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[SCCP] Resolve indirect branch target when possible.

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Summary:
Resolve indirect branch target when possible.
This potentially eliminates more basicblocks and result in better evaluation for phi and other things.

Reviewers: davide, efriedma, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30322

llvm-svn: 299830
This commit is contained in:
Xin Tong 2017-04-10 00:33:25 +00:00
parent cbfb20a0d6
commit 5903821d9e
2 changed files with 147 additions and 8 deletions
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79
lib/Transforms/Scalar/SCCP.cpp
View File

@ -140,6 +140,14 @@ public:
return nullptr;
}
/// getBlockAddress - If this is a constant with a BlockAddress value, return
/// it, otherwise return null.
BlockAddress *getBlockAddress() const {
if (isConstant())
return dyn_cast<BlockAddress>(getConstant());
return nullptr;
}
void markForcedConstant(Constant *V) {
assert(isUnknown() && "Can't force a defined value!");
Val.setInt(forcedconstant);
@ -600,10 +608,32 @@ void SCCPSolver::getFeasibleSuccessors(TerminatorInst &TI,
return;
}
// TODO: This could be improved if the operand is a [cast of a] BlockAddress.
if (isa<IndirectBrInst>(&TI)) {
// Just mark all destinations executable!
Succs.assign(TI.getNumSuccessors(), true);
// In case of indirect branch and its address is a blockaddress, we mark
// the target as executable.
if (auto *IBR = dyn_cast<IndirectBrInst>(&TI)) {
// Casts are folded by visitCastInst.
LatticeVal IBRValue = getValueState(IBR->getAddress());
BlockAddress *Addr = IBRValue.getBlockAddress();
if (!Addr) { // Overdefined or unknown condition?
// All destinations are executable!
if (!IBRValue.isUnknown())
Succs.assign(TI.getNumSuccessors(), true);
return;
}
BasicBlock* T = Addr->getBasicBlock();
assert(Addr->getFunction() == T->getParent() &&
"Block address of a different function ?");
for (unsigned i = 0; i < IBR->getNumSuccessors(); ++i) {
// This is the target.
if (IBR->getDestination(i) == T) {
Succs[i] = true;
return;
}
}
// If we didn't find our destination in the IBR successor list, then we
// have undefined behavior. Its ok to assume no successor is executable.
return;
}
@ -656,10 +686,18 @@ bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
return SI->findCaseValue(CI).getCaseSuccessor() == To;
}
// Just mark all destinations executable!
// TODO: This could be improved if the operand is a [cast of a] BlockAddress.
if (isa<IndirectBrInst>(TI))
return true;
// In case of indirect branch and its address is a blockaddress, we mark
// the target as executable.
if (auto *IBR = dyn_cast<IndirectBrInst>(TI)) {
LatticeVal IBRValue = getValueState(IBR->getAddress());
BlockAddress *Addr = IBRValue.getBlockAddress();
if (!Addr)
return !IBRValue.isUnknown();
// At this point, the indirectbr is branching on a blockaddress.
return Addr->getBasicBlock() == To;
}
DEBUG(dbgs() << "Unknown terminator instruction: " << *TI << '\n');
llvm_unreachable("SCCP: Don't know how to handle this terminator!");
@ -1484,6 +1522,31 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
return true;
}
if (auto *IBR = dyn_cast<IndirectBrInst>(TI)) {
// Indirect branch with no successor ?. Its ok to assume it branches
// to no target.
if (IBR->getNumSuccessors() < 1)
continue;
if (!getValueState(IBR->getAddress()).isUnknown())
continue;
// If the input to SCCP is actually branch on undef, fix the undef to
// the first successor of the indirect branch.
if (isa<UndefValue>(IBR->getAddress())) {
IBR->setAddress(BlockAddress::get(IBR->getSuccessor(0)));
markEdgeExecutable(&BB, IBR->getSuccessor(0));
return true;
}
// Otherwise, it is a branch on a symbolic value which is currently
// considered to be undef. Handle this by forcing the input value to the
// branch to the first successor.
markForcedConstant(IBR->getAddress(),
BlockAddress::get(IBR->getSuccessor(0)));
return true;
}
if (auto *SI = dyn_cast<SwitchInst>(TI)) {
if (!SI->getNumCases() || !getValueState(SI->getCondition()).isUnknown())
continue;

76
test/Transforms/SCCP/indirectbr.ll Normal file
View File

@ -0,0 +1,76 @@
; RUN: opt -S -sccp < %s | FileCheck %s
declare void @BB0_f()
declare void @BB1_f()
; Make sure we can eliminate what is in BB0 as we know that the indirectbr is going to BB1.
;
; CHECK-LABEL: define void @indbrtest1(
; CHECK-NOT: call void @BB0_f()
; CHECK: ret void
define void @indbrtest1() {
entry:
indirectbr i8* blockaddress(@indbrtest1, %BB1), [label %BB0, label %BB1]
BB0:
call void @BB0_f()
br label %BB1
BB1:
call void @BB1_f()
ret void
}
; Make sure we can eliminate what is in BB0 as we know that the indirectbr is going to BB1
; by looking through the casts. The casts should be folded away when they are visited
; before the indirectbr instruction.
;
; CHECK-LABEL: define void @indbrtest2(
; CHECK-NOT: call void @BB0_f()
; CHECK: ret void
define void @indbrtest2() {
entry:
%a = ptrtoint i8* blockaddress(@indbrtest2, %BB1) to i64
%b = inttoptr i64 %a to i8*
%c = bitcast i8* %b to i8*
indirectbr i8* %b, [label %BB0, label %BB1]
BB0:
call void @BB0_f()
br label %BB1
BB1:
call void @BB1_f()
ret void
}
; Make sure we can not eliminate BB0 as we do not know the target of the indirectbr.
;
; CHECK-LABEL: define void @indbrtest3(
; CHECK: call void @BB0_f()
; CHECK: ret void
define void @indbrtest3(i8** %Q) {
entry:
%t = load i8*, i8** %Q
indirectbr i8* %t, [label %BB0, label %BB1]
BB0:
call void @BB0_f()
br label %BB1
BB1:
call void @BB1_f()
ret void
}
; Make sure we eliminate BB1 as we pick the first successor on undef.
;
; CHECK-LABEL: define void @indbrtest4(
; CHECK: call void @BB0_f()
; CHECK: ret void
define void @indbrtest4(i8** %Q) {
entry:
indirectbr i8* undef, [label %BB0, label %BB1]
BB0:
call void @BB0_f()
br label %BB1
BB1:
call void @BB1_f()
ret void
}