Re: gcc behavior on memory exhaustion (bug 81818)

[Andrew Roberts <andrewm dot roberts at sky dot com>]

I've updated this bug [Bug 81818 - aarch64 uses 2-3x memory and 2x time 
of arm at -Os, -O2, -O3] with more info.

It looks to be that on ARM systems with limited RAM optimizations are 
being skipped, but not on AARCH64.

Is there a way I can check this is true?

I checked all the optimizations enabled at -O2 which where not in -O1, 
and checked memory usage of each. -fgcse came out the winner, using huge 
amounts of memory.

Comparing -O1, -O1 -fgcse and -O2, on Rasberry Pi 3, and odroid-c2.

ARM (1Gb RAM):

gcc -O1 -c testmap.cpp
Time=2:25.26 Mem=318976 PageFaults=0

gcc -O1 -fgcse -c testmap.cpp
Time=2:42.04 Mem=554872 PageFaults=123

gcc -O2 -c testmap.cpp
Time=2:31.89 Mem=262828 PageFaults=11   <== Is this really running all 
-O2 opts?

AARCH64 (1Gb RAM):

gcc -O1 -c testmap.cpp
Time=3:10.68 Mem=397248 PageFaults=3

gcc -O1 -fgcse -c testmap.cpp
Time=5:15.26 Mem=771276 PageFaults=166

gcc -O2 -c testmap.cpp
Time=11:06.06 Mem=820744 PageFaults=4266  <== blows up big time unlike ARM

AARCH64 (2Gb RAM, odroid-c2, different clock speed etc):

gcc -O1 -c ../testmap.cpp
Time=1:47.58 Mem=394896 PageFaults=0

gcc -O1 -fgcse -c ../testmap.cpp
Time=3:10.06 Mem=765460 PageFaults=0

gcc -O2 -c ../testmap.cpp
Time=3:05.06 Mem=906624 PageFaults=0   <== ok if enough RAM though

Andrew