  
+    frame-setup PUSH64r undef %rax, implicit-def %rsp, implicit %rsp
+    CFI_INSTRUCTION def_cfa_offset 16
+    TEST8ri %sil, 1, implicit-def %eflags, implicit killed %esi
+    JE_1 %bb.3.right, implicit killed %eflags
+  
+  bb.1.left:
+    successors: %bb.2.null(0x7ffff800), %bb.4.not_null(0x00000800)
+    liveins: %rdi
+  
+    %eax = FAULTING_LOAD_OP %bb.2.null, 1684, killed %rdi, 1, _, 0, _ :: (load 4 from %ir.ptr)
+    JMP_1 %bb.4.not_null
+  
+  bb.4.not_null:
+    liveins: %rdi, %eax
+  
+    %rcx = POP64r implicit-def %rsp, implicit %rsp
+    RETQ %eax
+
+  bb.2.null:
+    liveins: %rdi
+  
+    CALL64pcrel32 @stub, csr_64, implicit %rsp, implicit %rdi, implicit-def %rsp
+  
+  bb.3.right:
+    dead %edi = XOR32rr undef %edi, undef %edi, implicit-def dead %eflags, implicit-def %rdi
+    CALL64pcrel32 @stub, csr_64, implicit %rsp, implicit %rdi, implicit-def %rsp
+  
+...