[PATCH] D73849: [ValueTracking][InstCombine] Fix infinite min/max canonicalization loop (PR44541)

[Nikita Popov via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
Closed by commit rGa148b9e9909d: [InstCombine] Fix infinite min/max canonicalization loop (PR44541) (authored by nikic).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D73849/new/

https://reviews.llvm.org/D73849

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/pr44541.ll


Index: llvm/test/Transforms/InstCombine/pr44541.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/pr44541.ll
@@ -0,0 +1,25 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -instcombine -expensive-combines=0 -instcombine-infinite-loop-threshold=3 < %s | FileCheck %s
+
+; This test used to cause an infinite combine loop.
+
+define i16 @passthru(i16 returned %x) {
+; CHECK-LABEL: @passthru(
+; CHECK-NEXT:    ret i16 [[X:%.*]]
+;
+  ret i16 %x
+}
+
+define i16 @test(i16 %arg) {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:    [[ZERO:%.*]] = call i16 @passthru(i16 0)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt i16 [[ARG:%.*]], 0
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[TMP1]], i16 [[ARG]], i16 0
+; CHECK-NEXT:    ret i16 [[RET]]
+;
+  %zero = call i16 @passthru(i16 0)
+  %sub = sub nuw nsw i16 %arg, %zero
+  %cmp = icmp slt i16 %sub, 0
+  %ret = select i1 %cmp, i16 0, i16 %sub
+  ret i16 %ret
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1013,6 +1013,12 @@
       Cmp.getPredicate() == CanonicalPred)
     return nullptr;
 
+  // Bail out on unsimplified X-0 operand (due to some worklist management bug),
+  // as this may cause an infinite combine loop. Let the sub be folded first.
+  if (match(LHS, m_Sub(m_Value(), m_Zero())) ||
+      match(RHS, m_Sub(m_Value(), m_Zero())))
+    return nullptr;
+
   // Create the canonical compare and plug it into the select.
   Sel.setCondition(Builder.CreateICmp(CanonicalPred, LHS, RHS));
 


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