[llvm] f5f421e - [SCEV] Apply loop guards in reverse order.
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Thu Dec 16 02:53:16 PST 2021
Author: Florian Hahn
Date: 2021-12-16T10:52:37Z
New Revision: f5f421e0eefa492545c3848e318c21ed04cb1ddd
URL: https://github.com/llvm/llvm-project/commit/f5f421e0eefa492545c3848e318c21ed04cb1ddd
DIFF: https://github.com/llvm/llvm-project/commit/f5f421e0eefa492545c3848e318c21ed04cb1ddd.diff
LOG: [SCEV] Apply loop guards in reverse order.
This patch updates applyLoopGuards to first collect all conditions and
then applies them in reverse order. This ensures the SCEVs with the
shortest dependency chains are constructed first, limiting the required
stack size.
This fixes a crash reported in D113578.
Note that the order conditions are applied can impact the accuracy of
the result, mostly due to missing min/max simplifications when
constructing SCEVs.
The changed test highlights the impact of the evaluation order. I will
follow up with a SCEV patch to improve min/max simplifications to get
the same results for both orders.
Added:
Modified:
llvm/lib/Analysis/ScalarEvolution.cpp
llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 8ea448e34dac..0c3f32295ae1 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13959,11 +13959,12 @@ const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
ExprsToRewrite.push_back(LHS);
}
};
- // Starting at the loop predecessor, climb up the predecessor chain, as long
- // as there are predecessors that can be found that have unique successors
- // leading to the original header.
+ // First, collect conditions from dominating branches. Starting at the loop
+ // predecessor, climb up the predecessor chain, as long as there are
+ // predecessors that can be found that have unique successors leading to the
+ // original header.
// TODO: share this logic with isLoopEntryGuardedByCond.
- DenseMap<const SCEV *, const SCEV *> RewriteMap;
+ SmallVector<std::pair<Value *, bool>> Terms;
for (std::pair<const BasicBlock *, const BasicBlock *> Pair(
L->getLoopPredecessor(), L->getHeader());
Pair.first; Pair = getPredecessorWithUniqueSuccessorForBB(Pair.first)) {
@@ -13973,10 +13974,20 @@ const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
if (!LoopEntryPredicate || LoopEntryPredicate->isUnconditional())
continue;
- bool EnterIfTrue = LoopEntryPredicate->getSuccessor(0) == Pair.second;
+ Terms.emplace_back(LoopEntryPredicate->getCondition(),
+ LoopEntryPredicate->getSuccessor(0) == Pair.second);
+ }
+
+ // Now apply the information from the collected conditions to RewriteMap.
+ // Conditions are processed in reverse order, so the earliest conditions is
+ // processed first. This ensures the SCEVs with the shortest dependency chains
+ // are constructed first.
+ DenseMap<const SCEV *, const SCEV *> RewriteMap;
+ for (auto &E : reverse(Terms)) {
+ bool EnterIfTrue = E.second;
SmallVector<Value *, 8> Worklist;
SmallPtrSet<Value *, 8> Visited;
- Worklist.push_back(LoopEntryPredicate->getCondition());
+ Worklist.push_back(E.first);
while (!Worklist.empty()) {
Value *Cond = Worklist.pop_back_val();
if (!Visited.insert(Cond).second)
diff --git a/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll b/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
index 9289d38cb6f8..1a9dba0511cd 100644
--- a/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
+++ b/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
@@ -1480,12 +1480,12 @@ define i32 @sle_sgt_ult_umax_to_smax(i32 %num) {
; CHECK-LABEL: 'sle_sgt_ult_umax_to_smax'
; CHECK-NEXT: Classifying expressions for: @sle_sgt_ult_umax_to_smax
; CHECK-NEXT: %iv = phi i32 [ 0, %guard.3 ], [ %iv.next, %loop ]
-; CHECK-NEXT: --> {0,+,4}<nuw><%loop> U: [0,-3) S: [-2147483648,2147483645) Exits: (4 * ((-4 + %num) /u 4))<nuw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT: --> {0,+,4}<nuw><%loop> U: [0,25) S: [0,25) Exits: (4 * ((-4 + %num) /u 4))<nuw> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %iv.next = add nuw i32 %iv, 4
-; CHECK-NEXT: --> {4,+,4}<nuw><%loop> U: [4,-3) S: [-2147483648,2147483645) Exits: (4 + (4 * ((-4 + %num) /u 4))<nuw>) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT: --> {4,+,4}<nuw><%loop> U: [4,29) S: [4,29) Exits: (4 + (4 * ((-4 + %num) /u 4))<nuw>) LoopDispositions: { %loop: Computable }
; CHECK-NEXT: Determining loop execution counts for: @sle_sgt_ult_umax_to_smax
; CHECK-NEXT: Loop %loop: backedge-taken count is ((-4 + %num) /u 4)
-; CHECK-NEXT: Loop %loop: max backedge-taken count is 1073741823
+; CHECK-NEXT: Loop %loop: max backedge-taken count is 6
; CHECK-NEXT: Loop %loop: Predicated backedge-taken count is ((-4 + %num) /u 4)
; CHECK-NEXT: Predicates:
; CHECK: Loop %loop: Trip multiple is 1
@@ -1517,12 +1517,12 @@ define i32 @ult_sle_sgt_umax_to_smax(i32 %num) {
; CHECK-LABEL: 'ult_sle_sgt_umax_to_smax'
; CHECK-NEXT: Classifying expressions for: @ult_sle_sgt_umax_to_smax
; CHECK-NEXT: %iv = phi i32 [ 0, %guard.3 ], [ %iv.next, %loop ]
-; CHECK-NEXT: --> {0,+,4}<nuw><%loop> U: [0,25) S: [0,25) Exits: (4 * ((-4 + %num) /u 4))<nuw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT: --> {0,+,4}<nuw><%loop> U: [0,-3) S: [-2147483648,2147483645) Exits: (4 * ((-4 + %num) /u 4))<nuw> LoopDispositions: { %loop: Computable }
; CHECK-NEXT: %iv.next = add nuw i32 %iv, 4
-; CHECK-NEXT: --> {4,+,4}<nuw><%loop> U: [4,29) S: [4,29) Exits: (4 + (4 * ((-4 + %num) /u 4))<nuw>) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT: --> {4,+,4}<nuw><%loop> U: [4,-3) S: [-2147483648,2147483645) Exits: (4 + (4 * ((-4 + %num) /u 4))<nuw>) LoopDispositions: { %loop: Computable }
; CHECK-NEXT: Determining loop execution counts for: @ult_sle_sgt_umax_to_smax
; CHECK-NEXT: Loop %loop: backedge-taken count is ((-4 + %num) /u 4)
-; CHECK-NEXT: Loop %loop: max backedge-taken count is 6
+; CHECK-NEXT: Loop %loop: max backedge-taken count is 1073741823
; CHECK-NEXT: Loop %loop: Predicated backedge-taken count is ((-4 + %num) /u 4)
; CHECK-NEXT: Predicates:
; CHECK: Loop %loop: Trip multiple is 1
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