[llvm-dev] Significant performance difference with a split call to opt
Sébastien Michelland via llvm-dev
llvm-dev at lists.llvm.org
Fri Jun 14 12:11:15 PDT 2019
This is a follow-up from a question I asked last month. I'm evaluating
the performance of two pass sequences that resemble (but are not) -O3.
With -O3, -debug-pass=Structure prints several independent blocks that
seem to represent several calls to opt. I focused on two of these
blocks, say S1 and S2, and compared the following optimization methods:
1. Executing them separately, ie. opt -S1 | opt -S2
2. Executing them in a single call, ie. opt -S1 -S2
I built the test suite with each of these configurations, then measured
the performance of the compiled programs with perf, over 10 runs.
I'm attaching a plot of the speedup of method 1 over method 2. The
intervals represent the standard deviation of the performance measures.
As you can see, programs compiled with method 1 are significantly slower
than their counterparts compiled with method 2. However, if passes were
applied in order using function composition, their performance should be
I'd like to know if there is a way to recover this property in the pass
manager, or at least explain the difference. If needed, I can provide
scripts to reproduce the measurements.
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