Hi My name is Cong Hou, and I am a Noogler working in the compiler optimization team at Google.
When we were trying moving the vectorization from O3 to O2 in GCC 4.9, we met a bootstrap failure from comparison between stage 2&3. This failure is caused by a potential bug in GCC as stated below. In the file tree-vect-data-refs.c, there is a qsort() function call which sorts a group of data references using a comparison function called "dr_group_sort_cmp()". In this function, the iterative hash values of tree nodes are used for comparisons. For a declaration tree node, its UID participates in the calculation of the hash value. However, a specific declaration may have different UIDs whether the debug information is switched on/off. In consequence, the results of comparisons may vary in stage 2&3 during bootstrapping. As a solution, I think we need a function to compare two tree nodes that does not rely on UIDs. An apparent idea is comparing the tree code first, and if they have the same tree code, then compare their subnodes recursively. To resolve the issue we have now, I just composed a very basic comparison function which only compares tree codes, and this patch can make the bootstrap get passed. I have attached the patch here. I am wondering if this is a valid solution and appreciate if someone could give me any feedback. Thank you! Cong
patch.diff
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