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In Machine CSE pass, the source register of a COPY machine instruction can

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be propagated to all its users, and this propagation could increase the 
probability of finding common subexpressions. If the COPY has only one user,
the COPY itself can be removed.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215344 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jiangning Liu 2014-08-11 05:17:19 +00:00
parent 1505fa4376
commit 0679d2d0a4
2 changed files with 64 additions and 11 deletions
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30
lib/CodeGen/MachineCSE.cpp
View File

@ -79,7 +79,8 @@ namespace {
SmallVector<MachineInstr*, 64> Exps;
unsigned CurrVN;
bool PerformTrivialCoalescing(MachineInstr *MI, MachineBasicBlock *MBB);
bool PerformTrivialCopyPropagation(MachineInstr *MI,
MachineBasicBlock *MBB);
bool isPhysDefTriviallyDead(unsigned Reg,
MachineBasicBlock::const_iterator I,
MachineBasicBlock::const_iterator E) const;
@ -113,8 +114,12 @@ INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_END(MachineCSE, "machine-cse",
"Machine Common Subexpression Elimination", false, false)
bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
MachineBasicBlock *MBB) {
/// The source register of a COPY machine instruction can be propagated to all
/// its users, and this propagation could increase the probability of finding
/// common subexpressions. If the COPY has only one user, the COPY itself can
/// be removed.
bool MachineCSE::PerformTrivialCopyPropagation(MachineInstr *MI,
MachineBasicBlock *MBB) {
bool Changed = false;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = MI->getOperand(i);
@ -123,10 +128,7 @@ bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
unsigned Reg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(Reg))
continue;
if (!MRI->hasOneNonDBGUse(Reg))
// Only coalesce single use copies. This ensure the copy will be
// deleted.
continue;
bool OnlyOneUse = MRI->hasOneNonDBGUse(Reg);
MachineInstr *DefMI = MRI->getVRegDef(Reg);
if (!DefMI->isCopy())
continue;
@ -154,10 +156,14 @@ bool MachineCSE::PerformTrivialCoalescing(MachineInstr *MI,
continue;
DEBUG(dbgs() << "Coalescing: " << *DefMI);
DEBUG(dbgs() << "*** to: " << *MI);
// Propagate SrcReg of copies to MI.
MO.setReg(SrcReg);
MRI->clearKillFlags(SrcReg);
DefMI->eraseFromParent();
++NumCoalesces;
// Coalesce single use copies.
if (OnlyOneUse) {
DefMI->eraseFromParent();
++NumCoalesces;
}
Changed = true;
}
@ -454,13 +460,15 @@ bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
bool FoundCSE = VNT.count(MI);
if (!FoundCSE) {
// Look for trivial copy coalescing opportunities.
if (PerformTrivialCoalescing(MI, MBB)) {
// Using trivial copy propagation to find more CSE opportunities.
if (PerformTrivialCopyPropagation(MI, MBB)) {
Changed = true;
// After coalescing MI itself may become a copy.
if (MI->isCopyLike())
continue;
// Try again to see if CSE is possible.
FoundCSE = VNT.count(MI);
}
}

45
test/CodeGen/AArch64/machine_cse.ll Normal file
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@ -0,0 +1,45 @@
; RUN: llc < %s -mtriple=aarch64-linux-gnuabi -O2 | FileCheck %s
; marked as external to prevent possible optimizations
@a = external global i32
@b = external global i32
@c = external global i32
@d = external global i32
@e = external global i32
define void @combine-sign-comparisons-by-cse(i32 *%arg) {
; CHECK: cmp
; CHECK: b.ge
; CHECK-NOT: cmp
; CHECK: b.le
entry:
%a = load i32* @a, align 4
%b = load i32* @b, align 4
%c = load i32* @c, align 4
%d = load i32* @d, align 4
%e = load i32* @e, align 4
%cmp = icmp slt i32 %a, %e
br i1 %cmp, label %land.lhs.true, label %lor.lhs.false
land.lhs.true:
%cmp1 = icmp eq i32 %b, %c
br i1 %cmp1, label %return, label %if.end
lor.lhs.false:
%cmp2 = icmp sgt i32 %a, %e
br i1 %cmp2, label %land.lhs.true3, label %if.end
land.lhs.true3:
%cmp4 = icmp eq i32 %b, %d
br i1 %cmp4, label %return, label %if.end
if.end:
br label %return
return:
%retval.0 = phi i32 [ 0, %if.end ], [ 1, %land.lhs.true3 ], [ 1, %land.lhs.true ]
store i32 %a, i32 *%arg
ret void
}