I am profiling the compilation time of my code to determine why the compile time is so slow. I am using gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0 and have added the compiler flag -ftime-report.
What I notice is that the compilation units that are slow to compile spend a majority of time on the phase opt and generate stage. What exactly is this stage? How can I reduce the time taken by this phase.
For reference, this is what the output for one of the compilation units looks like.
Time variable usr sys wall GGC
phase setup : 0.00 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 1579 kB ( 0%)
phase parsing : 1.74 ( 20%) 0.71 ( 44%) 2.46 ( 24%) 311927 kB ( 36%)
phase lang. deferred : 1.33 ( 15%) 0.34 ( 21%) 1.67 ( 16%) 259524 kB ( 30%)
phase opt and generate : 5.68 ( 65%) 0.58 ( 36%) 6.26 ( 60%) 301021 kB ( 34%)
phase last asm : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 2 kB ( 0%)
|name lookup : 0.44 ( 5%) 0.12 ( 7%) 0.49 ( 5%) 15499 kB ( 2%)
|overload resolution : 0.76 ( 9%) 0.22 ( 13%) 0.92 ( 9%) 130607 kB ( 15%)
garbage collection : 0.33 ( 4%) 0.01 ( 1%) 0.34 ( 3%) 0 kB ( 0%)
dump files : 0.18 ( 2%) 0.04 ( 2%) 0.10 ( 1%) 0 kB ( 0%)
callgraph construction : 0.12 ( 1%) 0.03 ( 2%) 0.14 ( 1%) 6318 kB ( 1%)
callgraph optimization : 0.16 ( 2%) 0.04 ( 2%) 0.19 ( 2%) 82 kB ( 0%)
ipa function summary : 0.02 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 2289 kB ( 0%)
ipa dead code removal : 0.01 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 0 kB ( 0%)
ipa inheritance graph : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 29 kB ( 0%)
ipa virtual call target : 0.02 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 3 kB ( 0%)
ipa cp : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 1140 kB ( 0%)
ipa inlining heuristics : 0.04 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 2438 kB ( 0%)
ipa function splitting : 0.00 ( 0%) 0.01 ( 1%) 0.01 ( 0%) 451 kB ( 0%)
ipa profile : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
ipa pure const : 0.02 ( 0%) 0.00 ( 0%) 0.05 ( 0%) 40 kB ( 0%)
ipa icf : 0.01 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 4 kB ( 0%)
ipa SRA : 0.10 ( 1%) 0.00 ( 0%) 0.05 ( 0%) 9838 kB ( 1%)
cfg cleanup : 0.08 ( 1%) 0.01 ( 1%) 0.08 ( 1%) 1621 kB ( 0%)
trivially dead code : 0.03 ( 0%) 0.00 ( 0%) 0.06 ( 1%) 0 kB ( 0%)
df scan insns : 0.02 ( 0%) 0.01 ( 1%) 0.05 ( 0%) 18 kB ( 0%)
df multiple defs : 0.02 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 0 kB ( 0%)
df reaching defs : 0.06 ( 1%) 0.00 ( 0%) 0.04 ( 0%) 0 kB ( 0%)
df live regs : 0.19 ( 2%) 0.01 ( 1%) 0.25 ( 2%) 0 kB ( 0%)
df live&initialized regs : 0.05 ( 1%) 0.00 ( 0%) 0.06 ( 1%) 0 kB ( 0%)
df use-def / def-use chains : 0.03 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
df reg dead/unused notes : 0.08 ( 1%) 0.00 ( 0%) 0.07 ( 1%) 2152 kB ( 0%)
register information : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
alias analysis : 0.03 ( 0%) 0.00 ( 0%) 0.09 ( 1%) 5413 kB ( 1%)
alias stmt walking : 0.08 ( 1%) 0.00 ( 0%) 0.13 ( 1%) 738 kB ( 0%)
register scan : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 167 kB ( 0%)
rebuild jump labels : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
preprocessing : 0.15 ( 2%) 0.21 ( 13%) 0.39 ( 4%) 11918 kB ( 1%)
parser (global) : 0.29 ( 3%) 0.21 ( 13%) 0.51 ( 5%) 105494 kB ( 12%)
parser struct body : 0.18 ( 2%) 0.04 ( 2%) 0.22 ( 2%) 39504 kB ( 5%)
parser enumerator list : 0.01 ( 0%) 0.01 ( 1%) 0.00 ( 0%) 1305 kB ( 0%)
parser function body : 0.18 ( 2%) 0.04 ( 2%) 0.15 ( 1%) 9096 kB ( 1%)
parser inl. func. body : 0.27 ( 3%) 0.02 ( 1%) 0.39 ( 4%) 33105 kB ( 4%)
parser inl. meth. body : 0.21 ( 2%) 0.06 ( 4%) 0.25 ( 2%) 23541 kB ( 3%)
template instantiation : 1.61 ( 18%) 0.43 ( 26%) 2.05 ( 20%) 346006 kB ( 40%)
constant expression evaluation : 0.05 ( 1%) 0.03 ( 2%) 0.02 ( 0%) 1470 kB ( 0%)
early inlining heuristics : 0.00 ( 0%) 0.01 ( 1%) 0.03 ( 0%) 3751 kB ( 0%)
inline parameters : 0.06 ( 1%) 0.02 ( 1%) 0.05 ( 0%) 12991 kB ( 1%)
integration : 0.12 ( 1%) 0.04 ( 2%) 0.26 ( 3%) 53810 kB ( 6%)
tree gimplify : 0.06 ( 1%) 0.02 ( 1%) 0.11 ( 1%) 20691 kB ( 2%)
tree eh : 0.02 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 2821 kB ( 0%)
tree CFG construction : 0.02 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 8987 kB ( 1%)
tree CFG cleanup : 0.11 ( 1%) 0.02 ( 1%) 0.13 ( 1%) 208 kB ( 0%)
tree tail merge : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 880 kB ( 0%)
tree VRP : 0.17 ( 2%) 0.00 ( 0%) 0.18 ( 2%) 7001 kB ( 1%)
tree Early VRP : 0.05 ( 1%) 0.00 ( 0%) 0.05 ( 0%) 7256 kB ( 1%)
tree copy propagation : 0.00 ( 0%) 0.00 ( 0%) 0.05 ( 0%) 104 kB ( 0%)
tree PTA : 0.13 ( 1%) 0.05 ( 3%) 0.25 ( 2%) 1906 kB ( 0%)
tree PHI insertion : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 736 kB ( 0%)
tree SSA rewrite : 0.06 ( 1%) 0.01 ( 1%) 0.04 ( 0%) 6289 kB ( 1%)
tree SSA other : 0.00 ( 0%) 0.02 ( 1%) 0.03 ( 0%) 940 kB ( 0%)
tree SSA incremental : 0.08 ( 1%) 0.00 ( 0%) 0.03 ( 0%) 1717 kB ( 0%)
tree operand scan : 0.08 ( 1%) 0.00 ( 0%) 0.08 ( 1%) 19096 kB ( 2%)
dominator optimization : 0.18 ( 2%) 0.01 ( 1%) 0.15 ( 1%) 5240 kB ( 1%)
backwards jump threading : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 244 kB ( 0%)
tree SRA : 0.03 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 1712 kB ( 0%)
tree CCP : 0.10 ( 1%) 0.02 ( 1%) 0.10 ( 1%) 1097 kB ( 0%)
tree reassociation : 0.00 ( 0%) 0.01 ( 1%) 0.00 ( 0%) 50 kB ( 0%)
tree PRE : 0.15 ( 2%) 0.01 ( 1%) 0.18 ( 2%) 4977 kB ( 1%)
tree FRE : 0.13 ( 1%) 0.02 ( 1%) 0.12 ( 1%) 2498 kB ( 0%)
tree linearize phis : 0.02 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 563 kB ( 0%)
tree forward propagate : 0.09 ( 1%) 0.00 ( 0%) 0.10 ( 1%) 1071 kB ( 0%)
tree phiprop : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 11 kB ( 0%)
tree conservative DCE : 0.04 ( 0%) 0.01 ( 1%) 0.02 ( 0%) 133 kB ( 0%)
tree aggressive DCE : 0.04 ( 0%) 0.01 ( 1%) 0.04 ( 0%) 7238 kB ( 1%)
tree DSE : 0.00 ( 0%) 0.01 ( 1%) 0.03 ( 0%) 254 kB ( 0%)
tree loop invariant motion : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 17 kB ( 0%)
scev constant prop : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 112 kB ( 0%)
tree loop unswitching : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 349 kB ( 0%)
complete unrolling : 0.01 ( 0%) 0.01 ( 1%) 0.03 ( 0%) 1141 kB ( 0%)
tree slp vectorization : 0.01 ( 0%) 0.02 ( 1%) 0.03 ( 0%) 5032 kB ( 1%)
tree iv optimization : 0.02 ( 0%) 0.00 ( 0%) 0.04 ( 0%) 2110 kB ( 0%)
predictive commoning : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 302 kB ( 0%)
gimple CSE reciprocals : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
dominance computation : 0.14 ( 2%) 0.03 ( 2%) 0.16 ( 2%) 0 kB ( 0%)
out of ssa : 0.05 ( 1%) 0.00 ( 0%) 0.01 ( 0%) 55 kB ( 0%)
expand vars : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 1422 kB ( 0%)
expand : 0.03 ( 0%) 0.01 ( 1%) 0.10 ( 1%) 14790 kB ( 2%)
post expand cleanups : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 1273 kB ( 0%)
varconst : 0.00 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 8 kB ( 0%)
jump : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
forward prop : 0.02 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 1330 kB ( 0%)
CSE : 0.13 ( 1%) 0.00 ( 0%) 0.08 ( 1%) 664 kB ( 0%)
dead code elimination : 0.00 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 0 kB ( 0%)
dead store elim1 : 0.02 ( 0%) 0.00 ( 0%) 0.06 ( 1%) 1230 kB ( 0%)
dead store elim2 : 0.05 ( 1%) 0.00 ( 0%) 0.03 ( 0%) 1584 kB ( 0%)
loop init : 0.11 ( 1%) 0.02 ( 1%) 0.07 ( 1%) 8638 kB ( 1%)
loop versioning : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 40 kB ( 0%)
loop invariant motion : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 8 kB ( 0%)
CPROP : 0.12 ( 1%) 0.00 ( 0%) 0.06 ( 1%) 3321 kB ( 0%)
PRE : 0.08 ( 1%) 0.00 ( 0%) 0.05 ( 0%) 935 kB ( 0%)
CSE 2 : 0.07 ( 1%) 0.00 ( 0%) 0.08 ( 1%) 333 kB ( 0%)
branch prediction : 0.02 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 1178 kB ( 0%)
combiner : 0.21 ( 2%) 0.00 ( 0%) 0.15 ( 1%) 7070 kB ( 1%)
if-conversion : 0.02 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 464 kB ( 0%)
integrated RA : 0.25 ( 3%) 0.01 ( 1%) 0.30 ( 3%) 20626 kB ( 2%)
LRA non-specific : 0.10 ( 1%) 0.00 ( 0%) 0.09 ( 1%) 1243 kB ( 0%)
LRA virtuals elimination : 0.02 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 834 kB ( 0%)
LRA reload inheritance : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 195 kB ( 0%)
LRA create live ranges : 0.11 ( 1%) 0.01 ( 1%) 0.13 ( 1%) 234 kB ( 0%)
LRA hard reg assignment : 0.02 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
LRA rematerialization : 0.04 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 0 kB ( 0%)
reload : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 0 kB ( 0%)
reload CSE regs : 0.09 ( 1%) 0.00 ( 0%) 0.06 ( 1%) 2212 kB ( 0%)
load CSE after reload : 0.06 ( 1%) 0.00 ( 0%) 0.05 ( 0%) 559 kB ( 0%)
ree : 0.00 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 71 kB ( 0%)
thread pro- & epilogue : 0.03 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 939 kB ( 0%)
peephole 2 : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%) 170 kB ( 0%)
hard reg cprop : 0.00 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 15 kB ( 0%)
scheduling 2 : 0.15 ( 2%) 0.00 ( 0%) 0.16 ( 2%) 894 kB ( 0%)
machine dep reorg : 0.00 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 502 kB ( 0%)
reorder blocks : 0.04 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 1015 kB ( 0%)
shorten branches : 0.02 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 0 kB ( 0%)
final : 0.04 ( 0%) 0.00 ( 0%) 0.03 ( 0%) 3408 kB ( 0%)
straight-line strength reduction : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%) 21 kB ( 0%)
tree loop if-conversion : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%) 203 kB ( 0%)
rest of compilation : 0.10 ( 1%) 0.01 ( 1%) 0.13 ( 1%) 3241 kB ( 0%)
remove unused locals : 0.02 ( 0%) 0.00 ( 0%) 0.08 ( 1%) 3 kB ( 0%)
address taken : 0.04 ( 0%) 0.01 ( 1%) 0.04 ( 0%) 0 kB ( 0%)
TOTAL : 8.75 1.63 10.40 874064 kB
Edit I had a few people comment asking for the compiler flags, here they are:
-std=c++17 -Wall -Ofast -DNDEBUG -Wno-deprecated-declarations
