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Optimize code placement in loop to eliminate unconditional branches or move unconditional branch to the outside of the loop. e.g.

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///       A:                                                                                                                                                                 
///       ...                                                                                                                                                                
///       <fallthrough to B>                                                                                                                                                 
///                                                                                                                                                                          
///       B:  --> loop header                                                                                                                                                
///       ...                                                                                                                                                                
///       jcc <cond> C, [exit]                                                                                                                                               
///                                                                                                                                                                          
///       C:                                                                                                                                                                 
///       ...                                                                                                                                                                
///       jmp B                                                                                                                                                              
///                                                                                                                                                                          
/// ==>                                                                                                                                                                      
///                                                                                                                                                                          
///       A:                                                                                                                                                                 
///       ...                                                                                                                                                                
///       jmp B                                                                                                                                                              
///                                                                                                                                                                          
///       C:  --> new loop header                                                                                                                                            
///       ...                                                                                                                                                                
///       <fallthough to B>                                                                                                                                                  
///                                                                                                                                                                          
///       B:                                                                                                                                                                 
///       ...                                                                                                                                                                
///       jcc <cond> C, [exit] 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71209 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng 2009-05-08 06:34:09 +00:00
parent 4a228864f2
commit 45e0010e14
3 changed files with 371 additions and 17 deletions
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252
lib/CodeGen/CodePlacementOpt.cpp
View File

@ -16,15 +16,40 @@
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
static cl::opt<bool>
OptLoopBBPlacement("opt-loop-bb-placement",
cl::init(false), cl::Hidden,
cl::desc("Optimize block placements in loops"));
STATISTIC(NumHeaderAligned, "Number of loop header aligned");
STATISTIC(NumIntraElim, "Number of intra loop branches eliminated");
STATISTIC(NumIntraMoved, "Number of intra loop branches moved");
namespace {
class CodePlacementOpt : public MachineFunctionPass {
const MachineLoopInfo *MLI;
const TargetInstrInfo *TII;
const TargetLowering *TLI;
/// ChangedMBBs - BBs which are modified by OptimizeIntraLoopEdges.
SmallPtrSet<MachineBasicBlock*, 8> ChangedMBBs;
/// UncondJmpMBBs - A list of BBs which are in loops and end with
/// unconditional branches.
SmallVector<std::pair<MachineBasicBlock*,MachineBasicBlock*>, 4>
UncondJmpMBBs;
/// LoopHeaders - A list of BBs which are loop headers.
SmallVector<MachineBasicBlock*, 4> LoopHeaders;
public:
static char ID;
@ -37,12 +62,12 @@ namespace {
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineLoopInfo>();
AU.addPreserved<MachineLoopInfo>();
AU.addPreservedID(MachineDominatorsID);
MachineFunctionPass::getAnalysisUsage(AU);
}
private:
bool OptimizeIntraLoopEdges();
bool AlignLoops(MachineFunction &MF);
};
@ -53,32 +78,199 @@ FunctionPass *llvm::createCodePlacementOptPass() {
return new CodePlacementOpt();
}
/// OptimizeBackEdges - Place loop back edges to move unconditional branches
/// out of the loop.
///
/// A:
/// ...
/// <fallthrough to B>
///
/// B: --> loop header
/// ...
/// jcc <cond> C, [exit]
///
/// C:
/// ...
/// jmp B
///
/// ==>
///
/// A:
/// ...
/// jmp B
///
/// C: --> new loop header
/// ...
/// <fallthough to B>
///
/// B:
/// ...
/// jcc <cond> C, [exit]
///
bool CodePlacementOpt::OptimizeIntraLoopEdges() {
if (!OptLoopBBPlacement)
return false;
bool Changed = false;
for (unsigned i = 0, e = UncondJmpMBBs.size(); i != e; ++i) {
MachineBasicBlock *MBB = UncondJmpMBBs[i].first;
MachineBasicBlock *SuccMBB = UncondJmpMBBs[i].second;
MachineLoop *L = MLI->getLoopFor(MBB);
assert(L && "BB is expected to be in a loop!");
if (ChangedMBBs.count(MBB)) {
// BB has been modified, re-analyze.
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond) || !Cond.empty())
continue;
if (MLI->getLoopFor(TBB) != L || TBB->isLandingPad())
continue;
SuccMBB = TBB;
} else {
assert(MLI->getLoopFor(SuccMBB) == L &&
"Successor is not in the same loop!");
}
if (MBB->isLayoutSuccessor(SuccMBB)) {
// Successor is right after MBB, just eliminate the unconditional jmp.
// Can this happen?
TII->RemoveBranch(*MBB);
ChangedMBBs.insert(MBB);
++NumIntraElim;
continue;
}
// Now check if the predecessor is fallthrough from any BB. If there is,
// that BB should be from outside the loop since edge will become a jmp.
bool OkToMove = true;
MachineBasicBlock *FtMBB = 0, *FtTBB = 0, *FtFBB = 0;
SmallVector<MachineOperand, 4> FtCond;
for (MachineBasicBlock::pred_iterator PI = SuccMBB->pred_begin(),
PE = SuccMBB->pred_end(); PI != PE; ++PI) {
MachineBasicBlock *PredMBB = *PI;
if (PredMBB->isLayoutSuccessor(SuccMBB)) {
if (TII->AnalyzeBranch(*PredMBB, FtTBB, FtFBB, FtCond)) {
OkToMove = false;
break;
}
if (!FtTBB)
FtTBB = SuccMBB;
else if (!FtFBB) {
assert(FtFBB != SuccMBB && "Unexpected control flow!");
FtFBB = SuccMBB;
}
// A fallthrough.
FtMBB = PredMBB;
MachineLoop *PL = MLI->getLoopFor(PredMBB);
if (PL && (PL == L || PL->getLoopDepth() >= L->getLoopDepth())) {
OkToMove = false;
break;
}
}
}
if (!OkToMove)
continue;
// Is it profitable? If SuccMBB can fallthrough itself, that can be changed
// into a jmp.
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (TII->AnalyzeBranch(*SuccMBB, TBB, FBB, Cond))
continue;
if (!TBB && Cond.empty())
TBB = next(MachineFunction::iterator(SuccMBB));
else if (!FBB && !Cond.empty())
FBB = next(MachineFunction::iterator(SuccMBB));
// This calculate the cost of the transformation. Also, it finds the *only*
// intra-loop edge if there is one.
int Cost = 0;
bool HasOneIntraSucc = true;
MachineBasicBlock *IntraSucc = 0;
for (MachineBasicBlock::succ_iterator SI = SuccMBB->succ_begin(),
SE = SuccMBB->succ_end(); SI != SE; ++SI) {
MachineBasicBlock *SSMBB = *SI;
if (MLI->getLoopFor(SSMBB) == L) {
if (!IntraSucc)
IntraSucc = SSMBB;
else
HasOneIntraSucc = false;
}
if (SuccMBB->isLayoutSuccessor(SSMBB))
// This will become a jmp.
++Cost;
else if (MBB->isLayoutSuccessor(SSMBB))
// One of the successor will become the new fallthrough.
if (SSMBB == FBB) {
FBB = 0;
--Cost;
} else if (!FBB && SSMBB == TBB && Cond.empty()) {
TBB = 0;
--Cost;
} else if (!TII->ReverseBranchCondition(Cond)) {
TBB = FBB;
FBB = 0;
--Cost;
}
}
if (Cost)
continue;
// Now, let's move the successor to below the BB to eliminate the jmp.
SuccMBB->moveAfter(MBB);
TII->RemoveBranch(*MBB);
TII->RemoveBranch(*SuccMBB);
if (TBB)
TII->InsertBranch(*SuccMBB, TBB, FBB, Cond);
ChangedMBBs.insert(MBB);
ChangedMBBs.insert(SuccMBB);
if (FtMBB) {
TII->RemoveBranch(*FtMBB);
TII->InsertBranch(*FtMBB, FtTBB, FtFBB, FtCond);
ChangedMBBs.insert(FtMBB);
}
// If BB is the loop latch, we may have a new loop headr.
if (MBB == L->getLoopLatch()) {
assert(MLI->isLoopHeader(SuccMBB) &&
"Only succ of loop latch is not the header?");
if (HasOneIntraSucc && IntraSucc)
std::replace(LoopHeaders.begin(),LoopHeaders.end(), SuccMBB, IntraSucc);
}
}
++NumIntraMoved;
return Changed;
}
/// AlignLoops - Align loop headers to target preferred alignments.
///
bool CodePlacementOpt::AlignLoops(MachineFunction &MF) {
const TargetLowering *TLI = MF.getTarget().getTargetLowering();
if (!TLI)
const Function *F = MF.getFunction();
if (F->hasFnAttr(Attribute::OptimizeForSize))
return false;
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return false; // Don't care about loop alignment.
const Function *F = MF.getFunction();
if (F->hasFnAttr(Attribute::OptimizeForSize))
return false;
// Make sure blocks are numbered in order
MF.RenumberBlocks();
bool Changed = false;
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
MachineBasicBlock *MBB = I;
if (MLI->isLoopHeader(MBB)) {
MachineBasicBlock *PredBB = prior(I);
if (MLI->getLoopFor(MBB) == MLI->getLoopFor(PredBB))
// If previously BB is in the same loop, don't align this BB. We want
// to prevent adding noop's inside a loop.
continue;
MBB->setAlignment(Align);
for (unsigned i = 0, e = LoopHeaders.size(); i != e; ++i) {
MachineBasicBlock *HeaderMBB = LoopHeaders[i];
MachineBasicBlock *PredMBB = prior(MachineFunction::iterator(HeaderMBB));
if (MLI->getLoopFor(HeaderMBB) != MLI->getLoopFor(PredMBB)) {
// If previously BB is in the same loop, don't align this BB. We want
// to prevent adding noop's inside a loop.
HeaderMBB->setAlignment(Align);
Changed = true;
++NumHeaderAligned;
}
}
@ -90,8 +282,36 @@ bool CodePlacementOpt::runOnMachineFunction(MachineFunction &MF) {
if (MLI->empty())
return false; // No loops.
bool Changed = false;
TLI = MF.getTarget().getTargetLowering();
TII = MF.getTarget().getInstrInfo();
// Analyze the BBs first and keep track of loop headers and BBs that
// end with an unconditional jmp to another block in the same loop.
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
MachineBasicBlock *MBB = I;
if (MBB->isLandingPad())
continue;
MachineLoop *L = MLI->getLoopFor(MBB);
if (!L)
continue;
if (MLI->isLoopHeader(MBB))
LoopHeaders.push_back(MBB);
MachineBasicBlock *TBB = 0, *FBB = 0;
SmallVector<MachineOperand, 4> Cond;
if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond) || !Cond.empty())
continue;
if (MLI->getLoopFor(TBB) == L && !TBB->isLandingPad())
UncondJmpMBBs.push_back(std::make_pair(MBB, TBB));
}
bool Changed = OptimizeIntraLoopEdges();
Changed |= AlignLoops(MF);
ChangedMBBs.clear();
UncondJmpMBBs.clear();
LoopHeaders.clear();
return Changed;
}

2
lib/Target/X86/X86InstrInfo.cpp
View File

@ -1508,7 +1508,7 @@ bool X86InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
if (I->getOpcode() == X86::JMP) {
if (!AllowModify) {
TBB = I->getOperand(0).getMBB();
return false;
continue;
}
// If the block has any instructions after a JMP, delete them.

134
test/CodeGen/X86/code_placement.ll Normal file
View File

@ -0,0 +1,134 @@
; RUN: llvm-as < %s | llc -march=x86 -opt-loop-bb-placement | %prcontext jmp 1 | grep align
@Te0 = external global [256 x i32] ; <[256 x i32]*> [#uses=5]
@Te1 = external global [256 x i32] ; <[256 x i32]*> [#uses=4]
@Te3 = external global [256 x i32] ; <[256 x i32]*> [#uses=2]
define void @t(i8* nocapture %in, i8* nocapture %out, i32* nocapture %rk, i32 %r) nounwind ssp {
entry:
%0 = load i32* %rk, align 4 ; <i32> [#uses=1]
%1 = getelementptr i32* %rk, i64 1 ; <i32*> [#uses=1]
%2 = load i32* %1, align 4 ; <i32> [#uses=1]
%tmp15 = add i32 %r, -1 ; <i32> [#uses=1]
%tmp.16 = zext i32 %tmp15 to i64 ; <i64> [#uses=2]
br label %bb
bb: ; preds = %bb1, %entry
%indvar = phi i64 [ 0, %entry ], [ %indvar.next, %bb1 ] ; <i64> [#uses=3]
%s1.0 = phi i32 [ %2, %entry ], [ %56, %bb1 ] ; <i32> [#uses=2]
%s0.0 = phi i32 [ %0, %entry ], [ %43, %bb1 ] ; <i32> [#uses=2]
%tmp18 = shl i64 %indvar, 4 ; <i64> [#uses=4]
%rk26 = bitcast i32* %rk to i8* ; <i8*> [#uses=6]
%3 = lshr i32 %s0.0, 24 ; <i32> [#uses=1]
%4 = zext i32 %3 to i64 ; <i64> [#uses=1]
%5 = getelementptr [256 x i32]* @Te0, i64 0, i64 %4 ; <i32*> [#uses=1]
%6 = load i32* %5, align 4 ; <i32> [#uses=1]
%7 = lshr i32 %s1.0, 16 ; <i32> [#uses=1]
%8 = and i32 %7, 255 ; <i32> [#uses=1]
%9 = zext i32 %8 to i64 ; <i64> [#uses=1]
%10 = getelementptr [256 x i32]* @Te1, i64 0, i64 %9 ; <i32*> [#uses=1]
%11 = load i32* %10, align 4 ; <i32> [#uses=1]
%ctg2.sum2728 = or i64 %tmp18, 8 ; <i64> [#uses=1]
%12 = getelementptr i8* %rk26, i64 %ctg2.sum2728 ; <i8*> [#uses=1]
%13 = bitcast i8* %12 to i32* ; <i32*> [#uses=1]
%14 = load i32* %13, align 4 ; <i32> [#uses=1]
%15 = xor i32 %11, %6 ; <i32> [#uses=1]
%16 = xor i32 %15, %14 ; <i32> [#uses=3]
%17 = lshr i32 %s1.0, 24 ; <i32> [#uses=1]
%18 = zext i32 %17 to i64 ; <i64> [#uses=1]
%19 = getelementptr [256 x i32]* @Te0, i64 0, i64 %18 ; <i32*> [#uses=1]
%20 = load i32* %19, align 4 ; <i32> [#uses=1]
%21 = and i32 %s0.0, 255 ; <i32> [#uses=1]
%22 = zext i32 %21 to i64 ; <i64> [#uses=1]
%23 = getelementptr [256 x i32]* @Te3, i64 0, i64 %22 ; <i32*> [#uses=1]
%24 = load i32* %23, align 4 ; <i32> [#uses=1]
%ctg2.sum2930 = or i64 %tmp18, 12 ; <i64> [#uses=1]
%25 = getelementptr i8* %rk26, i64 %ctg2.sum2930 ; <i8*> [#uses=1]
%26 = bitcast i8* %25 to i32* ; <i32*> [#uses=1]
%27 = load i32* %26, align 4 ; <i32> [#uses=1]
%28 = xor i32 %24, %20 ; <i32> [#uses=1]
%29 = xor i32 %28, %27 ; <i32> [#uses=4]
%30 = lshr i32 %16, 24 ; <i32> [#uses=1]
%31 = zext i32 %30 to i64 ; <i64> [#uses=1]
%32 = getelementptr [256 x i32]* @Te0, i64 0, i64 %31 ; <i32*> [#uses=1]
%33 = load i32* %32, align 4 ; <i32> [#uses=2]
%exitcond = icmp eq i64 %indvar, %tmp.16 ; <i1> [#uses=1]
br i1 %exitcond, label %bb2, label %bb1
bb1: ; preds = %bb
%ctg2.sum31 = add i64 %tmp18, 16 ; <i64> [#uses=1]
%34 = getelementptr i8* %rk26, i64 %ctg2.sum31 ; <i8*> [#uses=1]
%35 = bitcast i8* %34 to i32* ; <i32*> [#uses=1]
%36 = lshr i32 %29, 16 ; <i32> [#uses=1]
%37 = and i32 %36, 255 ; <i32> [#uses=1]
%38 = zext i32 %37 to i64 ; <i64> [#uses=1]
%39 = getelementptr [256 x i32]* @Te1, i64 0, i64 %38 ; <i32*> [#uses=1]
%40 = load i32* %39, align 4 ; <i32> [#uses=1]
%41 = load i32* %35, align 4 ; <i32> [#uses=1]
%42 = xor i32 %40, %33 ; <i32> [#uses=1]
%43 = xor i32 %42, %41 ; <i32> [#uses=1]
%44 = lshr i32 %29, 24 ; <i32> [#uses=1]
%45 = zext i32 %44 to i64 ; <i64> [#uses=1]
%46 = getelementptr [256 x i32]* @Te0, i64 0, i64 %45 ; <i32*> [#uses=1]
%47 = load i32* %46, align 4 ; <i32> [#uses=1]
%48 = and i32 %16, 255 ; <i32> [#uses=1]
%49 = zext i32 %48 to i64 ; <i64> [#uses=1]
%50 = getelementptr [256 x i32]* @Te3, i64 0, i64 %49 ; <i32*> [#uses=1]
%51 = load i32* %50, align 4 ; <i32> [#uses=1]
%ctg2.sum32 = add i64 %tmp18, 20 ; <i64> [#uses=1]
%52 = getelementptr i8* %rk26, i64 %ctg2.sum32 ; <i8*> [#uses=1]
%53 = bitcast i8* %52 to i32* ; <i32*> [#uses=1]
%54 = load i32* %53, align 4 ; <i32> [#uses=1]
%55 = xor i32 %51, %47 ; <i32> [#uses=1]
%56 = xor i32 %55, %54 ; <i32> [#uses=1]
%indvar.next = add i64 %indvar, 1 ; <i64> [#uses=1]
br label %bb
bb2: ; preds = %bb
%tmp10 = shl i64 %tmp.16, 4 ; <i64> [#uses=2]
%ctg2.sum = add i64 %tmp10, 16 ; <i64> [#uses=1]
%tmp1213 = getelementptr i8* %rk26, i64 %ctg2.sum ; <i8*> [#uses=1]
%57 = bitcast i8* %tmp1213 to i32* ; <i32*> [#uses=1]
%58 = and i32 %33, -16777216 ; <i32> [#uses=1]
%59 = lshr i32 %29, 16 ; <i32> [#uses=1]
%60 = and i32 %59, 255 ; <i32> [#uses=1]
%61 = zext i32 %60 to i64 ; <i64> [#uses=1]
%62 = getelementptr [256 x i32]* @Te1, i64 0, i64 %61 ; <i32*> [#uses=1]
%63 = load i32* %62, align 4 ; <i32> [#uses=1]
%64 = and i32 %63, 16711680 ; <i32> [#uses=1]
%65 = or i32 %64, %58 ; <i32> [#uses=1]
%66 = load i32* %57, align 4 ; <i32> [#uses=1]
%67 = xor i32 %65, %66 ; <i32> [#uses=2]
%68 = lshr i32 %29, 8 ; <i32> [#uses=1]
%69 = zext i32 %68 to i64 ; <i64> [#uses=1]
%70 = getelementptr [256 x i32]* @Te0, i64 0, i64 %69 ; <i32*> [#uses=1]
%71 = load i32* %70, align 4 ; <i32> [#uses=1]
%72 = and i32 %71, -16777216 ; <i32> [#uses=1]
%73 = and i32 %16, 255 ; <i32> [#uses=1]
%74 = zext i32 %73 to i64 ; <i64> [#uses=1]
%75 = getelementptr [256 x i32]* @Te1, i64 0, i64 %74 ; <i32*> [#uses=1]
%76 = load i32* %75, align 4 ; <i32> [#uses=1]
%77 = and i32 %76, 16711680 ; <i32> [#uses=1]
%78 = or i32 %77, %72 ; <i32> [#uses=1]
%ctg2.sum25 = add i64 %tmp10, 20 ; <i64> [#uses=1]
%79 = getelementptr i8* %rk26, i64 %ctg2.sum25 ; <i8*> [#uses=1]
%80 = bitcast i8* %79 to i32* ; <i32*> [#uses=1]
%81 = load i32* %80, align 4 ; <i32> [#uses=1]
%82 = xor i32 %78, %81 ; <i32> [#uses=2]
%83 = lshr i32 %67, 24 ; <i32> [#uses=1]
%84 = trunc i32 %83 to i8 ; <i8> [#uses=1]
store i8 %84, i8* %out, align 1
%85 = lshr i32 %67, 16 ; <i32> [#uses=1]
%86 = trunc i32 %85 to i8 ; <i8> [#uses=1]
%87 = getelementptr i8* %out, i64 1 ; <i8*> [#uses=1]
store i8 %86, i8* %87, align 1
%88 = getelementptr i8* %out, i64 4 ; <i8*> [#uses=1]
%89 = lshr i32 %82, 24 ; <i32> [#uses=1]
%90 = trunc i32 %89 to i8 ; <i8> [#uses=1]
store i8 %90, i8* %88, align 1
%91 = lshr i32 %82, 16 ; <i32> [#uses=1]
%92 = trunc i32 %91 to i8 ; <i8> [#uses=1]
%93 = getelementptr i8* %out, i64 5 ; <i8*> [#uses=1]
store i8 %92, i8* %93, align 1
ret void
}