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Codegen: Tail Merge: Be less aggressive with special cases.

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This change makes it possible for tail-duplication and tail-merging to
be disjoint. By being less aggressive when merging during layout, there are no
overlapping cases between tail-duplication and tail-merging, provided the
thresholds are disjoint.

There is a remaining TODO to benchmark the succ_size() test for non-layout tail
merging.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278265 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Kyle Butt 2016-08-10 18:36:18 +00:00
parent 4425f12c5d
commit c04b985f53
3 changed files with 16 additions and 7 deletions
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17
lib/CodeGen/BranchFolding.cpp
View File

@ -597,7 +597,8 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB,
DenseMap<const MachineBasicBlock *, int> &FuncletMembership) {
DenseMap<const MachineBasicBlock *, int> &FuncletMembership,
bool AfterPlacement) {
// It is never profitable to tail-merge blocks from two different funclets.
if (!FuncletMembership.empty()) {
auto Funclet1 = FuncletMembership.find(MBB1);
@ -617,7 +618,11 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
// It's almost always profitable to merge any number of non-terminator
// instructions with the block that falls through into the common successor.
if (MBB1 == PredBB || MBB2 == PredBB) {
// This is true only for a single successor. For multiple successors, we are
// trading a conditional branch for an unconditional one.
// TODO: Re-visit successor size for non-layout tail merging.
if ((MBB1 == PredBB || MBB2 == PredBB) &&
(!AfterPlacement || MBB1->succ_size() == 1)) {
MachineBasicBlock::iterator I;
unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
if (CommonTailLen > NumTerms)
@ -635,9 +640,12 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
// If both blocks have an unconditional branch temporarily stripped out,
// count that as an additional common instruction for the following
// heuristics.
// heuristics. This heuristic is only accurate for single-succ blocks, so to
// make sure that during layout merging and duplicating don't crash, we check
// for that when merging during layout.
unsigned EffectiveTailLen = CommonTailLen;
if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
(MBB1->succ_size() == 1 || !AfterPlacement) &&
!MBB1->back().isBarrier() &&
!MBB2->back().isBarrier())
++EffectiveTailLen;
@ -682,7 +690,8 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
minCommonTailLength,
CommonTailLen, TrialBBI1, TrialBBI2,
SuccBB, PredBB,
FuncletMembership)) {
FuncletMembership,
AfterBlockPlacement)) {
if (CommonTailLen > maxCommonTailLength) {
SameTails.clear();
maxCommonTailLength = CommonTailLen;

4
test/CodeGen/ARM/ifcvt4.ll
View File

@ -1,8 +1,8 @@
; RUN: llc -mtriple=arm-eabi %s -o - | FileCheck %s
; CHECK-LABEL: t:
; CHECK: subgt
; CHECK: suble
; CHECK-DAG: subgt
; CHECK-DAG: suble
define i32 @t(i32 %a, i32 %b) {
entry:
%tmp1434 = icmp eq i32 %a, %b ; <i1> [#uses=1]

2
test/CodeGen/Hexagon/rdf-copy.ll
View File

@ -17,7 +17,7 @@
; CHECK: [[DST:r[0-9]+]] = [[SRC:r[0-9]+]]
; CHECK-DAG: memw([[SRC]]
; CHECK-NOT: memw([[DST]]
; CHECK-LABEL: LBB0_2
; CHECK: %if.end
target datalayout = "e-p:32:32:32-i64:64:64-i32:32:32-i16:16:16-i1:32:32-f64:64:64-f32:32:32-v64:64:64-v32:32:32-a0:0-n16:32"
target triple = "hexagon"