[PATCH] Increase min-lto-partition.

[Martin Liška]

Hi.

I played a bit with -flinker-output=nolto-rel of gimple-match.ii and I identified
that current default of min-lto-partition leads to too many LTRANS. We pay with
LTO overhead and so that user time is high.

Base is:
$ g++ -O2 /tmp/gimple-match.ii -c -fno-checking -o x.o
real	0m40.130s
user	0m39.911s

LGEN:

$ time g++ -O2 /tmp/gimple-match.ii -c -flto -fno-checking
real	0m8.709s
user	0m8.543s

WPA+LTRANS:

$ time gcc -flto=auto -flinker-output=nolto-rel gimple-match.o  -r -o gimple-match2.o --param lto-partitions=4  -fno-checking
real	0m11.220s
user	0m33.067s

$ time gcc -flto=auto -flinker-output=nolto-rel gimple-match.o  -r -o gimple-match2.o --param lto-partitions=6  -fno-checking
real	0m9.880s
user	0m35.599s

$ time gcc -flto=auto -flinker-output=nolto-rel gimple-match.o  -r -o gimple-match2.o --param lto-partitions=8  -fno-checking
real	0m6.681s
user	0m39.746s

default:
$ time gcc -flto=auto -flinker-output=nolto-rel gimple-match.o  -r -o gimple-match2.o -fno-checking
real	0m6.065s
user	1m22.698s

So I would recommend to set the param value to 75000, which leads to 6 partitions. That would be:

9+10s = 19s vs. 40s (total real time 44s). That seems reasonable to me.

Thoughts?
Thanks,
Martin

gcc/ChangeLog:

2020-03-13  Martin Liska  <mliska@suse.cz>

	* params.opt: Bump min-lto-partition in order to not create
	too many LTRANS.
---
  gcc/params.opt | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)


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