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[GuardWidening] Improve analysis of potential widening into hotter block, try 2

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The initial version was reverted because it looped infinitely if the likely successor
isn't properly dominated by the predecessor. In practice it means that we went up the
CFG through backedge and looped infinitely.

I also added some paranoid assertion checks to make sure that every other invariant
holds. I also found a hypothetical situation when we may go past the dominated block
while following the likely successors (it means that in fact the dominated block is
dynamically not reachable from dominating block) and explicitly prohibited this, though
I don't have a motivating test showing that it's a real problem.

https://reviews.llvm.org/D146276
This commit is contained in:
Max Kazantsev 2023-03-22 14:29:12 +07:00
parent f809eb4db2
commit 7b83a1438f
2 changed files with 69 additions and 37 deletions
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66
llvm/lib/Transforms/Scalar/GuardWidening.cpp
View File

@ -460,27 +460,67 @@ GuardWideningImpl::computeWideningScore(Instruction *DominatedInstr,
if (HoistingOutOfLoop)
return WS_Positive;
// Returns true if we might be hoisting above explicit control flow. Note
// that this completely ignores implicit control flow (guards, calls which
// throw, etc...). That choice appears arbitrary.
auto MaybeHoistingOutOfIf = [&]() {
auto *DominatingBlock = DominatingGuard->getParent();
auto *DominatedBlock = DominatedInstr->getParent();
if (isGuardAsWidenableBranch(DominatingGuard))
DominatingBlock = cast<BranchInst>(DominatingGuard)->getSuccessor(0);
// For a given basic block \p BB, return its successor which is guaranteed or
// highly likely will be taken as its successor.
auto GetLikelySuccessor = [](const BasicBlock * BB)->const BasicBlock * {
if (auto *UniqueSucc = BB->getUniqueSuccessor())
return UniqueSucc;
auto *Term = BB->getTerminator();
Value *Cond = nullptr;
const BasicBlock *IfTrue = nullptr, *IfFalse = nullptr;
using namespace PatternMatch;
if (!match(Term, m_Br(m_Value(Cond), m_BasicBlock(IfTrue),
m_BasicBlock(IfFalse))))
return nullptr;
// For constant conditions, only one dynamical successor is possible
if (auto *ConstCond = dyn_cast<ConstantInt>(Cond))
return ConstCond->isAllOnesValue() ? IfTrue : IfFalse;
// If one of successors ends with deopt, another one is likely.
if (IfFalse->getPostdominatingDeoptimizeCall())
return IfTrue;
if (IfTrue->getPostdominatingDeoptimizeCall())
return IfFalse;
// TODO: Use branch frequency metatada to allow hoisting through non-deopt
// branches?
return nullptr;
};
// Same Block?
// Returns true if we might be hoisting above explicit control flow into a
// considerably hotter block. Note that this completely ignores implicit
// control flow (guards, calls which throw, etc...). That choice appears
// arbitrary (we assume that implicit control flow exits are all rare).
auto MaybeHoistingToHotterBlock = [&]() {
const auto *DominatingBlock = DominatingGuard->getParent();
const auto *DominatedBlock = DominatedInstr->getParent();
// Descend as low as we can, always taking the likely successor.
assert(DT.isReachableFromEntry(DominatingBlock) && "Unreached code");
assert(DT.isReachableFromEntry(DominatedBlock) && "Unreached code");
assert(DT.dominates(DominatingBlock, DominatedBlock) && "No dominance");
while (DominatedBlock != DominatingBlock) {
auto *LikelySucc = GetLikelySuccessor(DominatingBlock);
// No likely successor?
if (!LikelySucc)
break;
// Only go down the dominator tree.
if (!DT.properlyDominates(DominatingBlock, LikelySucc))
break;
DominatingBlock = LikelySucc;
}
// Found?
if (DominatedBlock == DominatingBlock)
return false;
// Obvious successor (common loop header/preheader case)
if (DominatedBlock == DominatingBlock->getUniqueSuccessor())
return false;
// We followed the likely successor chain and went past the dominated
// block. It means that the dominated guard is in dead/very cold code.
if (!DT.dominates(DominatingBlock, DominatedBlock))
return true;
// TODO: diamond, triangle cases
if (!PDT) return true;
return !PDT->dominates(DominatedBlock, DominatingBlock);
};
return MaybeHoistingOutOfIf() ? WS_IllegalOrNegative : WS_Neutral;
return MaybeHoistingToHotterBlock() ? WS_IllegalOrNegative : WS_Neutral;
}
bool GuardWideningImpl::canBeHoistedTo(

40
llvm/test/Transforms/GuardWidening/two_forms_behavior_consistency.ll
View File

@ -42,30 +42,26 @@ define void @test_01(i32 %a, i32 %b, i32 %c, i32 %d) {
; BRANCH_FORM-NEXT: entry:
; BRANCH_FORM-NEXT: br label [[LOOP:%.*]]
; BRANCH_FORM: loop:
; BRANCH_FORM-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[GUARDED5:%.*]] ]
; BRANCH_FORM-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[GUARDED:%.*]] ]
; BRANCH_FORM-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
; BRANCH_FORM-NEXT: [[C1:%.*]] = icmp ult i32 [[IV]], [[A]]
; BRANCH_FORM-NEXT: [[C2:%.*]] = icmp ult i32 [[IV]], [[B]]
; BRANCH_FORM-NEXT: [[WIDE_CHK:%.*]] = and i1 [[C1]], [[C2]]
; BRANCH_FORM-NEXT: [[C3:%.*]] = icmp ult i32 [[IV]], [[C]]
; BRANCH_FORM-NEXT: [[WIDE_CHK13:%.*]] = and i1 [[WIDE_CHK]], [[C3]]
; BRANCH_FORM-NEXT: [[C4:%.*]] = icmp ult i32 [[IV]], [[D]]
; BRANCH_FORM-NEXT: [[WIDE_CHK14:%.*]] = and i1 [[WIDE_CHK13]], [[C4]]
; BRANCH_FORM-NEXT: [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM-NEXT: [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[WIDE_CHK]], [[WIDENABLE_COND]]
; BRANCH_FORM-NEXT: br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof [[PROF0:![0-9]+]]
; BRANCH_FORM-NEXT: [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[WIDE_CHK14]], [[WIDENABLE_COND]]
; BRANCH_FORM-NEXT: br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED]], label [[DEOPT:%.*]], !prof [[PROF0:![0-9]+]]
; BRANCH_FORM: deopt:
; BRANCH_FORM-NEXT: call void (...) @llvm.experimental.deoptimize.isVoid() [ "deopt"() ]
; BRANCH_FORM-NEXT: ret void
; BRANCH_FORM: guarded:
; BRANCH_FORM-NEXT: [[WIDENABLE_COND3:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM-NEXT: [[EXIPLICIT_GUARD_COND4:%.*]] = and i1 [[C2]], [[WIDENABLE_COND3]]
; BRANCH_FORM-NEXT: [[C3:%.*]] = icmp ult i32 [[IV]], [[C]]
; BRANCH_FORM-NEXT: [[C4:%.*]] = icmp ult i32 [[IV]], [[D]]
; BRANCH_FORM-NEXT: [[WIDE_CHK13:%.*]] = and i1 [[C3]], [[C4]]
; BRANCH_FORM-NEXT: [[WIDENABLE_COND7:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM-NEXT: [[EXIPLICIT_GUARD_COND8:%.*]] = and i1 [[WIDE_CHK13]], [[WIDENABLE_COND7]]
; BRANCH_FORM-NEXT: br i1 [[EXIPLICIT_GUARD_COND8]], label [[GUARDED5]], label [[DEOPT6:%.*]], !prof [[PROF0]]
; BRANCH_FORM: deopt6:
; BRANCH_FORM-NEXT: call void (...) @llvm.experimental.deoptimize.isVoid() [ "deopt"() ]
; BRANCH_FORM-NEXT: ret void
; BRANCH_FORM: guarded5:
; BRANCH_FORM-NEXT: [[EXIPLICIT_GUARD_COND8:%.*]] = and i1 [[C3]], [[WIDENABLE_COND7]]
; BRANCH_FORM-NEXT: [[WIDENABLE_COND11:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM-NEXT: [[EXIPLICIT_GUARD_COND12:%.*]] = and i1 [[C4]], [[WIDENABLE_COND11]]
; BRANCH_FORM-NEXT: [[LOOP_COND:%.*]] = call i1 @cond()
@ -78,30 +74,26 @@ define void @test_01(i32 %a, i32 %b, i32 %c, i32 %d) {
; BRANCH_FORM_LICM-NEXT: entry:
; BRANCH_FORM_LICM-NEXT: br label [[LOOP:%.*]]
; BRANCH_FORM_LICM: loop:
; BRANCH_FORM_LICM-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[GUARDED5:%.*]] ]
; BRANCH_FORM_LICM-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[GUARDED:%.*]] ]
; BRANCH_FORM_LICM-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
; BRANCH_FORM_LICM-NEXT: [[C1:%.*]] = icmp ult i32 [[IV]], [[A]]
; BRANCH_FORM_LICM-NEXT: [[C2:%.*]] = icmp ult i32 [[IV]], [[B]]
; BRANCH_FORM_LICM-NEXT: [[WIDE_CHK:%.*]] = and i1 [[C1]], [[C2]]
; BRANCH_FORM_LICM-NEXT: [[C3:%.*]] = icmp ult i32 [[IV]], [[C]]
; BRANCH_FORM_LICM-NEXT: [[WIDE_CHK13:%.*]] = and i1 [[WIDE_CHK]], [[C3]]
; BRANCH_FORM_LICM-NEXT: [[C4:%.*]] = icmp ult i32 [[IV]], [[D]]
; BRANCH_FORM_LICM-NEXT: [[WIDE_CHK14:%.*]] = and i1 [[WIDE_CHK13]], [[C4]]
; BRANCH_FORM_LICM-NEXT: [[WIDENABLE_COND:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM_LICM-NEXT: [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[WIDE_CHK]], [[WIDENABLE_COND]]
; BRANCH_FORM_LICM-NEXT: br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED:%.*]], label [[DEOPT:%.*]], !prof [[PROF0:![0-9]+]]
; BRANCH_FORM_LICM-NEXT: [[EXIPLICIT_GUARD_COND:%.*]] = and i1 [[WIDE_CHK14]], [[WIDENABLE_COND]]
; BRANCH_FORM_LICM-NEXT: br i1 [[EXIPLICIT_GUARD_COND]], label [[GUARDED]], label [[DEOPT:%.*]], !prof [[PROF0:![0-9]+]]
; BRANCH_FORM_LICM: deopt:
; BRANCH_FORM_LICM-NEXT: call void (...) @llvm.experimental.deoptimize.isVoid() [ "deopt"() ]
; BRANCH_FORM_LICM-NEXT: ret void
; BRANCH_FORM_LICM: guarded:
; BRANCH_FORM_LICM-NEXT: [[WIDENABLE_COND3:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM_LICM-NEXT: [[EXIPLICIT_GUARD_COND4:%.*]] = and i1 [[C2]], [[WIDENABLE_COND3]]
; BRANCH_FORM_LICM-NEXT: [[C3:%.*]] = icmp ult i32 [[IV]], [[C]]
; BRANCH_FORM_LICM-NEXT: [[C4:%.*]] = icmp ult i32 [[IV]], [[D]]
; BRANCH_FORM_LICM-NEXT: [[WIDE_CHK13:%.*]] = and i1 [[C3]], [[C4]]
; BRANCH_FORM_LICM-NEXT: [[WIDENABLE_COND7:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM_LICM-NEXT: [[EXIPLICIT_GUARD_COND8:%.*]] = and i1 [[WIDE_CHK13]], [[WIDENABLE_COND7]]
; BRANCH_FORM_LICM-NEXT: br i1 [[EXIPLICIT_GUARD_COND8]], label [[GUARDED5]], label [[DEOPT6:%.*]], !prof [[PROF0]]
; BRANCH_FORM_LICM: deopt6:
; BRANCH_FORM_LICM-NEXT: call void (...) @llvm.experimental.deoptimize.isVoid() [ "deopt"() ]
; BRANCH_FORM_LICM-NEXT: ret void
; BRANCH_FORM_LICM: guarded5:
; BRANCH_FORM_LICM-NEXT: [[EXIPLICIT_GUARD_COND8:%.*]] = and i1 [[C3]], [[WIDENABLE_COND7]]
; BRANCH_FORM_LICM-NEXT: [[WIDENABLE_COND11:%.*]] = call i1 @llvm.experimental.widenable.condition()
; BRANCH_FORM_LICM-NEXT: [[EXIPLICIT_GUARD_COND12:%.*]] = and i1 [[C4]], [[WIDENABLE_COND11]]
; BRANCH_FORM_LICM-NEXT: [[LOOP_COND:%.*]] = call i1 @cond()