[llvm] [SCEV][LV] Invalidate LCSSA exit phis more thoroughly (PR #69909)
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Mon Oct 23 03:01:53 PDT 2023
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis
Author: Nikita Popov (nikic)
<details>
<summary>Changes</summary>
This an alternative to #<!-- -->69886. The basic problem is that SCEV can look through trivial LCSSA phis. When the phi node later becomes non-trivial, we do invalidate it, but this doesn't catch uses that are not covered by the IR use-def walk, such as those in BECounts.
Fix this by adding a special invalidation method for LCSSA phis, which will also invalidate all the SCEVUnknowns used by the LCSSA phi node.
We should probably also use this invalidation method in other places that add predecessors to exit blocks, such as loop unrolling and loop peeling.
---
Full diff: https://github.com/llvm/llvm-project/pull/69909.diff
4 Files Affected:
- (modified) llvm/include/llvm/Analysis/ScalarEvolution.h (+4)
- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+29)
- (modified) llvm/lib/Transforms/Vectorize/LoopVectorize.cpp (+1-1)
- (added) llvm/test/Transforms/LoopVectorize/pr66616.ll (+95)
``````````diff
diff --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
index 2765f1286d8bce5..ab2007abe6b3d5d 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolution.h
+++ b/llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -943,6 +943,10 @@ class ScalarEvolution {
/// def-use chain linking it to a loop.
void forgetValue(Value *V);
+ /// Forget LCSSA Phi node to which a new predecessor was added, such that
+ /// it may no longer be trivial.
+ void forgetLcssaPhiWithNewPredecessor(PHINode *V);
+
/// Called when the client has changed the disposition of values in
/// this loop.
///
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 4850a6aa5625d42..17edce0aff61758 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8408,6 +8408,35 @@ void ScalarEvolution::forgetValue(Value *V) {
forgetMemoizedResults(ToForget);
}
+void ScalarEvolution::forgetLcssaPhiWithNewPredecessor(PHINode *V) {
+ // If SCEV looked through a trivial LCSSA phi node, we might have SCEV's
+ // directly using a SCEVUnknown defined in the loop. After an extra
+ // predecessor is added, this is no longer valid. Find all SCEVUnknown's
+ // defined in the loop and invalidate any SCEV's making use of them.
+ if (isSCEVable(V->getType())) {
+ if (const SCEV *S = getExistingSCEV(V)) {
+ struct SCEVUnknownCollector {
+ SmallVector<const SCEV *, 8> Unknowns;
+
+ bool follow(const SCEV *S) {
+ // This could be restricted to unknowns defined in the loop.
+ if (isa<SCEVUnknown>(S))
+ Unknowns.push_back(S);
+ return true;
+ }
+ bool isDone() const { return false; }
+ };
+
+ SCEVUnknownCollector C;
+ visitAll(S, C);
+ forgetMemoizedResults(C.Unknowns);
+ }
+ }
+
+ // Also perform the normal invalidation.
+ forgetValue(V);
+}
+
void ScalarEvolution::forgetLoopDispositions() { LoopDispositions.clear(); }
void ScalarEvolution::forgetBlockAndLoopDispositions(Value *V) {
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 0807d2a7e5a2671..d17412f52981e0d 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -3552,7 +3552,7 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State,
OrigLoop->getExitBlocks(ExitBlocks);
for (BasicBlock *Exit : ExitBlocks)
for (PHINode &PN : Exit->phis())
- PSE.getSE()->forgetValue(&PN);
+ PSE.getSE()->forgetLcssaPhiWithNewPredecessor(&PN);
VPBasicBlock *LatchVPBB = Plan.getVectorLoopRegion()->getExitingBasicBlock();
Loop *VectorLoop = LI->getLoopFor(State.CFG.VPBB2IRBB[LatchVPBB]);
diff --git a/llvm/test/Transforms/LoopVectorize/pr66616.ll b/llvm/test/Transforms/LoopVectorize/pr66616.ll
new file mode 100644
index 000000000000000..2fb7f88e5341e5f
--- /dev/null
+++ b/llvm/test/Transforms/LoopVectorize/pr66616.ll
@@ -0,0 +1,95 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
+; RUN: opt -passes="print<scalar-evolution>,loop-vectorize" --verify-scev -S < %s -force-vector-width=4 2>/dev/null | FileCheck %s
+
+; Make sure users of SCEVUnknowns from the scalar loop are invalidated.
+
+define void @pr66616(ptr %ptr) {
+; CHECK-LABEL: define void @pr66616(
+; CHECK-SAME: ptr [[PTR:%.*]]) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
+; CHECK: vector.ph:
+; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
+; CHECK: vector.body:
+; CHECK-NEXT: [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT: [[TMP0:%.*]] = load i32, ptr [[PTR]], align 4
+; CHECK-NEXT: [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[TMP0]], i64 0
+; CHECK-NEXT: [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer
+; CHECK-NEXT: [[TMP1:%.*]] = add <4 x i32> [[BROADCAST_SPLAT]], <i32 1, i32 1, i32 1, i32 1>
+; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 4
+; CHECK-NEXT: [[TMP2:%.*]] = icmp eq i32 [[INDEX_NEXT]], 256
+; CHECK-NEXT: br i1 [[TMP2]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK: middle.block:
+; CHECK-NEXT: [[TMP3:%.*]] = extractelement <4 x i32> [[TMP1]], i32 3
+; CHECK-NEXT: br i1 true, label [[PREHEADER:%.*]], label [[SCALAR_PH]]
+; CHECK: scalar.ph:
+; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i8 [ 0, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT: br label [[LOOP_1:%.*]]
+; CHECK: loop.1:
+; CHECK-NEXT: [[IV_1:%.*]] = phi i8 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INC:%.*]], [[LOOP_1]] ]
+; CHECK-NEXT: [[LOAD:%.*]] = load i32, ptr [[PTR]], align 4
+; CHECK-NEXT: [[ADD3:%.*]] = add i32 [[LOAD]], 1
+; CHECK-NEXT: [[INC]] = add i8 [[IV_1]], 1
+; CHECK-NEXT: [[COND1:%.*]] = icmp eq i8 [[INC]], 0
+; CHECK-NEXT: br i1 [[COND1]], label [[PREHEADER]], label [[LOOP_1]], !llvm.loop [[LOOP3:![0-9]+]]
+; CHECK: preheader:
+; CHECK-NEXT: [[ADD3_LCSSA:%.*]] = phi i32 [ [[ADD3]], [[LOOP_1]] ], [ [[TMP3]], [[MIDDLE_BLOCK]] ]
+; CHECK-NEXT: [[TMP4:%.*]] = sub i32 0, [[ADD3_LCSSA]]
+; CHECK-NEXT: [[TMP5:%.*]] = zext i32 [[TMP4]] to i64
+; CHECK-NEXT: [[TMP6:%.*]] = add nuw nsw i64 [[TMP5]], 1
+; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[TMP6]], 4
+; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label [[SCALAR_PH2:%.*]], label [[VECTOR_PH3:%.*]]
+; CHECK: vector.ph3:
+; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[TMP6]], 4
+; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[TMP6]], [[N_MOD_VF]]
+; CHECK-NEXT: [[DOTCAST:%.*]] = trunc i64 [[N_VEC]] to i32
+; CHECK-NEXT: [[IND_END:%.*]] = add i32 [[ADD3_LCSSA]], [[DOTCAST]]
+; CHECK-NEXT: [[IND_END5:%.*]] = getelementptr i8, ptr [[PTR]], i64 [[N_VEC]]
+; CHECK-NEXT: br label [[VECTOR_BODY7:%.*]]
+; CHECK: vector.body7:
+; CHECK-NEXT: [[INDEX8:%.*]] = phi i64 [ 0, [[VECTOR_PH3]] ], [ [[INDEX_NEXT9:%.*]], [[VECTOR_BODY7]] ]
+; CHECK-NEXT: [[INDEX_NEXT9]] = add nuw i64 [[INDEX8]], 4
+; CHECK-NEXT: [[TMP7:%.*]] = icmp eq i64 [[INDEX_NEXT9]], [[N_VEC]]
+; CHECK-NEXT: br i1 [[TMP7]], label [[MIDDLE_BLOCK1:%.*]], label [[VECTOR_BODY7]], !llvm.loop [[LOOP4:![0-9]+]]
+; CHECK: middle.block1:
+; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[TMP6]], [[N_VEC]]
+; CHECK-NEXT: br i1 [[CMP_N]], label [[EXIT:%.*]], label [[SCALAR_PH2]]
+; CHECK: scalar.ph2:
+; CHECK-NEXT: [[BC_RESUME_VAL4:%.*]] = phi i32 [ [[IND_END]], [[MIDDLE_BLOCK1]] ], [ [[ADD3_LCSSA]], [[PREHEADER]] ]
+; CHECK-NEXT: [[BC_RESUME_VAL6:%.*]] = phi ptr [ [[IND_END5]], [[MIDDLE_BLOCK1]] ], [ [[PTR]], [[PREHEADER]] ]
+; CHECK-NEXT: br label [[LOOP_2:%.*]]
+; CHECK: loop.2:
+; CHECK-NEXT: [[IV_2:%.*]] = phi i32 [ [[IV_2_I:%.*]], [[LOOP_2]] ], [ [[BC_RESUME_VAL4]], [[SCALAR_PH2]] ]
+; CHECK-NEXT: [[IV_3:%.*]] = phi ptr [ [[IV_3_I:%.*]], [[LOOP_2]] ], [ [[BC_RESUME_VAL6]], [[SCALAR_PH2]] ]
+; CHECK-NEXT: [[IV_2_I]] = add i32 [[IV_2]], 1
+; CHECK-NEXT: [[IV_3_I]] = getelementptr i8, ptr [[IV_3]], i64 1
+; CHECK-NEXT: [[COND2:%.*]] = icmp eq i32 [[IV_2]], 0
+; CHECK-NEXT: br i1 [[COND2]], label [[EXIT]], label [[LOOP_2]], !llvm.loop [[LOOP5:![0-9]+]]
+; CHECK: exit:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %loop.1
+
+loop.1:
+ %iv.1 = phi i8 [ 0, %entry ], [ %inc, %loop.1 ]
+ %load = load i32, ptr %ptr, align 4
+ %add3 = add i32 %load, 1
+ %inc = add i8 %iv.1, 1
+ %cond1 = icmp eq i8 %inc, 0
+ br i1 %cond1, label %preheader, label %loop.1
+
+preheader:
+ br label %loop.2
+
+loop.2:
+ %iv.2 = phi i32 [ %iv.2.i, %loop.2 ], [ %add3, %preheader ]
+ %iv.3 = phi ptr [ %iv.3.i, %loop.2 ], [ %ptr, %preheader ]
+ %iv.2.i = add i32 %iv.2, 1
+ %iv.3.i = getelementptr i8, ptr %iv.3, i64 1
+ %cond2 = icmp eq i32 %iv.2, 0
+ br i1 %cond2, label %exit, label %loop.2
+
+exit:
+ ret void
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/69909
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