1 files changed, 53 insertions, 0 deletions
diff --git a/llvm/test/Transforms/CodeGenPrepare/bitreverse-hang.ll b/llvm/test/Transforms/CodeGenPrepare/bitreverse-hang.ll
new file mode 100644
index 00000000000..c81dcc15cae
--- /dev/null
+++ b/llvm/test/Transforms/CodeGenPrepare/bitreverse-hang.ll
@@ -0,0 +1,53 @@
+; RUN: opt < %s -loop-unroll -codegenprepare -S | FileCheck %s
+
+; This test is a worst-case scenario for bitreversal/byteswap detection.
+; After loop unrolling (the unrolled loop is unreadably large so it has been kept
+; rolled here), we have a binary tree of OR operands (as bitreversal detection
+; looks straight through shifts):
+;
+;  OR
+;  | \
+;  |  LSHR
+;  | /
+;  OR
+;  | \
+;  |  LSHR
+;  | /
+;  OR
+;
+; This results in exponential runtime. The loop here is 32 iterations which will
+; totally hang if we don't deal with this case cleverly.
+
+@b = common global i32 0, align 4
+
+; CHECK: define i32 @fn1
+define i32 @fn1() #0 {
+entry:
+  %b.promoted = load i32, i32* @b, align 4, !tbaa !2
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %or4 = phi i32 [ %b.promoted, %entry ], [ %or, %for.body ]
+  %i.03 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %shr = lshr i32 %or4, 1
+  %or = or i32 %shr, %or4
+  %inc = add nuw nsw i32 %i.03, 1
+  %exitcond = icmp eq i32 %inc, 32
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  store i32 %or, i32* @b, align 4, !tbaa !2
+  ret i32 undef
+}
+
+attributes #0 = { norecurse nounwind ssp uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"PIC Level", i32 2}
+!1 = !{!"clang version 3.8.0 (http://llvm.org/git/clang.git eb70f4e9cc9a4dc3dd57b032fb858d56b4b64a0e)"}
+!2 = !{!3, !3, i64 0}
+!3 = !{!"int", !4, i64 0}
+!4 = !{!"omnipotent char", !5, i64 0}
+!5 = !{!"Simple C/C++ TBAA"}