[llvm] [IVDescriptors] Reject FindLast candidates with loop invariant conditions (PR #184340)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Graham Hunter (huntergr-arm)

<details>
<summary>Changes</summary>

Fixes #<!-- -->182152


---
Full diff: https://github.com/llvm/llvm-project/pull/184340.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/IVDescriptors.cpp (+8-4) 
- (modified) llvm/test/Transforms/LoopVectorize/find-last.ll (+34) 


``````````diff
diff --git a/llvm/lib/Analysis/IVDescriptors.cpp b/llvm/lib/Analysis/IVDescriptors.cpp
index 814e32c2abf7b..26fdd4ab7695e 100644
--- a/llvm/lib/Analysis/IVDescriptors.cpp
+++ b/llvm/lib/Analysis/IVDescriptors.cpp
@@ -924,11 +924,15 @@ RecurrenceDescriptor::isFindPattern(Loop *TheLoop, PHINode *OrigPhi,
   //   select(cmp(), phi, value) or
   //   select(cmp(), value, phi)
   // TODO: Match selects with multi-use cmp conditions.
-  Value *NonRdxPhi = nullptr;
-  if (!match(I, m_CombineOr(m_Select(m_OneUse(m_Cmp()), m_Value(NonRdxPhi),
+  Value *Cmp = nullptr;
+  if (!match(I, m_CombineOr(m_Select(m_OneUse(m_Value(Cmp, m_Cmp())), m_Value(),
                                      m_Specific(OrigPhi)),
-                            m_Select(m_OneUse(m_Cmp()), m_Specific(OrigPhi),
-                                     m_Value(NonRdxPhi)))))
+                            m_Select(m_OneUse(m_Value(Cmp, m_Cmp())),
+                                     m_Specific(OrigPhi), m_Value()))))
+    return InstDesc(false, I);
+
+  // If the condition is loop invariant, bail out.
+  if (TheLoop->isLoopInvariant(Cmp))
     return InstDesc(false, I);
 
   return InstDesc(I, RecurKind::FindLast);
diff --git a/llvm/test/Transforms/LoopVectorize/find-last.ll b/llvm/test/Transforms/LoopVectorize/find-last.ll
index 59811eec8b298..d3aa74d9479a8 100644
--- a/llvm/test/Transforms/LoopVectorize/find-last.ll
+++ b/llvm/test/Transforms/LoopVectorize/find-last.ll
@@ -198,3 +198,37 @@ loop:
 exit:
   ret i32 %sel
 }
+
+;; Negative tests
+
+define i32 @loop_inv_select_condition_issue_182152(i32 %iv.start) {
+; CHECK-LABEL: @loop_inv_select_condition_issue_182152(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[INV_COND:%.*]] = icmp eq i32 0, 0
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[IV_START:%.*]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[RDX:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[SELECT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[SELECT]] = select i1 [[INV_COND]], i32 [[RDX]], i32 [[IV]]
+; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], 1
+; CHECK-NEXT:    [[EXIT_COND:%.*]] = icmp sgt i32 [[IV]], 0
+; CHECK-NEXT:    br i1 [[EXIT_COND]], label [[EXIT:%.*]], label [[LOOP]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[SELECT_LCSSA:%.*]] = phi i32 [ [[SELECT]], [[LOOP]] ]
+; CHECK-NEXT:    ret i32 [[SELECT_LCSSA]]
+;
+entry:
+  %inv_cond = icmp eq i32 0, 0
+  br label %loop
+
+loop:
+  %iv = phi i32 [ %iv.start, %entry ], [ %iv.next, %loop ]
+  %rdx = phi i32 [ 0, %entry ], [ %select, %loop ]
+  %select = select i1 %inv_cond, i32 %rdx, i32 %iv
+  %iv.next = add i32 %iv, 1
+  %exit.cond = icmp sgt i32 %iv, 0
+  br i1 %exit.cond, label %exit, label %loop
+
+exit:
+  ret i32 %select
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/184340