#4223: LLVM slower then NCG, C example
-------------------------------+--------------------------------------------
Reporter: dterei | Owner: dterei
Type: bug | Status: new
Priority: normal | Component: Compiler (LLVM)
Version: 6.12.3 | Keywords:
Os: Linux | Testcase:
Architecture: x86_64 (amd64) | Failure: Runtime performance bug
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Comment(by dterei):
OK found the problem.
The LLVM opt tool (which is the llvm optimiser) actually makes the code
worse. Here are the timings again, (on a different 64 bit machine then
before [msrc64]):
{{{
* fasm: 1.993s
* viac: 1.965s
* llvm (O0): 1.935s
* llvm (O1): 2.358s
* llvm (O2): 2.366s
* llvm (O3): 2.366s
}}}
Will have to narrow down further which optimisation pass is causing the
problem. This also raises the question again though of what flags are a
good default to pass to the opt tool for Haskell code. Here we see it
significantly degrades the performance of the code. And for the nofib
benchmark suite it doesn't seem to have any effect. Yet Don found some
cases where it made significant differences!... what to do? Perhaps a good
default is actually to not use opt for now, since here it hinders the code
and for nofib there is no difference. Then users can manually choose
optimisation passes anyway for the specific code, which is what good C
programmers generally have to do anyway...
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4223#comment:2>
GHC <http://www.haskell.org/ghc/>
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