[LLVMdev] Reducing impact of DSE (original) (raw)

Bruno Cardoso Lopes bruno.cardoso at gmail.com
Tue Feb 3 12:35:58 PST 2015

Hi David,

If you have really large basic blocks, you could be hitting pr22348, see this thread: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150112/253207.html

On Tue, Feb 3, 2015 at 6:17 PM, David Jones <djones at xtreme-eda.com> wrote:

I am writing a compiler using LLVM 3.2 as the back end.

Given a certain (IMHO pathological, but otherwise representative of the things I will be expected to compile) input, LLVM is spending a lot of time in dead store elimination: /tools/llvm/3.2na/bin/opt test.bc -O2 -time-passes -o opt.bc reports: 119s (71%) in DSE Bug 15000 reports a performance issue with DSE. The bug is marked fixed, so I decided to try using LLVM 3.5.1: /tools/llvm/3.5.1rel/bin/opt test.bc -O2 -time-passes -o opt.bc Total Execution Time: 151.3811 seconds (151.4667 wall clock) ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Name --- 98.8245 ( 65.3%) 0.0000 ( 0.0%) 98.8245 ( 65.3%) 98.9236 ( 65.3%) Dead Store Elimination I have no insight as to why DSE remains so poor. What constructs are particularly hard on DSE, which would result in what I'm seeing? Is there anything I can do? Is there an easy way for me to programmatically disable the DSE pass? NOTE: by "programmatically", I am looking for a way to disable DSE when using LLVM as a library. Passing command line options isn't a very clean solution in that case.

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-- Bruno Cardoso Lopes http://www.brunocardoso.cc

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