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[SCEV] Prove implications for SCEVUnknown Phis 

This patch teaches SCEV how to prove implications for SCEVUnknown nodes that are Phis.
If we need to prove `Pred` for `LHS, RHS`, and `LHS` is a Phi with possible incoming values
`L1, L2, ..., LN`, then if we prove `Pred` for `(L1, RHS), (L2, RHS), ..., (LN, RHS)` then we can also
prove it for `(LHS, RHS)`. If both `LHS` and `RHS` are Phis from the same block, it is sufficient
to prove the predicate for values that come from the same predecessor block.

The typical case that it handles is that we sometimes need to prove that `Phi(Len, Len - 1) >= 0`
given that `Len > 0`. The new logic was added to `isImpliedViaOperations` and only uses it and
non-recursive reasoning to prove the facts we need, so it should not hurt compile time a lot.

Differential Revision: https://reviews.llvm.org/D44001
Reviewed By: anna

llvm-svn: 329150
This commit is contained in:
Max Kazantsev 2018-04-04 05:46:47 +00:00
parent c18fe4cf41
commit 613af1f7ca
4 changed files with 314 additions and 0 deletions
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15
llvm/include/llvm/Analysis/ScalarEvolution.h
View File

@ -1142,6 +1142,9 @@ private:
/// Mark SCEVUnknown Phis currently being processed by getRangeRef.
SmallPtrSet<const PHINode *, 6> PendingPhiRanges;
// Mark SCEVUnknown Phis currently being processed by isImpliedViaMerge.
SmallPtrSet<const PHINode *, 6> PendingMerges;
/// Set to true by isLoopBackedgeGuardedByCond when we're walking the set of
/// conditions dominating the backedge of a loop.
bool WalkingBEDominatingConds = false;
@ -1667,6 +1670,18 @@ private:
const SCEV *FoundLHS,
const SCEV *FoundRHS);
/// Test whether the condition described by Pred, LHS, and RHS is true
/// whenever the condition described by Pred, FoundLHS, and FoundRHS is
/// true.
///
/// This routine tries to figure out predicate for Phis which are SCEVUnknown
/// if it is true for every possible incoming value from their respective
/// basic blocks.
bool isImpliedViaMerge(ICmpInst::Predicate Pred,
const SCEV *LHS, const SCEV *RHS,
const SCEV *FoundLHS, const SCEV *FoundRHS,
unsigned Depth);
/// If we know that the specified Phi is in the header of its containing
/// loop, we know the loop executes a constant number of times, and the PHI
/// node is just a recurrence involving constants, fold it.

118
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -9518,6 +9518,115 @@ bool ScalarEvolution::isImpliedCondOperandsViaNoOverflow(
getConstant(FoundRHSLimit));
}
bool ScalarEvolution::isImpliedViaMerge(ICmpInst::Predicate Pred,
const SCEV *LHS, const SCEV *RHS,
const SCEV *FoundLHS,
const SCEV *FoundRHS, unsigned Depth) {
const PHINode *LPhi = nullptr, *RPhi = nullptr;
auto ClearOnExit = make_scope_exit([&]() {
if (LPhi) {
bool Erased = PendingMerges.erase(LPhi);
assert(Erased && "Failed to erase LPhi!");
(void)Erased;
}
if (RPhi) {
bool Erased = PendingMerges.erase(RPhi);
assert(Erased && "Failed to erase RPhi!");
(void)Erased;
}
});
// Find respective Phis and check that they are not being pending.
if (const SCEVUnknown *LU = dyn_cast<SCEVUnknown>(LHS))
if (auto *Phi = dyn_cast<PHINode>(LU->getValue())) {
if (!PendingMerges.insert(Phi).second)
return false;
LPhi = Phi;
}
if (const SCEVUnknown *RU = dyn_cast<SCEVUnknown>(RHS))
if (auto *Phi = dyn_cast<PHINode>(RU->getValue())) {
// If we detect a loop of Phi nodes being processed by this method, for
// example:
//
// %a = phi i32 [ %some1, %preheader ], [ %b, %latch ]
// %b = phi i32 [ %some2, %preheader ], [ %a, %latch ]
//
// we don't want to deal with a case that complex, so return conservative
// answer false.
if (!PendingMerges.insert(Phi).second)
return false;
RPhi = Phi;
}
// If none of LHS, RHS is a Phi, nothing to do here.
if (!LPhi && !RPhi)
return false;
// If there is a SCEVUnknown Phi we are interested in, make it left.
if (!LPhi) {
std::swap(LHS, RHS);
std::swap(FoundLHS, FoundRHS);
std::swap(LPhi, RPhi);
Pred = ICmpInst::getSwappedPredicate(Pred);
}
assert(LPhi && "LPhi should definitely be a SCEVUnknown Phi!");
const BasicBlock *LBB = LPhi->getParent();
const SCEVAddRecExpr *RAR = dyn_cast<SCEVAddRecExpr>(RHS);
auto ProvedEasily = [&](const SCEV *S1, const SCEV *S2) {
return isKnownViaNonRecursiveReasoning(Pred, S1, S2) ||
isImpliedCondOperandsViaRanges(Pred, S1, S2, FoundLHS, FoundRHS) ||
isImpliedViaOperations(Pred, S1, S2, FoundLHS, FoundRHS, Depth);
};
if (RPhi && RPhi->getParent() == LBB) {
// Case one: RHS is also a SCEVUnknown Phi from the same basic block.
// If we compare two Phis from the same block, and for each entry block
// the predicate is true for incoming values from this block, then the
// predicate is also true for the Phis.
for (const BasicBlock *IncBB : predecessors(LBB)) {
const SCEV *L = getSCEV(LPhi->getIncomingValueForBlock(IncBB));
const SCEV *R = getSCEV(RPhi->getIncomingValueForBlock(IncBB));
if (!ProvedEasily(L, R))
return false;
}
} else if (RAR && RAR->getLoop()->getHeader() == LBB) {
// Case two: RHS is also a Phi from the same basic block, and it is an
// AddRec. It means that there is a loop which has both AddRec and Unknown
// PHIs, for it we can compare incoming values of AddRec from preheader and
// latch with their respective incoming values of LPhi.
assert(LPhi->getNumIncomingValues() == 2 &&
"Phi node standing in loop header does not have exactly 2 inputs?");
auto *RLoop = RAR->getLoop();
auto *Preheader = RLoop->getLoopPreheader();
assert(Preheader && "Loop with AddRec with no preheader?");
const SCEV *L1 = getSCEV(LPhi->getIncomingValueForBlock(Preheader));
if (!ProvedEasily(L1, RAR->getStart()))
return false;
auto *Latch = RLoop->getLoopLatch();
assert(Latch && "Loop with AddRec with no latch?");
const SCEV *L2 = getSCEV(LPhi->getIncomingValueForBlock(Latch));
if (!ProvedEasily(L2, RAR->getPostIncExpr(*this)))
return false;
} else {
// In all other cases go over inputs of LHS and compare each of them to RHS,
// the predicate is true for (LHS, RHS) if it is true for all such pairs.
// At this point RHS is either a non-Phi, or it is a Phi from some block
// different from LBB.
for (const BasicBlock *IncBB : predecessors(LBB)) {
// Check that RHS is available in this block.
if (!dominates(RHS, IncBB))
return false;
const SCEV *L = getSCEV(LPhi->getIncomingValueForBlock(IncBB));
if (!ProvedEasily(L, RHS))
return false;
}
}
return true;
}
bool ScalarEvolution::isImpliedCondOperands(ICmpInst::Predicate Pred,
const SCEV *LHS, const SCEV *RHS,
const SCEV *FoundLHS,
@ -9671,6 +9780,7 @@ bool ScalarEvolution::isImpliedViaOperations(ICmpInst::Predicate Pred,
};
// Acquire values from extensions.
auto *OrigLHS = LHS;
auto *OrigFoundLHS = FoundLHS;
LHS = GetOpFromSExt(LHS);
FoundLHS = GetOpFromSExt(FoundLHS);
@ -9778,6 +9888,12 @@ bool ScalarEvolution::isImpliedViaOperations(ICmpInst::Predicate Pred,
}
}
// If our expression contained SCEVUnknown Phis, and we split it down and now
// need to prove something for them, try to prove the predicate for every
// possible incoming values of those Phis.
if (isImpliedViaMerge(Pred, OrigLHS, RHS, OrigFoundLHS, FoundRHS, Depth + 1))
return true;
return false;
}
@ -10863,6 +10979,7 @@ ScalarEvolution::ScalarEvolution(ScalarEvolution &&Arg)
ValueExprMap(std::move(Arg.ValueExprMap)),
PendingLoopPredicates(std::move(Arg.PendingLoopPredicates)),
PendingPhiRanges(std::move(Arg.PendingPhiRanges)),
PendingMerges(std::move(Arg.PendingMerges)),
MinTrailingZerosCache(std::move(Arg.MinTrailingZerosCache)),
BackedgeTakenCounts(std::move(Arg.BackedgeTakenCounts)),
PredicatedBackedgeTakenCounts(
@ -10907,6 +11024,7 @@ ScalarEvolution::~ScalarEvolution() {
assert(PendingLoopPredicates.empty() && "isImpliedCond garbage");
assert(PendingPhiRanges.empty() && "getRangeRef garbage");
assert(PendingMerges.empty() && "isImpliedViaMerge garbage");
assert(!WalkingBEDominatingConds && "isLoopBackedgeGuardedByCond garbage!");
assert(!ProvingSplitPredicate && "ProvingSplitPredicate garbage!");
}

143
llvm/test/Transforms/IRCE/decrementing-loop.ll
View File

@ -119,5 +119,148 @@ define void @test_02(i32* %a, i32* %b, i32* %a_len_ptr, i32* %b_len_ptr) {
ret void
}
; Check that we can figure out that IV is non-negative via implication through
; Phi node.
define void @test_03(i32* %a, i32* %a_len_ptr, i1 %cond) {
; CHECK-LABEL: test_03
; CHECK: mainloop:
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK: br i1 true, label %in.bounds, label %out.of.bounds
; CHECK: loop.preloop:
entry:
%len.a = load i32, i32* %a_len_ptr, !range !0
%len.minus.one = sub nsw i32 %len.a, 1
%len.minus.two = sub nsw i32 %len.a, 2
br i1 %cond, label %if.true, label %if.false
if.true:
br label %merge
if.false:
br label %merge
merge:
%starting.value = phi i32 [ %len.minus.two, %if.true ], [ %len.minus.one, %if.false ]
%first.itr.check = icmp sgt i32 %len.a, 3
br i1 %first.itr.check, label %loop, label %exit
loop:
%idx = phi i32 [ %starting.value, %merge ] , [ %idx.next, %in.bounds ]
%idx.next = sub i32 %idx, 1
%rc = icmp ult i32 %idx.next, %len.a
br i1 %rc, label %in.bounds, label %out.of.bounds, !prof !1
in.bounds:
%el.a = getelementptr i32, i32* %a, i32 %idx.next
%v = load i32, i32* %el.a
%loop.cond = icmp slt i32 %idx, 2
br i1 %loop.cond, label %exit, label %loop
out.of.bounds:
ret void
exit:
ret void
}
; Check that we can figure out that IV is non-negative via implication through
; two Phi nodes.
define void @test_04(i32* %a, i32* %a_len_ptr, i1 %cond) {
; CHECK-LABEL: test_04
; CHECK: mainloop:
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK: br i1 true, label %in.bounds, label %out.of.bounds
; CHECK: loop.preloop:
entry:
%len.a = load i32, i32* %a_len_ptr, !range !0
%len.minus.one = sub nsw i32 %len.a, 1
%len.plus.one = add nsw i32 %len.a, 1
%len.minus.two = sub nsw i32 %len.a, 2
br i1 %cond, label %if.true, label %if.false
if.true:
br label %merge
if.false:
br label %merge
merge:
%starting.value = phi i32 [ %len.minus.two, %if.true ], [ %len.minus.one, %if.false ]
%len.phi = phi i32 [ %len.a, %if.true ], [ %len.plus.one, %if.false ]
%first.itr.check = icmp sgt i32 %len.a, 3
br i1 %first.itr.check, label %loop, label %exit
loop:
%idx = phi i32 [ %starting.value, %merge ] , [ %idx.next, %in.bounds ]
%idx.next = sub i32 %idx, 1
%rc = icmp ult i32 %idx.next, %len.phi
br i1 %rc, label %in.bounds, label %out.of.bounds, !prof !1
in.bounds:
%el.a = getelementptr i32, i32* %a, i32 %idx.next
%v = load i32, i32* %el.a
%loop.cond = icmp slt i32 %idx, 2
br i1 %loop.cond, label %exit, label %loop
out.of.bounds:
ret void
exit:
ret void
}
; Check that we can figure out that IV is non-negative via implication through
; two Phi nodes, one being AddRec.
define void @test_05(i32* %a, i32* %a_len_ptr, i1 %cond) {
; CHECK-LABEL: test_05
; CHECK: mainloop:
; CHECK-NEXT: br label %loop
; CHECK: loop:
; CHECK: br i1 true, label %in.bounds, label %out.of.bounds
; CHECK: loop.preloop:
entry:
%len.a = load i32, i32* %a_len_ptr, !range !0
%len.minus.one = sub nsw i32 %len.a, 1
%len.plus.one = add nsw i32 %len.a, 1
%len.minus.two = sub nsw i32 %len.a, 2
br label %merge
merge:
%starting.value = phi i32 [ %len.minus.two, %entry ], [ %len.minus.one, %merge ]
%len.phi = phi i32 [ %len.a, %entry ], [ %len.phi.next, %merge ]
%len.phi.next = add nsw i32 %len.phi, 1
br i1 true, label %first.iter.check, label %merge
first.iter.check:
%first.itr.check = icmp sgt i32 %len.a, 3
br i1 %first.itr.check, label %loop, label %exit
loop:
%idx = phi i32 [ %starting.value, %first.iter.check ] , [ %idx.next, %in.bounds ]
%idx.next = sub i32 %idx, 1
%rc = icmp ult i32 %idx.next, %len.phi
br i1 %rc, label %in.bounds, label %out.of.bounds, !prof !1
in.bounds:
%el.a = getelementptr i32, i32* %a, i32 %idx.next
%v = load i32, i32* %el.a
%loop.cond = icmp slt i32 %idx, 2
br i1 %loop.cond, label %exit, label %loop
out.of.bounds:
ret void
exit:
ret void
}
!0 = !{i32 0, i32 2147483647}
!1 = !{!"branch_weights", i32 64, i32 4}

38
llvm/test/Transforms/IndVarSimplify/promote-iv-to-eliminate-casts.ll
View File

@ -188,5 +188,43 @@ loop:
br i1 %loopcond, label %loopexit, label %loop
}
define void @promote_latch_condition_decrementing_loop_04(i32* %p, i32* %a, i1 %cond) {
; CHECK-LABEL: @promote_latch_condition_decrementing_loop_04(
; CHECK-NOT: trunc
entry:
%len = load i32, i32* %p, align 4, !range !0
%len.minus.1 = add nsw i32 %len, -1
br i1 %cond, label %if.true, label %if.false
if.true:
br label %merge
if.false:
br label %merge
merge:
%iv_start = phi i32 [ %len, %if.true ], [%len.minus.1, %if.false ]
%zero_check = icmp eq i32 %len, 0
br i1 %zero_check, label %loopexit, label %preheader
preheader:
br label %loop
loopexit:
ret void
loop:
%iv = phi i32 [ %iv.next, %loop ], [ %iv_start, %preheader ]
; CHECK: %indvars.iv = phi i64
%iv.wide = zext i32 %iv to i64
%el = getelementptr inbounds i32, i32* %a, i64 %iv.wide
store atomic i32 0, i32* %el unordered, align 4
%iv.next = add nsw i32 %iv, -1
; CHECK: %loopcond = icmp slt i64 %indvars.iv, 1
%loopcond = icmp slt i32 %iv, 1
br i1 %loopcond, label %loopexit, label %loop
}
!0 = !{i32 0, i32 2147483647}