[PATCH] D154575: [X86] Prevent infinite loop in SelectionDAG when lowering negations

[Maurice Heumann via Phabricator via llvm-commits]

momo5502 added inline comments.


================
Comment at: llvm/test/CodeGen/X86/select-neg.ll:16
+  %.neg = add i32 %0, 1
+  ret i32 %.neg
+}
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RKSimon wrote:
> Can this not be reduced any further?
I'm afraid not. The ADD and the first XOR are needed for sure.
I think the second XOR and the PtrToInt is needed as that bug only gets triggered due to a reassociation step that switches the two XORs.

SelectionDAG loops between 3 states:
1. SUB -> ADD
2. XOR reassociation
3. ADD -> SUB

I was not able to trigger the infinte loop without ressociation step inbetween and I was not able to trigger that reassociation without the PtrToInt.

However, I extracted and reduced that bug from a 1.5k line LL file, so I was pretty happy with a 5 instruction sample :D


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154575/new/

https://reviews.llvm.org/D154575