[llvm-dev] Where is opt spending its time?

[David Jones via llvm-dev]

I am trying to improve my application's compile-time performance.

On a given workload, I take 68 seconds to compile some code. If I disable
the LLVM code generation (i.e. I will generate IR instructions, but skip
the LLVM optimization and instruction selection steps) then my compile time
drops to 3 seconds.  If I write out the LLVM IR (just to prove that I am
generating it) then my compile time is 4 seconds. We're spending >90% of
the time in LLVM code generation.

To try to determine if there's anything I can do, I ran:

 time /tools/llvm/3.7.1dbg/bin/opt -O1 -filetype=obj -o opt.o my_ir.ll
-time-passes

and I get:

===-------------------------------------------------------------------------===
                      ... Pass execution timing report ...
===-------------------------------------------------------------------------===
  Total Execution Time: 19.1382 seconds (19.1587 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---
--- Name ---
   4.4755 ( 23.5%)   0.0000 (  0.0%)   4.4755 ( 23.4%)   4.4806 ( 23.4%)
Dead Store Elimination
   3.6255 ( 19.0%)   0.0000 (  0.0%)   3.6255 ( 18.9%)   3.6282 ( 18.9%)
Combine redundant instructions
   1.2138 (  6.4%)   0.0040 (  5.0%)   1.2178 (  6.4%)   1.2185 (  6.4%)
SROA
...
real    1m7.783s
user    1m7.548s
sys     0m0.183s

So: opt reports that it took 19 seconds, but overall, the run took 88
seconds. The system in question is a 6-core AMD K10 with 8GB of memory. The
system is not running anything else at the time.

What activity accounts for the unaccounted-for time?

For my application, IR verification has pathological performance (I ought
to file a bug on that), therefore I disable it. It is not clear if the IR
verifier is running in my opt runs. There is no line item for it.

It is also not clear if opt does instruction selection. I tried specifying
-filetype=null but that makes no difference to the run time.
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