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

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Summary:

Fix a bug of preducessors.

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

llvm-svn: 369191
This commit is contained in:
Kang Zhang 2019-08-17 14:37:05 +00:00
parent bdd80e6afb
commit cd9e0c5c7a
2 changed files with 44 additions and 8 deletions
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40
lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -2711,6 +2711,7 @@ 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.
@ -2730,9 +2731,45 @@ 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);
}
}
}
}
for (auto BB: EmptyBB) {
MLI->removeBlock(BB);
FunctionChain.remove(BB);
BlockToChain.erase(BB);
F->erase(BB);
}
}
void MachineBlockPlacement::alignBlocks() {
@ -3052,6 +3089,9 @@ 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();

12
test/CodeGen/PowerPC/block-placement.mir
View File

@ -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
...