maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

Tail merge at any size when there are two potentials blocks and one 

can be made to fall through into the other.

llvm-svn: 86909
This commit is contained in:
Dan Gohman 2009-11-12 00:39:10 +00:00
parent a970d39ebf
commit 09478e975d
2 changed files with 61 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
llvm/lib/CodeGen/BranchFolding.cpp
View File

@ -495,6 +495,15 @@ static bool ProfitableToMerge(MachineBasicBlock *MBB1,
return true;
}
// If one of the blocks can be completely merged and happens to be in
// a position where the other could fall through into it, merge any number
// of instructions, because it can be done without a branch.
// TODO: If the blocks are not adjacent, move one of them so that they are?
if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
return true;
if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
return true;
// If both blocks have an unconditional branch temporarily stripped out,
// treat that as an additional common instruction.
if (MBB1 != PredBB && MBB2 != PredBB &&
@ -716,16 +725,31 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
getParent()->begin();
unsigned commonTailIndex = SameTails.size();
for (unsigned i=0; i<SameTails.size(); i++) {
MachineBasicBlock *MBB = SameTails[i].getBlock();
if (MBB == EntryBB)
continue;
if (MBB == PredBB) {
commonTailIndex = i;
break;
// If there are two blocks, check to see if one can be made to fall through
// into the other.
if (SameTails.size() == 2 &&
SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
SameTails[1].tailIsWholeBlock())
commonTailIndex = 1;
else if (SameTails.size() == 2 &&
SameTails[1].getBlock()->isLayoutSuccessor(
SameTails[0].getBlock()) &&
SameTails[0].tailIsWholeBlock())
commonTailIndex = 0;
else {
// Otherwise just pick one, favoring the fall-through predecessor if
// there is one.
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
MachineBasicBlock *MBB = SameTails[i].getBlock();
if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
continue;
if (MBB == PredBB) {
commonTailIndex = i;
break;
}
if (SameTails[i].tailIsWholeBlock())
commonTailIndex = i;
}
if (MBB->begin() == SameTails[i].getTailStartPos())
commonTailIndex = i;
}
if (commonTailIndex == SameTails.size() ||
@ -1049,7 +1073,7 @@ bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB,
continue;
// Don't duplicate into a fall-through predecessor unless its the
// only predecessor.
if (&*next(MachineFunction::iterator(PredBB)) == TailBB &&
if (PredBB->isLayoutSuccessor(TailBB) &&
PrevFallsThrough &&
TailBB->pred_size() != 1)
continue;

27
llvm/test/CodeGen/X86/tail-opts.ll
View File

@ -266,3 +266,30 @@ bb3: ; preds = %bb2, %bb1, %lvalue_
declare fastcc i32 @lvalue_p(%union.tree_node* nocapture) nounwind readonly
declare fastcc %union.tree_node* @default_conversion(%union.tree_node*) nounwind
; If one tail merging candidate falls through into the other,
; tail merging is likely profitable regardless of how few
; instructions are involved. This function should have only
; one ret instruction.
; CHECK: foo:
; CHECK: call func
; CHECK-NEXT: .LBB5_2:
; CHECK-NEXT: addq $8, %rsp
; CHECK-NEXT: ret
define void @foo(i1* %V) nounwind {
entry:
%t0 = icmp eq i1* %V, null
br i1 %t0, label %return, label %bb
bb:
call void @func()
ret void
return:
ret void
}
declare void @func()