I profiled the compilation of my code with g++ -ftime-report to try to find a way to speed it up.
Here is the output :
Time variable usr sys wall GGC
phase setup : 0.00 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 1353 kB ( 0%)
phase parsing : 2.06 ( 5%) 1.13 ( 50%) 3.30 ( 8%) 565836 kB ( 30%)
phase lang. deferred : 0.30 ( 1%) 0.06 ( 3%) 0.36 ( 1%) 65727 kB ( 4%)
phase opt and generate : 37.96 ( 94%) 1.07 ( 47%) 39.03 ( 91%) 1224911 kB ( 66%)
|name lookup : 0.23 ( 1%) 0.06 ( 3%) 0.34 ( 1%) 18602 kB ( 1%)
|overload resolution : 0.36 ( 1%) 0.10 ( 4%) 0.41 ( 1%) 83103 kB ( 4%)
garbage collection : 0.42 ( 1%) 0.00 ( 0%) 0.43 ( 1%) 0 kB ( 0%)
dump files : 0.02 ( 0%) 0.01 ( 0%) 0.06 ( 0%) 0 kB ( 0%)
callgraph construction : 0.18 ( 0%) 0.01 ( 0%) 0.17 ( 0%) 12930 kB ( 1%)
callgraph optimization : 0.10 ( 0%) 0.01 ( 0%) 0.05 ( 0%) 371 kB ( 0%)
ipa function summary : 0.07 ( 0%) 0.00 ( 0%) 0.08 ( 0%) 1110 kB ( 0%)
ipa dead code removal : 0.03 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
ipa devirtualization : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 134 kB ( 0%)
ipa cp : 0.10 ( 0%) 0.01 ( 0%) 0.12 ( 0%) 8595 kB ( 0%)
ipa inlining heuristics : 3.18 ( 8%) 0.00 ( 0%) 3.20 ( 7%) 19108 kB ( 1%)
ipa function splitting : 0.21 ( 1%) 0.00 ( 0%) 0.19 ( 0%) 286 kB ( 0%)
ipa reference : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
ipa pure const : 0.05 ( 0%) 0.01 ( 0%) 0.06 ( 0%) 17 kB ( 0%)
ipa icf : 0.12 ( 0%) 0.00 ( 0%) 0.12 ( 0%) 1 kB ( 0%)
ipa SRA : 0.33 ( 1%) 0.03 ( 1%) 0.27 ( 1%) 23892 kB ( 1%)
ipa free inline summary : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
cfg construction : 0.04 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 2185 kB ( 0%)
cfg cleanup : 0.63 ( 2%) 0.02 ( 1%) 0.65 ( 2%) 4734 kB ( 0%)
trivially dead code : 0.16 ( 0%) 0.00 ( 0%) 0.09 ( 0%) 0 kB ( 0%)
df scan insns : 0.24 ( 1%) 0.01 ( 0%) 0.22 ( 1%) 8 kB ( 0%)
df multiple defs : 0.21 ( 1%) 0.01 ( 0%) 0.23 ( 1%) 0 kB ( 0%)
df reaching defs : 0.75 ( 2%) 0.00 ( 0%) 0.74 ( 2%) 0 kB ( 0%)
df live regs : 2.00 ( 5%) 0.00 ( 0%) 2.05 ( 5%) 0 kB ( 0%)
df live&initialized regs : 0.69 ( 2%) 0.00 ( 0%) 0.76 ( 2%) 0 kB ( 0%)
df must-initialized regs : 0.61 ( 2%) 0.24 ( 11%) 0.83 ( 2%) 0 kB ( 0%)
df use-def / def-use chains : 0.25 ( 1%) 0.00 ( 0%) 0.26 ( 1%) 0 kB ( 0%)
df reg dead/unused notes : 0.87 ( 2%) 0.00 ( 0%) 0.79 ( 2%) 14516 kB ( 1%)
register information : 0.10 ( 0%) 0.00 ( 0%) 0.15 ( 0%) 0 kB ( 0%)
alias analysis : 0.40 ( 1%) 0.00 ( 0%) 0.34 ( 1%) 28831 kB ( 2%)
alias stmt walking : 0.72 ( 2%) 0.07 ( 3%) 0.64 ( 1%) 5194 kB ( 0%)
register scan : 0.05 ( 0%) 0.00 ( 0%) 0.05 ( 0%) 217 kB ( 0%)
rebuild jump labels : 0.08 ( 0%) 0.00 ( 0%) 0.10 ( 0%) 0 kB ( 0%)
preprocessing : 0.60 ( 1%) 0.59 ( 26%) 1.38 ( 3%) 194467 kB ( 10%)
parser (global) : 0.22 ( 1%) 0.23 ( 10%) 0.38 ( 1%) 102668 kB ( 6%)
parser struct body : 0.27 ( 1%) 0.06 ( 3%) 0.35 ( 1%) 62614 kB ( 3%)
parser function body : 0.35 ( 1%) 0.09 ( 4%) 0.38 ( 1%) 70207 kB ( 4%)
parser inl. func. body : 0.06 ( 0%) 0.04 ( 2%) 0.07 ( 0%) 7795 kB ( 0%)
parser inl. meth. body : 0.16 ( 0%) 0.04 ( 2%) 0.22 ( 1%) 32985 kB ( 2%)
template instantiation : 0.64 ( 2%) 0.14 ( 6%) 0.78 ( 2%) 160006 kB ( 9%)
constant expression evaluation : 0.01 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 348 kB ( 0%)
early inlining heuristics : 0.12 ( 0%) 0.01 ( 0%) 0.09 ( 0%) 50683 kB ( 3%)
inline parameters : 0.16 ( 0%) 0.00 ( 0%) 0.16 ( 0%) 9128 kB ( 0%)
integration : 1.01 ( 3%) 0.13 ( 6%) 1.20 ( 3%) 272019 kB ( 15%)
tree gimplify : 0.09 ( 0%) 0.02 ( 1%) 0.10 ( 0%) 43912 kB ( 2%)
tree eh : 0.15 ( 0%) 0.00 ( 0%) 0.17 ( 0%) 49453 kB ( 3%)
tree CFG construction : 0.02 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 24163 kB ( 1%)
tree CFG cleanup : 0.77 ( 2%) 0.02 ( 1%) 0.86 ( 2%) 570 kB ( 0%)
tree tail merge : 0.10 ( 0%) 0.00 ( 0%) 0.09 ( 0%) 1409 kB ( 0%)
tree VRP : 0.68 ( 2%) 0.00 ( 0%) 0.76 ( 2%) 30167 kB ( 2%)
tree Early VRP : 0.08 ( 0%) 0.00 ( 0%) 0.08 ( 0%) 4515 kB ( 0%)
tree copy propagation : 0.19 ( 0%) 0.00 ( 0%) 0.20 ( 0%) 286 kB ( 0%)
tree PTA : 0.65 ( 2%) 0.00 ( 0%) 0.69 ( 2%) 5326 kB ( 0%)
tree PHI insertion : 0.01 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 5166 kB ( 0%)
tree SSA rewrite : 0.29 ( 1%) 0.02 ( 1%) 0.27 ( 1%) 28108 kB ( 2%)
tree SSA other : 0.04 ( 0%) 0.01 ( 0%) 0.05 ( 0%) 357 kB ( 0%)
tree SSA incremental : 0.38 ( 1%) 0.02 ( 1%) 0.39 ( 1%) 13003 kB ( 1%)
tree operand scan : 0.27 ( 1%) 0.05 ( 2%) 0.21 ( 0%) 41554 kB ( 2%)
dominator optimization : 0.62 ( 2%) 0.03 ( 1%) 0.70 ( 2%) 26865 kB ( 1%)
backwards jump threading : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 1082 kB ( 0%)
tree SRA : 0.07 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 757 kB ( 0%)
isolate eroneous paths : 0.02 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
tree CCP : 0.38 ( 1%) 0.01 ( 0%) 0.38 ( 1%) 6460 kB ( 0%)
tree split crit edges : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 6860 kB ( 0%)
tree reassociation : 0.07 ( 0%) 0.00 ( 0%) 0.06 ( 0%) 95 kB ( 0%)
tree PRE : 0.49 ( 1%) 0.07 ( 3%) 0.59 ( 1%) 29233 kB ( 2%)
tree FRE : 0.31 ( 1%) 0.06 ( 3%) 0.37 ( 1%) 5463 kB ( 0%)
tree code sinking : 0.05 ( 0%) 0.00 ( 0%) 0.05 ( 0%) 1998 kB ( 0%)
tree linearize phis : 0.06 ( 0%) 0.00 ( 0%) 0.06 ( 0%) 235 kB ( 0%)
tree backward propagate : 0.02 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
tree forward propagate : 0.15 ( 0%) 0.00 ( 0%) 0.19 ( 0%) 3598 kB ( 0%)
tree phiprop : 0.00 ( 0%) 0.01 ( 0%) 0.01 ( 0%) 162 kB ( 0%)
tree conservative DCE : 0.17 ( 0%) 0.04 ( 2%) 0.18 ( 0%) 121 kB ( 0%)
tree aggressive DCE : 0.09 ( 0%) 0.00 ( 0%) 0.18 ( 0%) 3761 kB ( 0%)
tree buildin call DCE : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 38 kB ( 0%)
tree DSE : 0.10 ( 0%) 0.01 ( 0%) 0.13 ( 0%) 2485 kB ( 0%)
PHI merge : 0.02 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 202 kB ( 0%)
complete unrolling : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 242 kB ( 0%)
tree slp vectorization : 0.14 ( 0%) 0.00 ( 0%) 0.16 ( 0%) 40876 kB ( 2%)
tree iv optimization : 0.02 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 484 kB ( 0%)
tree SSA uncprop : 0.02 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 0 kB ( 0%)
tree switch conversion : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
gimple CSE sin/cos : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 3 kB ( 0%)
gimple widening/fma detection : 0.04 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 0 kB ( 0%)
tree strlen optimization : 0.05 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 192 kB ( 0%)
dominance frontiers : 0.06 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 0 kB ( 0%)
dominance computation : 0.70 ( 2%) 0.00 ( 0%) 0.78 ( 2%) 0 kB ( 0%)
control dependences : 0.03 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
out of ssa : 0.09 ( 0%) 0.00 ( 0%) 0.09 ( 0%) 1047 kB ( 0%)
expand vars : 0.61 ( 2%) 0.00 ( 0%) 0.59 ( 1%) 11361 kB ( 1%)
expand : 0.24 ( 1%) 0.02 ( 1%) 0.27 ( 1%) 110705 kB ( 6%)
post expand cleanups : 0.11 ( 0%) 0.00 ( 0%) 0.09 ( 0%) 11138 kB ( 1%)
varconst : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 9 kB ( 0%)
forward prop : 0.28 ( 1%) 0.00 ( 0%) 0.32 ( 1%) 7463 kB ( 0%)
CSE : 0.47 ( 1%) 0.01 ( 0%) 0.50 ( 1%) 6406 kB ( 0%)
dead code elimination : 0.15 ( 0%) 0.00 ( 0%) 0.16 ( 0%) 0 kB ( 0%)
dead store elim1 : 0.25 ( 1%) 0.00 ( 0%) 0.25 ( 1%) 7807 kB ( 0%)
dead store elim2 : 0.15 ( 0%) 0.01 ( 0%) 0.13 ( 0%) 12268 kB ( 1%)
loop init : 0.30 ( 1%) 0.00 ( 0%) 0.32 ( 1%) 3678 kB ( 0%)
loop invariant motion : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 1 kB ( 0%)
loop fini : 0.01 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 0 kB ( 0%)
CPROP : 0.49 ( 1%) 0.00 ( 0%) 0.49 ( 1%) 12212 kB ( 1%)
PRE : 3.67 ( 9%) 0.04 ( 2%) 3.69 ( 9%) 17514 kB ( 1%)
CSE 2 : 0.28 ( 1%) 0.01 ( 0%) 0.29 ( 1%) 2791 kB ( 0%)
branch prediction : 0.13 ( 0%) 0.00 ( 0%) 0.13 ( 0%) 823 kB ( 0%)
combiner : 0.48 ( 1%) 0.00 ( 0%) 0.51 ( 1%) 19027 kB ( 1%)
if-conversion : 0.08 ( 0%) 0.00 ( 0%) 0.07 ( 0%) 3838 kB ( 0%)
integrated RA : 1.09 ( 3%) 0.00 ( 0%) 1.14 ( 3%) 72103 kB ( 4%)
LRA non-specific : 0.52 ( 1%) 0.01 ( 0%) 0.47 ( 1%) 3373 kB ( 0%)
LRA virtuals elimination : 0.14 ( 0%) 0.00 ( 0%) 0.11 ( 0%) 9546 kB ( 1%)
LRA reload inheritance : 0.04 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 822 kB ( 0%)
LRA create live ranges : 0.30 ( 1%) 0.00 ( 0%) 0.39 ( 1%) 1330 kB ( 0%)
LRA hard reg assignment : 0.01 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 0 kB ( 0%)
LRA rematerialization : 0.10 ( 0%) 0.00 ( 0%) 0.05 ( 0%) 0 kB ( 0%)
reload CSE regs : 0.49 ( 1%) 0.00 ( 0%) 0.53 ( 1%) 15987 kB ( 1%)
load CSE after reload : 2.73 ( 7%) 0.00 ( 0%) 2.72 ( 6%) 11924 kB ( 1%)
ree : 0.07 ( 0%) 0.00 ( 0%) 0.10 ( 0%) 74 kB ( 0%)
thread pro- & epilogue : 0.18 ( 0%) 0.00 ( 0%) 0.22 ( 1%) 446 kB ( 0%)
if-conversion 2 : 0.08 ( 0%) 0.00 ( 0%) 0.06 ( 0%) 23 kB ( 0%)
split paths : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 16 kB ( 0%)
combine stack adjustments : 0.01 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 0 kB ( 0%)
peephole 2 : 0.09 ( 0%) 0.00 ( 0%) 0.11 ( 0%) 878 kB ( 0%)
hard reg cprop : 0.19 ( 0%) 0.00 ( 0%) 0.15 ( 0%) 11 kB ( 0%)
scheduling 2 : 1.08 ( 3%) 0.02 ( 1%) 1.13 ( 3%) 8034 kB ( 0%)
machine dep reorg : 0.09 ( 0%) 0.00 ( 0%) 0.09 ( 0%) 2338 kB ( 0%)
reorder blocks : 0.33 ( 1%) 0.00 ( 0%) 0.31 ( 1%) 4597 kB ( 0%)
shorten branches : 0.09 ( 0%) 0.00 ( 0%) 0.10 ( 0%) 0 kB ( 0%)
final : 0.19 ( 0%) 0.00 ( 0%) 0.20 ( 0%) 21653 kB ( 1%)
tree if-combine : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 386 kB ( 0%)
straight-line strength reduction : 0.04 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 26 kB ( 0%)
store merging : 0.02 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 868 kB ( 0%)
initialize rtl : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 12 kB ( 0%)
address lowering : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
early local passes : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
rest of compilation : 0.59 ( 1%) 0.00 ( 0%) 0.55 ( 1%) 7917 kB ( 0%)
remove unused locals : 0.17 ( 0%) 0.00 ( 0%) 0.16 ( 0%) 416 kB ( 0%)
address taken : 0.11 ( 0%) 0.01 ( 0%) 0.14 ( 0%) 0 kB ( 0%)
rebuild frequencies : 0.05 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 138 kB ( 0%)
repair loop structures : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 0 kB ( 0%)
TOTAL : 40.32 2.26 42.69 1857845 kB
My problem is that I don't understand a thing about all the terms in this report (ipa cp, tree eh,...). I would like at least to understand what is the phase opt and generate stage because it takes 94% of the compile time so it's definitely what I should tackle.
In gcc documentation, there's almost no information about this command https://gcc.gnu.org/onlinedocs/gcc/Developer-Options.html#Developer-Options
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
It's a bit surprising to have a description that non exhaustive for a so complicated command.