llvm dev - Nov 2018 - Transfer GVN-PRE pass to Machine IR (llvm bug 38917)

Hi all,

I'm working on fixing this bug (https://bugs.llvm.org/show_bug.cgi?id=38917,
"Failure to hoist repeated vextractf128 from switch cases") --
actually,
under-optimization. It's not related to special arch or AVX inst. set, it
will arise on many cases when llvm ir instr/intrinsic is splitting to
several machine instrs (like vextract128->VEXTRACTF128rr+VPEXTRDmr) on this
specific CFG (defined by switch-case structure). To hoist common
subexpession successfully on such CFG, one need to use method eliminating
partitial redundancy like GVN-PRE. I'm to implement this as separate MIR
pass (actually I'm to copy and adopt llvm ir GVN-PRE pass).

Meanwhile I want to consult llvm dev -- is it a good decision? May anyone
has pros and cons of it?

  Thanks,
   Anton
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