[LLVMdev] -gcolumn-info and PR 14106

Diego Novillo
Thu, 26 Jun 2014 16:46:12 -0000

For -Rpass, and other related uses, I am looking at enabling column info by
default. David pointed me at PR 14106, which seems to be the original
motivation for introducing -gcolumn-info. However, I am finding no
differences when using it on this test.  I've tried building with/without
-gcolumn-info and found almost no difference in compile time (+0.4%):

$ /usr/bin/time clang -w -fno-builtin -O2 -g -gcolumn-info test-tgmath2.i
474.38user 2.10system 7:58.00elapsed 99%CPU

$ /usr/bin/time clang -w -fno-builtin -O2 -g  test-tgmath2.i
472.63user 2.02system 7:56.11elapsed 99%CPU

I'm running clang from trunk @211693.

The size of all debug sections (according to readelf) are:

- with -g -gcolumn-info: 836,177 bytes
- with -g: 826,552 bytes

That's a growth of about 1% in debug info size.

These numbers are in line with a comparative build I did of our internal
codebase. The build included a massive number of C and C++ files. For C
files, total file size grows by 1% on average. For C++ files the average
growth is around 0.2%. Build times are unchanged as well.

Does anyone remember any other edge case I may want to try? It seems to me
that these differences are not really worth the effort of having a flag
controlling column information.


Thanks.  Diego.
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