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[SCCP] Directly remove non-feasible edges 

Non-feasible control-flow edges are currently removed by replacing
the branch condition with a constant and then calling
ConstantFoldTerminator. This happens in a rather roundabout manner,
by inspecting the users (effectively: predecessors) of unreachable
blocks, and further complicated by the need to explicitly materialize
the condition for "forced" edges. I would like to extend SCCP to
discard switch conditions that are non-feasible based on range
information, but this is incompatible with the current approach
(as there is no single constant we could use.)

Instead, this patch explicitly removes non-feasible edges. It
currently only needs to handle the case where there is a single
feasible edge. The llvm_unreachable() branch will need to be
implemented for the aforementioned switch improvement.

Differential Revision: https://reviews.llvm.org/D84264
This commit is contained in:
Nikita Popov 2020-07-21 21:26:30 +02:00
parent d2b8fcff34
commit 9394c3ec88
7 changed files with 68 additions and 96 deletions
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116
llvm/lib/Transforms/Scalar/SCCP.cpp
View File

@ -276,7 +276,7 @@ public:
// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible.
bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
bool isEdgeFeasible(BasicBlock *From, BasicBlock *To) const;
std::vector<ValueLatticeElement> getStructLatticeValueFor(Value *V) const {
std::vector<ValueLatticeElement> StructValues;
@ -705,7 +705,7 @@ void SCCPSolver::getFeasibleSuccessors(Instruction &TI,
// isEdgeFeasible - Return true if the control flow edge from the 'From' basic
// block to the 'To' basic block is currently feasible.
bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) const {
// Check if we've called markEdgeExecutable on the edge yet. (We could
// be more aggressive and try to consider edges which haven't been marked
// yet, but there isn't any need.)
@ -1807,39 +1807,51 @@ static void findReturnsToZap(Function &F,
}
}
// Update the condition for terminators that are branching on indeterminate
// values, forcing them to use a specific edge.
static void forceIndeterminateEdge(Instruction* I, SCCPSolver &Solver) {
BasicBlock *Dest = nullptr;
Constant *C = nullptr;
if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
if (!isa<ConstantInt>(SI->getCondition())) {
// Indeterminate switch; use first case value.
Dest = SI->case_begin()->getCaseSuccessor();
C = SI->case_begin()->getCaseValue();
}
} else if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
if (!isa<ConstantInt>(BI->getCondition())) {
// Indeterminate branch; use false.
Dest = BI->getSuccessor(1);
C = ConstantInt::getFalse(BI->getContext());
}
} else if (IndirectBrInst *IBR = dyn_cast<IndirectBrInst>(I)) {
if (!isa<BlockAddress>(IBR->getAddress()->stripPointerCasts())) {
// Indeterminate indirectbr; use successor 0.
Dest = IBR->getSuccessor(0);
C = BlockAddress::get(IBR->getSuccessor(0));
}
} else {
llvm_unreachable("Unexpected terminator instruction");
static bool removeNonFeasibleEdges(const SCCPSolver &Solver, BasicBlock *BB,
DomTreeUpdater &DTU) {
SmallPtrSet<BasicBlock *, 8> FeasibleSuccessors;
bool HasNonFeasibleEdges = false;
for (BasicBlock *Succ : successors(BB)) {
if (Solver.isEdgeFeasible(BB, Succ))
FeasibleSuccessors.insert(Succ);
else
HasNonFeasibleEdges = true;
}
if (C) {
assert(Solver.isEdgeFeasible(I->getParent(), Dest) &&
"Didn't find feasible edge?");
(void)Dest;
I->setOperand(0, C);
// All edges feasible, nothing to do.
if (!HasNonFeasibleEdges)
return false;
// SCCP can only determine non-feasible edges for br, switch and indirectbr.
Instruction *TI = BB->getTerminator();
assert((isa<BranchInst>(TI) || isa<SwitchInst>(TI) ||
isa<IndirectBrInst>(TI)) &&
"Terminator must be a br, switch or indirectbr");
if (FeasibleSuccessors.size() == 1) {
// Replace with an unconditional branch to the only feasible successor.
BasicBlock *OnlyFeasibleSuccessor = *FeasibleSuccessors.begin();
SmallVector<DominatorTree::UpdateType, 8> Updates;
bool HaveSeenOnlyFeasibleSuccessor = false;
for (BasicBlock *Succ : successors(BB)) {
if (Succ == OnlyFeasibleSuccessor && !HaveSeenOnlyFeasibleSuccessor) {
// Don't remove the edge to the only feasible successor the first time
// we see it. We still do need to remove any multi-edges to it though.
HaveSeenOnlyFeasibleSuccessor = true;
continue;
}
Succ->removePredecessor(BB);
Updates.push_back({DominatorTree::Delete, BB, Succ});
}
DTU.applyUpdatesPermissive(Updates);
BranchInst::Create(OnlyFeasibleSuccessor, BB);
TI->eraseFromParent();
} else {
llvm_unreachable("Either all successors are feasible, or exactly one is");
}
return true;
}
bool llvm::runIPSCCP(
@ -1952,45 +1964,11 @@ bool llvm::runIPSCCP(
/*UseLLVMTrap=*/false,
/*PreserveLCSSA=*/false, &DTU);
// Now that all instructions in the function are constant folded,
// use ConstantFoldTerminator to get rid of in-edges, record DT updates and
// delete dead BBs.
for (BasicBlock *DeadBB : BlocksToErase) {
// If there are any PHI nodes in this successor, drop entries for BB now.
for (Value::user_iterator UI = DeadBB->user_begin(),
UE = DeadBB->user_end();
UI != UE;) {
// Grab the user and then increment the iterator early, as the user
// will be deleted. Step past all adjacent uses from the same user.
auto *I = dyn_cast<Instruction>(*UI);
do { ++UI; } while (UI != UE && *UI == I);
for (BasicBlock &BB : F)
removeNonFeasibleEdges(Solver, &BB, DTU);
// Ignore blockaddress users; BasicBlock's dtor will handle them.
if (!I) continue;
// If we have forced an edge for an indeterminate value, then force the
// terminator to fold to that edge.
forceIndeterminateEdge(I, Solver);
BasicBlock *InstBB = I->getParent();
bool Folded = ConstantFoldTerminator(InstBB,
/*DeleteDeadConditions=*/false,
/*TLI=*/nullptr, &DTU);
assert(Folded &&
"Expect TermInst on constantint or blockaddress to be folded");
(void) Folded;
// If we folded the terminator to an unconditional branch to another
// dead block, replace it with Unreachable, to avoid trying to fold that
// branch again.
BranchInst *BI = cast<BranchInst>(InstBB->getTerminator());
if (BI && BI->isUnconditional() &&
!Solver.isBlockExecutable(BI->getSuccessor(0))) {
InstBB->getTerminator()->eraseFromParent();
new UnreachableInst(InstBB->getContext(), InstBB);
}
}
// Mark dead BB for deletion.
for (BasicBlock *DeadBB : BlocksToErase)
DTU.deleteBB(DeadBB);
}
for (BasicBlock &BB : F) {
for (BasicBlock::iterator BI = BB.begin(), E = BB.end(); BI != E;) {

4
llvm/test/Transforms/SCCP/conditions-ranges.ll
View File

@ -231,12 +231,12 @@ define void @f7_nested_conds(i32* %a, i32 %b) {
; CHECK-NEXT: [[C_1:%.*]] = icmp ne i32 [[A_V]], 0
; CHECK-NEXT: br i1 [[C_1]], label [[TRUE:%.*]], label [[FALSE:%.*]]
; CHECK: false:
; CHECK-NEXT: br i1 true, label [[TRUE_2:%.*]], label [[TRUE]]
; CHECK-NEXT: br label [[TRUE_2:%.*]]
; CHECK: true.2:
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: ret void
; CHECK: true:
; CHECK-NEXT: store i32 [[B:%.*]], i32* [[A]]
; CHECK-NEXT: store i32 [[B:%.*]], i32* [[A]], align 4
; CHECK-NEXT: ret void
;
entry:

2
llvm/test/Transforms/SCCP/predicateinfo-cond.ll
View File

@ -105,7 +105,7 @@ define void @pr46814(i32 %a) {
; CHECK-NEXT: [[C3:%.*]] = and i1 [[C1]], [[C2]]
; CHECK-NEXT: br i1 [[C3]], label [[IF_1:%.*]], label [[EXIT:%.*]]
; CHECK: if.1:
; CHECK-NEXT: br i1 true, label [[IF_2:%.*]], label [[EXIT]]
; CHECK-NEXT: br label [[IF_2:%.*]]
; CHECK: if.2:
; CHECK-NEXT: br i1 true, label [[EXIT]], label [[EXIT]]
; CHECK: exit:

8
llvm/test/Transforms/SCCP/resolvedundefsin-tracked-fn.ll
View File

@ -136,13 +136,12 @@ define internal i1 @test2_g(%t1* %h, i32 %i) {
; CHECK-LABEL: define {{[^@]+}}@test2_g
; CHECK-SAME: (%t1* [[H:%.*]], i32 [[I:%.*]])
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 true, label [[LAND_RHS:%.*]], label [[LAND_END:%.*]]
; CHECK-NEXT: br label [[LAND_RHS:%.*]]
; CHECK: land.rhs:
; CHECK-NEXT: [[CALL:%.*]] = call i32 (...) @test2_j()
; CHECK-NEXT: [[TOBOOL1:%.*]] = icmp ne i32 [[CALL]], 0
; CHECK-NEXT: br label [[LAND_END]]
; CHECK-NEXT: br label [[LAND_END:%.*]]
; CHECK: land.end:
; CHECK-NEXT: [[TMP0:%.*]] = phi i1 [ false, [[ENTRY:%.*]] ], [ [[TOBOOL1]], [[LAND_RHS]] ]
; CHECK-NEXT: ret i1 undef
;
entry:
@ -196,10 +195,9 @@ define internal i32 @test3_k(i8 %h, i32 %i) {
; CHECK-NEXT: [[TMP1:%.*]] = inttoptr i64 [[CONV]] to %t1*
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[PHI:%.*]] = phi i1 [ undef, [[ENTRY:%.*]] ], [ false, [[LOOP]] ]
; CHECK-NEXT: [[CALL:%.*]] = call i1 @test3_g(%t1* [[TMP1]], i32 0)
; CHECK-NEXT: call void @use.1(i1 false)
; CHECK-NEXT: br i1 false, label [[LOOP]], label [[EXIT:%.*]]
; CHECK-NEXT: br label [[EXIT:%.*]]
; CHECK: exit:
; CHECK-NEXT: ret i32 undef
;

12
llvm/test/Transforms/SCCP/switch-constantfold-crash.ll
View File

@ -5,11 +5,11 @@
define void @barney() {
; CHECK-LABEL: @barney(
; CHECK-NEXT: bb:
; CHECK-NEXT: br label %bb9
; CHECK-NEXT: br label [[BB9:%.*]]
; CHECK: bb6:
; CHECK-NEXT: unreachable
; CHECK: bb9:
; CHECK-NEXT: unreachable
; CHECK-NEXT: br label [[BB6:%.*]]
;
bb:
br label %bb9
@ -29,9 +29,9 @@ bb9: ; preds = %bb
define void @blam() {
; CHECK-LABEL: @blam(
; CHECK-NEXT: bb:
; CHECK-NEXT: br label %bb16
; CHECK-NEXT: br label [[BB16:%.*]]
; CHECK: bb16:
; CHECK-NEXT: br label %bb38
; CHECK-NEXT: br label [[BB38:%.*]]
; CHECK: bb38:
; CHECK-NEXT: unreachable
;
@ -62,9 +62,9 @@ bb38: ; preds = %bb16
define void @hoge() {
; CHECK-LABEL: @hoge(
; CHECK-NEXT: bb:
; CHECK-NEXT: br label %bb2
; CHECK-NEXT: br label [[BB2:%.*]]
; CHECK: bb2:
; CHECK-NEXT: unreachable
; CHECK-NEXT: br label [[BB3:%.*]]
; CHECK: bb3:
; CHECK-NEXT: unreachable
;

8
llvm/test/Transforms/SCCP/switch.ll
View File

@ -23,15 +23,11 @@ define i32 @test_duplicate_successors_phi(i1 %c, i32 %x) {
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 [[C:%.*]], label [[SWITCH:%.*]], label [[END:%.*]]
; CHECK: switch:
; CHECK-NEXT: switch i32 -1, label [[SWITCH_DEFAULT:%.*]] [
; CHECK-NEXT: i32 0, label [[END]]
; CHECK-NEXT: i32 1, label [[END]]
; CHECK-NEXT: ]
; CHECK-NEXT: br label [[SWITCH_DEFAULT:%.*]]
; CHECK: switch.default:
; CHECK-NEXT: ret i32 -1
; CHECK: end:
; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ [[X:%.*]], [[ENTRY:%.*]] ], [ 1, [[SWITCH]] ], [ 1, [[SWITCH]] ]
; CHECK-NEXT: ret i32 [[PHI]]
; CHECK-NEXT: ret i32 [[X:%.*]]
;
entry:
br i1 %c, label %switch, label %end

14
llvm/test/Transforms/SCCP/widening.ll
View File

@ -216,11 +216,11 @@ define void @rotated_loop_2(i32 %x) {
; IPSCCP: bb3:
; IPSCCP-NEXT: br label [[EXIT]]
; IPSCCP: exit:
; IPSCCP-NEXT: [[P:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 3, [[BB1]] ], [ 2, [[BB2]] ], [ 5, [[BB3]] ], [ [[A:%.*]], [[EXIT]] ]
; IPSCCP-NEXT: [[A]] = add i32 [[P]], 1
; IPSCCP-NEXT: [[P:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 3, [[BB1]] ], [ 2, [[BB2]] ], [ 5, [[BB3]] ]
; IPSCCP-NEXT: [[A:%.*]] = add i32 [[P]], 1
; IPSCCP-NEXT: call void @use(i1 true)
; IPSCCP-NEXT: call void @use(i1 false)
; IPSCCP-NEXT: br i1 false, label [[EXIT]], label [[EXIT_1:%.*]]
; IPSCCP-NEXT: br label [[EXIT_1:%.*]]
; IPSCCP: exit.1:
; IPSCCP-NEXT: ret void
;
@ -451,10 +451,10 @@ define void @foo(i64* %arg) {
; SCCP-NEXT: [[TMP7:%.*]] = sub i64 3, [[TMP6]]
; SCCP-NEXT: [[TMP8:%.*]] = shl i64 [[TMP7]], 1
; SCCP-NEXT: [[TMP9:%.*]] = trunc i64 [[TMP8]] to i32
; SCCP-NEXT: [[TMP10:%.*]] = zext i32 [[TMP9]] to i64
; SCCP-NEXT: [[TMP0:%.*]] = zext i32 [[TMP9]] to i64
; SCCP-NEXT: br label [[BB11:%.*]]
; SCCP: bb11:
; SCCP-NEXT: [[TMP12:%.*]] = phi i64 [ [[TMP10]], [[BB4]] ], [ [[TMP17:%.*]], [[BB18:%.*]] ]
; SCCP-NEXT: [[TMP12:%.*]] = phi i64 [ [[TMP0]], [[BB4]] ], [ [[TMP17:%.*]], [[BB18:%.*]] ]
; SCCP-NEXT: br label [[BB13:%.*]]
; SCCP: bb13:
; SCCP-NEXT: [[C_1:%.*]] = icmp eq i64 [[TMP12]], 6
@ -489,10 +489,10 @@ define void @foo(i64* %arg) {
; IPSCCP-NEXT: [[TMP7:%.*]] = sub i64 3, [[TMP6]]
; IPSCCP-NEXT: [[TMP8:%.*]] = shl i64 [[TMP7]], 1
; IPSCCP-NEXT: [[TMP9:%.*]] = trunc i64 [[TMP8]] to i32
; IPSCCP-NEXT: [[TMP10:%.*]] = zext i32 [[TMP9]] to i64
; IPSCCP-NEXT: [[TMP0:%.*]] = zext i32 [[TMP9]] to i64
; IPSCCP-NEXT: br label [[BB11:%.*]]
; IPSCCP: bb11:
; IPSCCP-NEXT: [[TMP12:%.*]] = phi i64 [ [[TMP10]], [[BB4]] ], [ [[TMP17:%.*]], [[BB18:%.*]] ]
; IPSCCP-NEXT: [[TMP12:%.*]] = phi i64 [ [[TMP0]], [[BB4]] ], [ [[TMP17:%.*]], [[BB18:%.*]] ]
; IPSCCP-NEXT: br label [[BB13:%.*]]
; IPSCCP: bb13:
; IPSCCP-NEXT: [[C_1:%.*]] = icmp eq i64 [[TMP12]], 6