[llvm-branch-commits] [llvm] be3a066 - [SCEV] Improve handling of pointer compares involving subtractions.
Florian Hahn via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Sat Feb 20 08:25:32 PST 2021
Author: Florian Hahn
Date: 2021-02-20T16:25:18Z
New Revision: be3a066ebfe86371a159267e1c7287a86614e041
URL: https://github.com/llvm/llvm-project/commit/be3a066ebfe86371a159267e1c7287a86614e041
DIFF: https://github.com/llvm/llvm-project/commit/be3a066ebfe86371a159267e1c7287a86614e041.diff
LOG: [SCEV] Improve handling of pointer compares involving subtractions.
This patch improves handling of pointer comparisons involving
subtractions, by using knowledge that the involved variables are != 0
and signed positive.
It uses applyLoopGuards to add != 0 info to the involved expressions (by
re-writing them as umax expression. It also adds a new fold to move zext
into umax expressions and to move udiv expression inside subtraction, if
the operands guarantee a positive result.
Proof for zext/umax fold: https://alive2.llvm.org/ce/z/t75T53
Proof for getUDivExpr extension:a https://alive2.llvm.org/ce/z/H_G2Q0
Proof for isKnownPredicateSubIdiom: https://alive2.llvm.org/ce/z/Gfe8mS
Added:
Modified:
llvm/include/llvm/Analysis/ScalarEvolution.h
llvm/lib/Analysis/ScalarEvolution.cpp
llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
llvm/test/Transforms/IndVarSimplify/simplify-pointer-arithmetic.ll
Removed:
################################################################################
diff --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
index c35c1db7dfe0..1c9f9b36c94c 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolution.h
+++ b/llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -1876,6 +1876,11 @@ class ScalarEvolution {
bool isKnownPredicateViaConstantRanges(ICmpInst::Predicate Pred,
const SCEV *LHS, const SCEV *RHS);
+ /// Test if the given expression is known to satisfy the condition described
+ /// by Pred by decomposing a subtraction.
+ bool isKnownPredicateViaSubIdiom(ICmpInst::Predicate Pred, const SCEV *LHS,
+ const SCEV *RHS);
+
/// Try to prove the condition described by "LHS Pred RHS" by ruling out
/// integer overflow.
///
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index b8d55e6eb68a..6c82c5eece12 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -1833,6 +1833,12 @@ ScalarEvolution::getZeroExtendExpr(const SCEV *Op, Type *Ty, unsigned Depth) {
}
}
+ // zext (A umax B) --> (zext A) umax (zext B)
+ if (auto *SM = dyn_cast<SCEVUMaxExpr>(Op)) {
+ return getUMaxExpr(getZeroExtendExpr(SM->getOperand(0), Ty),
+ getZeroExtendExpr(SM->getOperand(1), Ty));
+ }
+
// The cast wasn't folded; create an explicit cast node.
// Recompute the insert position, as it may have been invalidated.
if (const SCEV *S = UniqueSCEVs.FindNodeOrInsertPos(ID, IP)) return S;
@@ -3104,6 +3110,34 @@ const SCEV *ScalarEvolution::getUDivExpr(const SCEV *LHS,
getEffectiveSCEVType(RHS->getType()) &&
"SCEVUDivExpr operand types don't match!");
+ // Try to shift udiv across an addition even when no wrapping info is
+ // present, using the fact that (B - A) will be in [0, B+1), if
+ // B s>= A and A s>= 0.
+ //
+ // (-C1 + X) /u C2 can be transformed to (C1 /u C3) + (X /u C2), if
+ // * C1 % C2 == 0
+ // * X % C3 == 0
+ // * X s>= C1
+ // * C1 s>= 0
+ //
+ // If C1 and C2 are constants, at least one of udiv expression can be
+ // eliminated. This pattern is commonly created for trip counts
+ // involving pointer IVs, where a multiple of the element width
+ // is subtracted.
+ const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(LHS);
+ const SCEVConstant *C = dyn_cast<SCEVConstant>(RHS);
+ if (Add && C && Add->getNumOperands() == 2) {
+ unsigned MultTrailing = C->getAPInt().countTrailingZeros();
+ auto *NegOp0 = getNegativeSCEV(Add->getOperand(0));
+ if (GetMinTrailingZeros(Add->getOperand(0)) >= MultTrailing &&
+ GetMinTrailingZeros(Add->getOperand(1)) >= MultTrailing &&
+ isKnownNonNegative(NegOp0) &&
+ isKnownPredicate(CmpInst::ICMP_SGE, Add->getOperand(1), NegOp0)) {
+ return getMinusSCEV(getUDivExactExpr(Add->getOperand(1), RHS),
+ getUDivExactExpr(NegOp0, RHS));
+ }
+ }
+
FoldingSetNodeID ID;
ID.AddInteger(scUDivExpr);
ID.AddPointer(LHS);
@@ -9113,7 +9147,6 @@ ScalarEvolution::howFarToZero(const SCEV *V, const Loop *L, bool ControlsExit,
// First compute the unsigned distance from zero in the direction of Step.
bool CountDown = StepC->getAPInt().isNegative();
const SCEV *Distance = CountDown ? Start : getNegativeSCEV(Start);
-
// Handle unitary steps, which cannot wraparound.
// 1*N = -Start; -1*N = Start (mod 2^BW), so:
// N = Distance (as unsigned)
@@ -10095,7 +10128,10 @@ bool ScalarEvolution::isLoopEntryGuardedByCond(const Loop *L,
"LHS is not available at Loop Entry");
assert(isAvailableAtLoopEntry(RHS, L) &&
"RHS is not available at Loop Entry");
- return isBasicBlockEntryGuardedByCond(L->getHeader(), Pred, LHS, RHS);
+ if (isBasicBlockEntryGuardedByCond(L->getHeader(), Pred, LHS, RHS))
+ return true;
+ return isBasicBlockEntryGuardedByCond(
+ L->getHeader(), Pred, applyLoopGuards(LHS, L), applyLoopGuards(RHS, L));
}
bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
@@ -10934,10 +10970,28 @@ static bool isKnownPredicateExtendIdiom(ICmpInst::Predicate Pred,
return false;
}
+bool ScalarEvolution::isKnownPredicateViaSubIdiom(ICmpInst::Predicate Pred,
+ const SCEV *LHS,
+ const SCEV *RHS) {
+ // Handle Y - X <= Y, if X s>= 0 and Y >= X. In that case, the result of (Y -
+ // X) will be in [0, Y+1) expression won't wrap in the unsigned sense.
+ auto *Add = dyn_cast<SCEVAddExpr>(LHS);
+ if (Add && Pred == CmpInst::ICMP_ULE) {
+ auto *X = Add->getOperand(0);
+ auto *Y = Add->getOperand(1);
+ if (Y == RHS && isKnownNonPositive(X) && isKnownNonNegative(Y) &&
+ isKnownPredicateViaConstantRanges(CmpInst::ICMP_UGE, Y,
+ getNegativeSCEV(X)))
+ return true;
+ }
+ return false;
+}
+
bool
ScalarEvolution::isKnownViaNonRecursiveReasoning(ICmpInst::Predicate Pred,
const SCEV *LHS, const SCEV *RHS) {
return isKnownPredicateExtendIdiom(Pred, LHS, RHS) ||
+ isKnownPredicateViaSubIdiom(Pred, LHS, RHS) ||
isKnownPredicateViaConstantRanges(Pred, LHS, RHS) ||
IsKnownPredicateViaMinOrMax(*this, Pred, LHS, RHS) ||
IsKnownPredicateViaAddRecStart(*this, Pred, LHS, RHS) ||
@@ -11223,12 +11277,13 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
return ExitLimit(getCouldNotCompute() /* ExactNotTaken */, MaxBECount,
false /*MaxOrZero*/, Predicates);
}
+
// If the backedge is taken at least once, then it will be taken
// (End-Start)/Stride times (rounded up to a multiple of Stride), where Start
// is the LHS value of the less-than comparison the first time it is evaluated
// and End is the RHS.
const SCEV *BECountIfBackedgeTaken =
- computeBECount(getMinusSCEV(End, Start), Stride, false);
+ computeBECount(getMinusSCEV(End, Start), Stride, false);
// If the loop entry is guarded by the result of the backedge test of the
// first loop iteration, then we know the backedge will be taken at least
// once and so the backedge taken count is as above. If not then we use the
diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index ae1fff0fa844..643605ff352c 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1521,8 +1521,8 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
// Can we prove that some other exit must be taken strictly before this
// one?
- if (SE->isLoopEntryGuardedByCond(L, CmpInst::ICMP_ULT,
- MaxExitCount, ExitCount)) {
+ if (SE->isLoopEntryGuardedByCond(L, CmpInst::ICMP_ULT, MaxExitCount,
+ ExitCount)) {
foldExit(L, ExitingBB, false, DeadInsts);
Changed = true;
continue;
diff --git a/llvm/test/Transforms/IndVarSimplify/simplify-pointer-arithmetic.ll b/llvm/test/Transforms/IndVarSimplify/simplify-pointer-arithmetic.ll
index 6ca5f43bb7d7..6560ca655ad7 100644
--- a/llvm/test/Transforms/IndVarSimplify/simplify-pointer-arithmetic.ll
+++ b/llvm/test/Transforms/IndVarSimplify/simplify-pointer-arithmetic.ll
@@ -20,10 +20,7 @@ define i1 @can_simplify_ult_i32_ptr_len_zext(i32* %p.base, i32 %len) {
; CHECK-NEXT: ret i1 false
; CHECK: header:
; CHECK-NEXT: [[P:%.*]] = phi i32* [ [[P_INC:%.*]], [[LATCH:%.*]] ], [ [[P_BASE]], [[HEADER_PREHEADER]] ]
-; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_INC:%.*]], [[LATCH]] ], [ 0, [[HEADER_PREHEADER]] ]
-; CHECK-NEXT: [[I_INC]] = add nuw nsw i64 [[I]], 1
-; CHECK-NEXT: [[I_ULT_EXT:%.*]] = icmp ult i64 [[I]], [[EXT]]
-; CHECK-NEXT: br i1 [[I_ULT_EXT]], label [[LATCH]], label [[TRAP_LOOPEXIT:%.*]]
+; CHECK-NEXT: br i1 true, label [[LATCH]], label [[TRAP_LOOPEXIT:%.*]]
; CHECK: latch:
; CHECK-NEXT: [[P_INC]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: [[C:%.*]] = icmp ne i32* [[P_INC]], [[P_END]]
@@ -179,10 +176,7 @@ define i1 @can_simplify_ule_i32_ptr_len_zext(i32* %p.base, i32 %len) {
; CHECK-NEXT: ret i1 false
; CHECK: header:
; CHECK-NEXT: [[P:%.*]] = phi i32* [ [[P_INC:%.*]], [[LATCH:%.*]] ], [ [[P_BASE]], [[HEADER_PREHEADER]] ]
-; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_INC:%.*]], [[LATCH]] ], [ 1, [[HEADER_PREHEADER]] ]
-; CHECK-NEXT: [[I_INC]] = add nuw nsw i64 [[I]], 1
-; CHECK-NEXT: [[I_ULT_EXT:%.*]] = icmp ule i64 [[I]], [[EXT]]
-; CHECK-NEXT: br i1 [[I_ULT_EXT]], label [[LATCH]], label [[TRAP_LOOPEXIT:%.*]]
+; CHECK-NEXT: br i1 true, label [[LATCH]], label [[TRAP_LOOPEXIT:%.*]]
; CHECK: latch:
; CHECK-NEXT: [[P_INC]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: [[C:%.*]] = icmp ne i32* [[P_INC]], [[P_END]]
@@ -234,10 +228,7 @@ define i1 @can_simplify_uge_i32_ptr_len_zext(i32* %p.base, i32 %len) {
; CHECK-NEXT: ret i1 false
; CHECK: header:
; CHECK-NEXT: [[P:%.*]] = phi i32* [ [[P_INC:%.*]], [[LATCH:%.*]] ], [ [[P_BASE]], [[HEADER_PREHEADER]] ]
-; CHECK-NEXT: [[I:%.*]] = phi i64 [ [[I_INC:%.*]], [[LATCH]] ], [ 0, [[HEADER_PREHEADER]] ]
-; CHECK-NEXT: [[I_INC]] = add nuw nsw i64 [[I]], 1
-; CHECK-NEXT: [[I_UGE_EXT:%.*]] = icmp uge i64 [[I]], [[EXT]]
-; CHECK-NEXT: br i1 [[I_UGE_EXT]], label [[TRAP_LOOPEXIT:%.*]], label [[LATCH]]
+; CHECK-NEXT: br i1 false, label [[TRAP_LOOPEXIT:%.*]], label [[LATCH]]
; CHECK: latch:
; CHECK-NEXT: [[P_INC]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: [[C:%.*]] = icmp ne i32* [[P_INC]], [[P_END]]
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