[llvm] r371177 - [CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks
Kang Zhang via llvm-commits
llvm-commits at lists.llvm.org
Fri Sep 6 01:16:19 PDT 2019
Author: zhangkang
Date: Fri Sep 6 01:16:18 2019
New Revision: 371177
URL: http://llvm.org/viewvc/llvm-project?rev=371177&view=rev
Log:
[CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks
Summary:
Fix a bug of not update the jump table and recommit it again.
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.mir
Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=371177&r1=371176&r2=371177&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Fri Sep 6 01:16:18 2019
@@ -38,6 +38,7 @@
#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"
@@ -2712,6 +2713,7 @@ 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.
@@ -2731,9 +2733,50 @@ void MachineBlockPlacement::optimizeBran
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() {
@@ -3053,6 +3096,9 @@ 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.mir
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/block-placement.mir?rev=371177&r1=371176&r2=371177&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/block-placement.mir (original)
+++ llvm/trunk/test/CodeGen/PowerPC/block-placement.mir Fri Sep 6 01:16:18 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 16):
- ; 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
...
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