[SCEV] Do not attempt to collect loop guards for loops without predecessor.

[juliannagele]

Attempting to collect loop guards for loops without a predecessor can lead to non-terminating recursion trying to construct a SCEV.

Fixes #122913.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: Julian Nagele (juliannagele)

Changes

Attempting to collect loop guards for loops without a predecessor can lead to non-terminating recursion trying to construct a SCEV.

Fixes #122913.

---

Full diff: https://github.com/llvm/llvm-project/pull/123662.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+2)
• (modified) llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll (+28)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 7673c354817579..210c7cab965edb 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15328,6 +15328,8 @@ ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
   BasicBlock *Header = L->getHeader();
   BasicBlock *Pred = L->getLoopPredecessor();
   LoopGuards Guards(SE);
+  if (!Pred)
+    return Guards;
   SmallPtrSet<const BasicBlock *, 8> VisitedBlocks;
   collectFromBlock(SE, Guards, Header, Pred, VisitedBlocks);
   return Guards;
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 46dccf454f21ac..6b100bfe5a8701 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
@@ -336,3 +336,31 @@ exit:
   ret void
 
 }
+
+; Checks correct traversal for loops without a unique predecessor
+; outside the loop.
+define void @pr122913() {
+; CHECK-LABEL: pr122913
+; CHECK-NEXT:  Determining loop execution counts for: @pr122913
+; CHECK-NEXT:  Loop %header: backedge-taken count is i1 false
+; CHECK-NEXT:  Loop %header: constant max backedge-taken count is i1 false
+; CHECK-NEXT:  Loop %header: symbolic max backedge-taken count is i1 false
+; CHECK-NEXT:  Loop %header: Trip multiple is 1
+entry:
+  br label %header
+
+bb:
+  br i1 1, label %exit, label %header
+
+header:
+  %0 = phi i32 [ %1, %body ], [ 0, %bb ], [ 0, %entry ]
+  br label %body
+
+body:
+  %1 = add i32 %0, 1
+  %2 = icmp ult i32 %1, 0
+  br i1 %2, label %header, label %exit
+
+exit:
+  ret void
+}

[nikic]

LG

[fhahn]

LGTM, thanks!

[fhahn]

Can the crash also be reproduced without one of the predecessors being unreachable?

Would be good to add a test case where bb is reachable if possible

[danilaml]

define void @pr122913() {
entry:
  br i1 1, label %bb, label %header

bb:
  br i1 1, label %exit, label %header

header:
  %0 = phi i32 [ %1, %body ], [ 0, %bb ], [ 0, %entry ]
  br label %body

body:
  %1 = add i32 %0, 1
  %2 = icmp ult i32 %1, 0
  br i1 %2, label %header, label %exit

exit:
  ret void
}

Seems to crash in the same way: https://godbolt.org/z/8rdhM11vh

[juliannagele]

Thanks! -- updated the test