[llvm] r368997 - [CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Fri Aug 16 06:20:31 PDT 2019
Hi,
I’ve reverted this change in r369104, as it seems to cause a verifier failure.
It looks like this commit adds invalid predecessors to MBBs. The example below fails the verifier after MachineBlockPlacement (run llc -verify-machineinstrs):
@global.4 = external constant i8*
declare i32 @zot(...)
define i16* @snork.67() personality i8* bitcast (i32 (...)* @zot to i8*) {
bb:
invoke void undef()
to label %bb5 unwind label %bb4
bb4: ; preds = %bb
%tmp = landingpad { i8*, i32 }
catch i8* null
unreachable
bb5: ; preds = %bb
%tmp6 = load i32, i32* null, align 4
%tmp7 = icmp eq i32 %tmp6, 0
br i1 %tmp7, label %bb14, label %bb8
bb8: ; preds = %bb11, %bb5
invoke void undef()
to label %bb9 unwind label %bb11
bb9: ; preds = %bb8
%tmp10 = invoke i16* undef()
to label %bb14 unwind label %bb11
bb11: ; preds = %bb9, %bb8
%tmp12 = landingpad { i8*, i32 }
cleanup
catch i8* bitcast (i8** @global.4 to i8*)
%tmp13 = icmp ult i64 undef, undef
br i1 %tmp13, label %bb8, label %bb14
bb14: ; preds = %bb11, %bb9, %bb5
%tmp15 = phi i16* [ null, %bb5 ], [ null, %bb11 ], [ %tmp10, %bb9 ]
ret i16* %tmp15
}
Cheers,
Florian
> On Aug 15, 2019, at 14:05, Kang Zhang via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>
> Author: zhangkang
> Date: Thu Aug 15 06:05:16 2019
> New Revision: 368997
>
> URL: http://llvm.org/viewvc/llvm-project?rev=368997&view=rev
> Log:
> [CodeGen] Do the Simple Early Return in block-placement pass to optimize the blocks
>
> Summary:
>
> This patch has trigger a bug of r368339, and the r368339 has been reverted, So upstream this patch 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=368997&r1=368996&r2=368997&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
> +++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Thu Aug 15 06:05:16 2019
> @@ -2711,6 +2711,7 @@ 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.
> @@ -2730,9 +2731,42 @@ 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);
> + }
> +
> + 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() {
> @@ -3052,6 +3086,9 @@ 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=368997&r1=368996&r2=368997&view=diff
> ==============================================================================
> --- llvm/trunk/test/CodeGen/PowerPC/block-placement.mir (original)
> +++ llvm/trunk/test/CodeGen/PowerPC/block-placement.mir Thu Aug 15 06:05:16 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
> ...
>
>
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