On Wednesday, 28 June 2017 at 16:52:26 UTC, Iain Buclaw wrote:
You probably want to tone down on optimizations as well. -O3
will be doing a lot of work, sometimes for little or no gain.
In most cases, -O2 -finline-functions is good enough, which can
be abbreviated further as simply -Os. [for full list of
enabled/disabled passes: gdc -Q -Os --help=optimizers]
You can see a breakdown of what areas the compiler spends the
most time in with -ftime-report
Compiling with -O3
------------------
phase opt and generate : 50.74 (96%) usr 21.24 (99%) sys
72.94 (97%) wall 2426962 kB (94%) ggc
TOTAL : 53.02 21.49
75.55 2589984 kB
real 1m21.339s
user 0m57.086s
sys 0m22.297s
arm-none-eabi-size binary/firmware
text data bss dec hex filename
6228 0 153600 159828 27054 binary/firmware
Compiling with -O2 -finline-functions
-------------------------------------
phase opt and generate : 50.71 (96%) usr 20.58 (98%) sys
72.04 (97%) wall 2381419 kB (94%) ggc
TOTAL : 52.89 20.93
74.63 2544441 kB
real 1m20.755s
user 0m56.857s
sys 0m21.826s
arm-none-eabi-size binary/firmware
text data bss dec hex filename
5912 0 153600 159512 26f18 binary/firmware
Compiling with -O0
------------------
phase opt and generate : 22.95 (91%) usr 5.42 (94%) sys
28.38 (92%) wall 1777106 kB (92%) ggc
TOTAL : 25.14 5.74
30.94 1940102 kB
real 0m36.476s
user 0m29.600s
sys 0m6.647s
arm-none-eabi-size binary/firmware
text data bss dec hex filename
45250 0 153600 198850 308c2 binary/firmware
-------------------------------------------------------------------------
The vast majority of time is spent in "phase opt and generate".
A few observations:
* Elapsed time isn't much different between -O3 and -O2
-finline-functions
* -O2 -finline-functions gave me a smaller binary :)
* -O0 reduced time significantly, but "phase opt and generate"
still takes an awfully long time relative to everything else
What exactly is "phase opt and generate"? I'm assuming "opt"
means optimizer, but why is it taking such a long time even with
-O0? Maybe it's the "generate" part of that that's the most
significant.
With -O0 there's still quite a few things enabled, so maybe I'll
start appending a "-fno" to each one and see if I can find a
culprit.
-O0 -Q --help=optimizers
-faggressive-loop-optimizations [enabled]
-fauto-inc-dec [enabled]
-fdce [enabled]
-fdelete-null-pointer-checks [enabled]
-fdse [enabled]
-fearly-inlining [enabled]
-ffp-contract=[off|on|fast] fast
-ffp-int-builtin-inexact [enabled]
-ffunction-cse [enabled]
-fgcse-lm [enabled]
-finline [enabled]
-finline-atomics [enabled]
-fira-hoist-pressure [enabled]
-fira-share-save-slots [enabled]
-fira-share-spill-slots [enabled]
-fivopts [enabled]
-fjump-tables [enabled]
-flifetime-dse [enabled]
-fmath-errno [enabled]
-fpeephole [enabled]
-fplt [enabled]
-fprefetch-loop-arrays [enabled]
-fprintf-return-value [enabled]
-freg-struct-return [enabled]
-frename-registers [enabled]
-frtti [enabled]
-fsched-critical-path-heuristic [enabled]
-fsched-dep-count-heuristic [enabled]
-fsched-group-heuristic [enabled]
-fsched-interblock [enabled]
-fsched-last-insn-heuristic [enabled]
-fsched-rank-heuristic [enabled]
-fsched-spec [enabled]
-fsched-spec-insn-heuristic [enabled]
-fsched-stalled-insns-dep [enabled]
-fschedule-fusion [enabled]
-fshort-enums [enabled]
-fshrink-wrap-separate [enabled]
-fsigned-zeros [enabled]
-fsimd-cost-model=[unlimited|dynamic|cheap] unlimited
-fsplit-ivs-in-unroller [enabled]
-fssa-backprop [enabled]
-fstack-reuse=[all|named_vars|none] all
-fstdarg-opt [enabled]
-fstrict-volatile-bitfields [enabled]
-fno-threadsafe-statics [enabled]
-ftrapping-math [enabled]
-ftree-cselim [enabled]
-ftree-forwprop [enabled]
-ftree-loop-if-convert [enabled]
-ftree-loop-im [enabled]
-ftree-loop-ivcanon [enabled]
-ftree-loop-optimize [enabled]
-ftree-phiprop [enabled]
-ftree-reassoc [enabled]
-ftree-scev-cprop [enabled]
-fvar-tracking [enabled]
-fvar-tracking-assignments [enabled]
-fweb [enabled]
