[llvm] [SCEV] Fix a hang introduced by collectForPHI (PR #158153)
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Fri Sep 12 08:16:09 PDT 2025
https://github.com/preames updated https://github.com/llvm/llvm-project/pull/158153
>From c1c6c64ab46271b5b8182b7366d3d2c629c9798d Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Thu, 11 Sep 2025 14:03:27 -0700
Subject: [PATCH 1/4] [SCEV] Fix an compiler hang introduced by collectForPHI
If we have a phi where one of it's source blocks is an unreachable
block, we don't want to traverse back into the unreachable region.
Doing so allows e.g. finding a trivial self loop when walking back
the predecessor chain.
---
llvm/lib/Analysis/ScalarEvolution.cpp | 9 ++++++
...t-guard-info-with-multiple-predecessors.ll | 31 +++++++++++++++++++
2 files changed, 40 insertions(+)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 34e497d9ea3cb..8c48ffbf1a049 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15445,6 +15445,12 @@ void ScalarEvolution::LoopGuards::collectFromPHI(
const BasicBlock *InBlock = Phi.getIncomingBlock(IncomingIdx);
if (!VisitedBlocks.insert(InBlock).second)
return {nullptr, scCouldNotCompute};
+
+ // Avoid analyzing unreachable blocks so that we don't get trapped
+ // traversing cycles with ill-formed dominance or infinite cycles
+ if (!SE.DT.isReachableFromEntry(InBlock))
+ return {nullptr, scCouldNotCompute};
+
auto [G, Inserted] = IncomingGuards.try_emplace(InBlock, LoopGuards(SE));
if (Inserted)
collectFromBlock(SE, G->second, Phi.getParent(), InBlock, VisitedBlocks,
@@ -15499,6 +15505,9 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
const BasicBlock *Block, const BasicBlock *Pred,
SmallPtrSetImpl<const BasicBlock *> &VisitedBlocks, unsigned Depth) {
+
+ assert(SE.DT.isReachableFromEntry(Block) && SE.DT.isReachableFromEntry(Pred));
+
SmallVector<const SCEV *> ExprsToRewrite;
auto CollectCondition = [&](ICmpInst::Predicate Predicate, const SCEV *LHS,
const SCEV *RHS,
diff --git a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
index 28035b05303db..13762e3fa0474 100644
--- a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
+++ b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
@@ -364,3 +364,34 @@ body:
exit:
ret void
}
+
+define void @hang_due_to_unreachable_phi_inblock() personality ptr null {
+ br label %1
+
+1: ; preds = %1, %0
+ %2 = invoke ptr null(i64 0)
+ to label %1 unwind label %3
+
+3: ; preds = %1
+ %4 = landingpad { ptr, i32 }
+ cleanup
+ br label %7
+
+5: ; No predecessors!
+ %6 = landingpad { ptr, i32 }
+ cleanup
+ br label %7
+
+7: ; preds = %5, %3
+ %8 = phi ptr [ null, %5 ], [ null, %3 ]
+ br i1 false, label %13, label %9
+
+9: ; preds = %9, %7
+ %10 = phi ptr [ %11, %9 ], [ null, %7 ]
+ %11 = getelementptr i8, ptr %10, i64 24
+ %12 = icmp eq ptr %10, null
+ br i1 %12, label %13, label %9
+
+13: ; preds = %9, %7
+ resume { ptr, i32 } zeroinitializer
+}
>From a7133dd7a21bd6a1398175c038fd6c1173c2ed12 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Fri, 12 Sep 2025 08:05:38 -0700
Subject: [PATCH 2/4] Minimal change to actually hand on old opt
---
...dge-taken-count-guard-info-with-multiple-predecessors.ll | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
index 13762e3fa0474..75b1d9fe2242d 100644
--- a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
+++ b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
@@ -377,7 +377,11 @@ define void @hang_due_to_unreachable_phi_inblock() personality ptr null {
cleanup
br label %7
-5: ; No predecessors!
+self-loop: ; preds = %1, %0
+ %dead = invoke ptr null(i64 0)
+ to label %self-loop unwind label %5
+
+5: ; preds = %self-loop
%6 = landingpad { ptr, i32 }
cleanup
br label %7
>From 7dee654bf9b52c4b005600b4f35abc9959205152 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Fri, 12 Sep 2025 08:07:39 -0700
Subject: [PATCH 3/4] instnamer for hand reduce
---
...t-guard-info-with-multiple-predecessors.ll | 45 ++++++++++---------
1 file changed, 23 insertions(+), 22 deletions(-)
diff --git a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
index 75b1d9fe2242d..d71c32f1736e6 100644
--- a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
+++ b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
@@ -366,36 +366,37 @@ exit:
}
define void @hang_due_to_unreachable_phi_inblock() personality ptr null {
- br label %1
+bb:
+ br label %bb1
-1: ; preds = %1, %0
- %2 = invoke ptr null(i64 0)
- to label %1 unwind label %3
+bb1: ; preds = %bb1, %bb
+ %i = invoke ptr null()
+ to label %bb1 unwind label %bb2
-3: ; preds = %1
- %4 = landingpad { ptr, i32 }
+bb2: ; preds = %bb1
+ %i3 = landingpad { ptr, i32 }
cleanup
- br label %7
+ br label %bb6
-self-loop: ; preds = %1, %0
- %dead = invoke ptr null(i64 0)
- to label %self-loop unwind label %5
+self-loop: ; preds = %self-loop
+ %dead = invoke ptr null()
+ to label %self-loop unwind label %bb4
-5: ; preds = %self-loop
- %6 = landingpad { ptr, i32 }
+bb4: ; preds = %self-loop
+ %i5 = landingpad { ptr, i32 }
cleanup
- br label %7
+ br label %bb6
-7: ; preds = %5, %3
- %8 = phi ptr [ null, %5 ], [ null, %3 ]
- br i1 false, label %13, label %9
+bb6: ; preds = %bb4, %bb2
+ %i7 = phi ptr [ null, %bb4 ], [ null, %bb2 ]
+ br i1 false, label %bb12, label %bb8
-9: ; preds = %9, %7
- %10 = phi ptr [ %11, %9 ], [ null, %7 ]
- %11 = getelementptr i8, ptr %10, i64 24
- %12 = icmp eq ptr %10, null
- br i1 %12, label %13, label %9
+bb8: ; preds = %bb8, %bb6
+ %i9 = phi ptr [ %i10, %bb8 ], [ null, %bb6 ]
+ %i10 = getelementptr i8, ptr %i9, i64 24
+ %i11 = icmp eq ptr %i9, null
+ br i1 %i11, label %bb12, label %bb8
-13: ; preds = %9, %7
+bb12: ; preds = %bb8, %bb6
resume { ptr, i32 } zeroinitializer
}
>From 074ecdcc506a886df7ffec9bf9e6d59e78f1a260 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Fri, 12 Sep 2025 08:12:21 -0700
Subject: [PATCH 4/4] Further hand reduction
---
...t-guard-info-with-multiple-predecessors.ll | 20 +++++--------------
1 file changed, 5 insertions(+), 15 deletions(-)
diff --git a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
index d71c32f1736e6..564ce6b7d622f 100644
--- a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
+++ b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll
@@ -367,15 +367,6 @@ exit:
define void @hang_due_to_unreachable_phi_inblock() personality ptr null {
bb:
- br label %bb1
-
-bb1: ; preds = %bb1, %bb
- %i = invoke ptr null()
- to label %bb1 unwind label %bb2
-
-bb2: ; preds = %bb1
- %i3 = landingpad { ptr, i32 }
- cleanup
br label %bb6
self-loop: ; preds = %self-loop
@@ -387,16 +378,15 @@ bb4: ; preds = %self-loop
cleanup
br label %bb6
-bb6: ; preds = %bb4, %bb2
- %i7 = phi ptr [ null, %bb4 ], [ null, %bb2 ]
- br i1 false, label %bb12, label %bb8
+bb6: ; preds = %bb4, %bb
+ %i7 = phi ptr [ null, %bb4 ], [ null, %bb ]
+ br label %bb8
bb8: ; preds = %bb8, %bb6
- %i9 = phi ptr [ %i10, %bb8 ], [ null, %bb6 ]
- %i10 = getelementptr i8, ptr %i9, i64 24
+ %i9 = phi ptr [ null, %bb8 ], [ null, %bb6 ]
%i11 = icmp eq ptr %i9, null
br i1 %i11, label %bb12, label %bb8
bb12: ; preds = %bb8, %bb6
- resume { ptr, i32 } zeroinitializer
+ ret void
}
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