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Reapply [BranchFolding] Restrict tail merging loop blocks after MBP 

Fixed a bug in the test case.

To fix PR28104, this patch restricts tail merging to blocks that belong to the
same loop after MBP.

llvm-svn: 278575
This commit is contained in:
Haicheng Wu 2016-08-12 23:13:38 +00:00
parent 2868fa171a
commit 7c4535d1e7
3 changed files with 116 additions and 10 deletions
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32
llvm/lib/CodeGen/BranchFolding.cpp
View File

@ -1005,6 +1005,24 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
MachineBasicBlock *IBB = &*I;
MachineBasicBlock *PredBB = &*std::prev(I);
MergePotentials.clear();
MachineLoop *ML;
// Bail if merging after placement and IBB is the loop header because
// -- If merging predecessors that belong to the same loop as IBB, the
// common tail of merged predecessors may become the loop top if block
// placement is called again and the predecessors may branch to this common
// tail and require more branches. This can be relaxed if
// MachineBlockPlacement::findBestLoopTop is more flexible.
// --If merging predecessors that do not belong to the same loop as IBB, the
// loop info of IBB's loop and the other loops may be affected. Calling the
// block placement again may make big change to the layout and eliminate the
// reason to do tail merging here.
if (AfterBlockPlacement && MLI) {
ML = MLI->getLoopFor(IBB);
if (ML && IBB == ML->getHeader())
continue;
}
for (MachineBasicBlock *PBB : I->predecessors()) {
if (MergePotentials.size() == TailMergeThreshold)
break;
@ -1024,16 +1042,12 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
if (PBB->hasEHPadSuccessor())
continue;
// Bail out if the loop header (IBB) is not the top of the loop chain
// after the block placement. Otherwise, the common tail of IBB's
// predecessors may become the loop top if block placement is called again
// and the predecessors may branch to this common tail.
// FIXME: Relaxed this check if the algorithm of finding loop top is
// changed in MBP.
// After block placement, only consider predecessors that belong to the
// same loop as IBB. The reason is the same as above when skipping loop
// header.
if (AfterBlockPlacement && MLI)
if (MachineLoop *ML = MLI->getLoopFor(IBB))
if (IBB == ML->getHeader() && ML == MLI->getLoopFor(PBB))
continue;
if (ML != MLI->getLoopFor(PBB))
continue;
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;

2
llvm/test/CodeGen/ARM/arm-and-tst-peephole.ll
View File

@ -49,7 +49,7 @@ tailrecurse.switch: ; preds = %tailrecurse
; V8-NEXT: beq
; V8-NEXT: %tailrecurse.switch
; V8: cmp
; V8-NEXT: beq
; V8-NEXT: bne
; V8-NEXT: b
; The trailing space in the last line checks that the branch is unconditional
switch i32 %and, label %sw.epilog [

92
llvm/test/CodeGen/X86/tail-merge-after-mbp.ll Normal file
View File

@ -0,0 +1,92 @@
; RUN: llc -mtriple=x86_64-linux -o - %s | FileCheck %s
%0 = type { %1, %3* }
%1 = type { %2* }
%2 = type { %2*, i8* }
%3 = type { i32, i32 (i32, i32)* }
declare i32 @Up(...)
declare i32 @f(i32, i32)
; check loop block BB#10 is not merged with LBB0_12
; check loop block LBB0_9 is not merged with BB#11, BB#13
define i32 @foo(%0* nocapture readonly, i32, i1 %c, i8* %p1, %2** %p2) {
; CHECK-LABEL: foo:
; CHECK: LBB0_9:
; CHECK-NEXT: movq (%r14), %rax
; CHECK-NEXT: testq %rax, %rax
; CHECK-NEXT: je
; CHECK-NEXT:# BB#10:
; CHECK-NEXT: cmpq $0, 8(%rax)
; CHECK-NEXT: jne
; CHECK-NEXT:# BB#11:
; CHECK-NEXT: movq (%r14), %rax
; CHECK-NEXT: testq %rax, %rax
; CHECK-NEXT: je
; CHECK-NEXT:LBB0_12:
; CHECK-NEXT: cmpq $0, 8(%rax)
; CHECK-NEXT: jne
; CHECK-NEXT:# BB#13:
; CHECK-NEXT: movq (%r14), %rax
; CHECK-NEXT: testq %rax, %rax
; CHECK-NEXT: jne
br i1 %c, label %34, label %3
; <label>:3: ; preds = %2
br i1 %c, label %7, label %4
; <label>:4: ; preds = %3
%5 = tail call i32 @f(i32 undef, i32 undef)
%6 = icmp eq i32 %5, 0
br i1 %6, label %7, label %34
; <label>:7: ; preds = %4, %3
%8 = icmp eq %2* null, null
br i1 %8, label %34, label %9
; <label>:9: ; preds = %7
%10 = icmp eq i8* %p1, null
br i1 %10, label %11, label %32
; <label>:11: ; preds = %9
%12 = load %2*, %2** %p2, align 8
%13 = icmp eq %2* %12, null
br i1 %13, label %34, label %14
; <label>:14: ; preds = %11
%15 = getelementptr inbounds %2, %2* %12, i64 0, i32 1
%16 = load i8*, i8** %15, align 8
%17 = icmp eq i8* %16, null
br i1 %17, label %18, label %32
; <label>:18: ; preds = %14
%19 = load %2*, %2** %p2, align 8
%20 = icmp eq %2* %19, null
br i1 %20, label %34, label %21
; <label>:21: ; preds = %18
%22 = getelementptr inbounds %2, %2* %19, i64 0, i32 1
%23 = load i8*, i8** %22, align 8
%24 = icmp eq i8* %23, null
br i1 %24, label %25, label %32
; <label>:25: ; preds = %28, %21
%26 = load %2*, %2** %p2, align 8
%27 = icmp eq %2* %26, null
br i1 %27, label %34, label %28
; <label>:28: ; preds = %25
%29 = getelementptr inbounds %2, %2* %26, i64 0, i32 1
%30 = load i8*, i8** %29, align 8
%31 = icmp eq i8* %30, null
br i1 %31, label %25, label %32
; <label>:32: ; preds = %28, %21, %14, %9
%33 = tail call i32 (...) @Up()
br label %34
; <label>:34: ; preds = %32, %25, %18, %11, %7, %4, %2
%35 = phi i32 [ 0, %2 ], [ %5, %4 ], [ 0, %7 ], [ 0, %11 ], [ 0, %32 ], [ 0, %18 ], [ 0, %25 ]
ret i32 %35
}