From 2b0433bae85bf26c965205545227af84f1d14f73 Mon Sep 17 00:00:00 2001
From: Reid Kleckner <rnk@google.com>
Date: Fri, 4 Oct 2019 22:24:21 +0000
Subject: [PATCH] Revert [CodeGen] Do the Simple Early Return in
 block-placement pass to optimize the blocks

This reverts r371177 (git commit f879c6875563c0a8cd838f1e13b14dd33558f1f8)

It caused PR43566 by removing empty, address-taken MachineBasicBlocks.
Such blocks may have references from blockaddress or other operands, and
need more consideration to be removed.

See the PR for a test case to use when relanding.

llvm-svn: 373805
---
 lib/CodeGen/MachineBlockPlacement.cpp    | 46 ------------------------
 test/CodeGen/PowerPC/block-placement.mir | 12 ++++---
 2 files changed, 8 insertions(+), 50 deletions(-)

diff --git a/lib/CodeGen/MachineBlockPlacement.cpp b/lib/CodeGen/MachineBlockPlacement.cpp
index 27438ecf0ad..ac19bc0bd8e 100644
--- a/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/lib/CodeGen/MachineBlockPlacement.cpp
@@ -38,7 +38,6 @@
 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
-#include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachinePostDominators.h"
@@ -2713,7 +2712,6 @@ void MachineBlockPlacement::optimizeBranches() {
   // cannot because all branches may not be analyzable.
   // E.g., the target may be able to remove an unconditional branch to
   // a fallthrough when it occurs after predicated terminators.
-  SmallVector<MachineBasicBlock*, 4> EmptyBB;
   for (MachineBasicBlock *ChainBB : FunctionChain) {
     Cond.clear();
     MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
@@ -2733,50 +2731,9 @@ void MachineBlockPlacement::optimizeBranches() {
         TII->removeBranch(*ChainBB);
         TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
         ChainBB->updateTerminator();
-      } else if (Cond.empty() && TBB && ChainBB != TBB && !TBB->empty() &&
-                 !TBB->canFallThrough()) {
-        // When ChainBB is unconditional branch to the TBB, and TBB has no
-        // fallthrough predecessor and fallthrough successor, try to merge
-        // ChainBB and TBB. This is legal under the one of following conditions:
-        // 1. ChainBB is empty except for an unconditional branch.
-        // 2. TBB has only one predecessor.
-        MachineFunction::iterator I(TBB);
-        if (((TBB == &*F->begin()) || !std::prev(I)->canFallThrough()) &&
-             (TailDup.isSimpleBB(ChainBB) || (TBB->pred_size() == 1))) {
-          TII->removeBranch(*ChainBB);
-          ChainBB->removeSuccessor(TBB);
-
-          // Update the CFG.
-          while (!TBB->pred_empty()) {
-            MachineBasicBlock *Pred = *(TBB->pred_end() - 1);
-            Pred->ReplaceUsesOfBlockWith(TBB, ChainBB);
-          }
-
-          while (!TBB->succ_empty()) {
-            MachineBasicBlock *Succ = *(TBB->succ_end() - 1);
-            ChainBB->addSuccessor(Succ, MBPI->getEdgeProbability(TBB, Succ));
-            TBB->removeSuccessor(Succ);
-          }
-
-          // Move all the instructions of TBB to ChainBB.
-          ChainBB->splice(ChainBB->end(), TBB, TBB->begin(), TBB->end());
-          EmptyBB.push_back(TBB);
-
-          // If TBB was the target of a jump table, update jump tables to go to
-          // the ChainBB instead.
-          if (MachineJumpTableInfo *MJTI = F->getJumpTableInfo())
-            MJTI->ReplaceMBBInJumpTables(TBB, ChainBB);
-        }
       }
     }
   }
-
-  for (auto BB: EmptyBB) {
-    MLI->removeBlock(BB);
-    FunctionChain.remove(BB);
-    BlockToChain.erase(BB);
-    F->erase(BB);
-  }
 }
 
 void MachineBlockPlacement::alignBlocks() {
@@ -3097,9 +3054,6 @@ bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
     }
   }
 
-  // optimizeBranches() may change the blocks, but we haven't updated the
-  // post-dominator tree. Because the post-dominator tree won't be used after
-  // this function and this pass don't preserve the post-dominator tree.
   optimizeBranches();
   alignBlocks();
 
diff --git a/test/CodeGen/PowerPC/block-placement.mir b/test/CodeGen/PowerPC/block-placement.mir
index 54bd9b8e923..9dc911f785b 100644
--- a/test/CodeGen/PowerPC/block-placement.mir
+++ b/test/CodeGen/PowerPC/block-placement.mir
@@ -209,10 +209,14 @@ body:             |
     BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 
   ; CHECK:      bb.5.if.else.i:
-  ; CHECK-NEXT:   renamable $x3 = LI8 1
-  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
+  ; CHECK:        successors: %bb.11(0x80000000)
+  ; CHECK:        B %bb.11
 
   ; CHECK:      bb.8.while.body.i (align 16):
-  ; CHECK:        successors: %bb.5(0x04000000), %bb.9(0x7c000000)
-  ; CHECK:        BCC 76, killed renamable $cr0, %bb.5
+  ; CHECK:        successors: %bb.11(0x04000000), %bb.9(0x7c000000)
+  ; CHECK:        BCC 76, killed renamable $cr0, %bb.11
+
+  ; CHECK:      bb.11:
+  ; CHECK:        renamable $x3 = LI8 1
+  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 ...