[llvm-dev] Any thoughts about preserving backend dependence?

Peizhao Ou via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 23 11:49:16 PST 2017

Hi folks,

This is Peizhao here, a graduate student from UC Irvine. My recent work
involves studying the performance of a dependence preserving compiler for
C/C++11. The motivation of doing this comes from Boehm and Demsky's paper
about outlawing OOTA behaviors for the C/C++11 memory model (

Basically I am trying to preserve a syntactic dependence of the original
program, and the way I am achieving this goal is to modify/disable IR->IR
passes and backend optimization passes too. For example, for the
instruction "%Y = mul i32 %X, 0", typically the IR pass "instcombine" would
optimize out this instruction and replace Y's usage with the value 0. Our
change to this pass would simply disable those dependence breaking
optimizations. Note that we also need to preserve the control dependence,
so we have also modified optimizations such as "simplifycfg", etc.

So far, I have finished modifying a select set of front-end passes (IR->IR)
and can generate a mostly optimized IR that preserves dependence. Our next
step is to modify the X86 backend optimizations. My current strategy for
the X86 backend is similar to that of the IR->IR passes, which is to pay
attention to those peephole optimizations and those that would break the
control dependence of a given IR.

Finally, I am wondering if you guys have comments on our approach to a
dependence-preserving backend. Or, is there anything you guys think I
should pay attention to in terms of dependence breaking (such as specific
passes)? From my understanding, but correct me if I am wrong, once you
generate the SelectionDAG, the X86 backend doesn't do too much to break the
control flow, so I could mostly focus on peephole optimizations within
basic blocks? Thanks, folks!:)

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