[llvm] r368509 - [CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks

[Kang Zhang via llvm-commits]

Author: zhangkang
Date: Sat Aug 10 02:58:52 2019
New Revision: 368509

URL: http://llvm.org/viewvc/llvm-project?rev=368509&view=rev
Log:
[CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks

Summary:

In `block-placement` pass, it will create some patterns for unconditional we can do the simple early retrun.
But the `early-ret` pass is before `block-placement`, we don't want to run it again.
This patch is to do the simple early return to optimize the blocks at the last of `block-placement`.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D63972

Modified:
    llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
    llvm/trunk/test/CodeGen/PowerPC/block-placement-1.mir
    llvm/trunk/test/CodeGen/PowerPC/block-placement.mir

Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=368509&r1=368508&r2=368509&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Sat Aug 10 02:58:52 2019
@@ -2755,6 +2755,7 @@ void MachineBlockPlacement::optimizeBran
   // 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.
@@ -2774,9 +2775,41 @@ void MachineBlockPlacement::optimizeBran
         TII->removeBranch(*ChainBB);
         TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
         ChainBB->updateTerminator();
+      } else if (Cond.empty() && TBB && ChainBB != TBB &&
+                 !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.
+          for (MachineBasicBlock::pred_iterator PI = TBB->pred_begin(),
+               PE = TBB->pred_end(); PI != PE; PI++)
+            (*PI)->ReplaceUsesOfBlockWith(TBB, ChainBB);
+
+          for (MachineBasicBlock *Succ : TBB->successors())
+            ChainBB->addSuccessor(Succ, MBPI->getEdgeProbability(TBB, Succ));
+
+          // Move all the instructions of TBB to ChainBB.
+          ChainBB->splice(ChainBB->end(), TBB, TBB->begin(), TBB->end());
+          EmptyBB.push_back(TBB);
+        }
       }
     }
   }
+
+  for (auto BB: EmptyBB) {
+    MLI->removeBlock(BB);
+    FunctionChain.remove(BB);
+    BlockToChain.erase(BB);
+    F->erase(BB);
+  }
 }
 
 void MachineBlockPlacement::alignBlocks() {
@@ -3096,6 +3129,9 @@ bool MachineBlockPlacement::runOnMachine
     }
   }
 
+  // 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();
 

Modified: llvm/trunk/test/CodeGen/PowerPC/block-placement-1.mir
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/block-placement-1.mir?rev=368509&r1=368508&r2=368509&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/block-placement-1.mir (original)
+++ llvm/trunk/test/CodeGen/PowerPC/block-placement-1.mir Sat Aug 10 02:58:52 2019
@@ -299,14 +299,8 @@ body:             |
   bb.11.unreachable:
 
   ; CHECK:      bb.4.catch4:
-  ; CHECK:        successors: %bb.11(0x7ffff800), %bb.6(0x00000800)
-  ; CHECK:        B %bb.11
+  ; CHECK:        successors: %bb.6(0x00000800)
 
   ; CHECK:      bb.1.for.body (align 4):
-  ; CHECK:        successors: %bb.2(0x7ffff800), %bb.3(0x00000800)
-  ; CHECK:        B %bb.2
-
-  ; CHECK:      bb.2..noexc:
-
-  ; CHECK:      bb.11.unreachable: 
+  ; CHECK:        successors: %bb.3(0x00000800)
 ...

Modified: llvm/trunk/test/CodeGen/PowerPC/block-placement.mir
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/block-placement.mir?rev=368509&r1=368508&r2=368509&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/block-placement.mir (original)
+++ llvm/trunk/test/CodeGen/PowerPC/block-placement.mir Sat Aug 10 02:58:52 2019
@@ -209,14 +209,10 @@ body:             |
     BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 
   ; CHECK:      bb.5.if.else.i:
-  ; CHECK:        successors: %bb.11(0x80000000)
-  ; CHECK:        B %bb.11
+  ; CHECK-NEXT:   renamable $x3 = LI8 1
+  ; CHECK-NEXT:   BLR8 implicit $lr8, implicit $rm, implicit killed $x3
 
   ; CHECK:      bb.8.while.body.i (align 4):
-  ; 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
+  ; CHECK:        successors: %bb.5(0x04000000), %bb.9(0x7c000000)
+  ; CHECK:        BCC 76, killed renamable $cr0, %bb.5
 ...