https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108705
Bug ID: 108705
Summary: Unexpected CPU time usage with LTO in ranger
propagation
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: rimvydas.jas at gmail dot com
Target Milestone: ---
Very trivialized reduced testcase that still works with
--enable-checking=release configured trunk.
$ cat hog.f90 # foo() and bar() are in separate units in original case
subroutine bar(n,m,p,s) ! in bar.f90
implicit none
integer :: n,m
real,intent(inout) :: p(n),s(*)
!real,intent(inout) :: p(:),s(:) ! gives slower growth
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
call foo(n,m,p,s)
! ...
!call foo(n,m,p,s)
end subroutine bar
subroutine foo(n,m,p,b) ! in foo.f90
implicit none
integer :: n,m,j
real,intent(inout) :: p(n),b(*)
do j=1,n
b(m+j-1)=p(j)
enddo
m=m+n ! <---- problematic part
end subroutine foo
$ gfortran -Wall -Wextra -O2 -flto -c hog.f90 # mimic ccBemfcW.ltrans23.o
$ lto1 -ftime-report -fchecking=0 hog.o # -fltrans /tmp/ccBemfcW.ltrans23.o
Reading object files: hog.o {GC 2518k} {heap 1028k}
Reading the symbol table:
Merging declarations: {GC 2520k} {heap 1028k}
Reading summaries: <odr> {GC 2520k} {heap 1028k} <profile_estimate> {GC 2520k}
{heap 1028k} <icf> {GC 2520k} {heap 1028k} <fnsummary> {GC 2530k} {heap
1168k} <pure-const> {GC 2530k} {heap 1168k} <modref> {GC 2532k} {heap 1168k}
{GC 2532k}
Merging symbols: {heap 1168k}Reading function bodies:
Performing interprocedural optimizations
<odr> {heap 1168k} <whole-program> {heap 1168k} <profile_estimate> {heap
1168k} <icf> {heap 1360k} <devirt> {heap 1360k} <cp> {heap 1360k} <cdtor> {heap
1360k} <fnsummary> {heap 1360k} <inline> {heap 1360k} <pure-const> {heap 1360k}
<modref> {heap 1360k} <free-fnsummary> {heap 1360k} <static-var> {heap 1360k}
<single-use> {heap 1360k} <comdats> {heap 1360k}Assembling functions:
<simdclone> {heap 1360k} foo in:foo_ bar in:bar_
Time variable usr sys wall
GGC
phase setup : 0.00 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
2583k ( 67%)
phase opt and generate :1490.95 (100%) 0.00 ( 0%)1491.02 (100%)
1230k ( 32%)
callgraph functions expansion :1490.95 (100%) 0.00 ( 0%)1491.02 (100%)
1201k ( 31%)
tree VRP : 5.08 ( 0%) 0.00 ( 0%) 5.07 ( 0%)
84k ( 2%)
dominator optimization :1485.85 (100%) 0.00 ( 0%)1485.92 (100%)
16k ( 0%)
tree canonical iv : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
71k ( 2%)
tree loop distribution : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
115k ( 3%)
dead store elim1 : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
2304 ( 0%)
combiner : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
42k ( 1%)
initialize rtl : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
12k ( 0%)
TOTAL :1490.95 0.00 1491.02
3846k
For 10+ calls adding extra new call adds to total time: 0.21, 1.07, 6.29,
38.74, 238.24, 1490.95, 9424.12, ... seconds of CPU time.
This is not a problem for non-LTO builds since objects need to be compiled only
once for all executables in the project. However with LTO it means that for
any executable having problematic subroutines in call graph *and* having such
unit pairs in the same ltrans partition would need to be compiled from scratch
over and over. In original case final LTO link of a first executable with
-flto=20 was still compiling last 3 ltrans partitions after 25h+ (gcc-12 is
fine).
It is quite hard to say where lto1 gets "stuck" (as in infinite loop, no new
output gets added to assembly outputs either). One has to ps(1) the full
command line, grab /tmp/ccBemfcW.ltrans23.o and manually invoke lto1 to see
where problematic code units could be. Also there are no support for
attributes to deal with such problems e.g.:
!GCC$ ATTRIBUTES noclone,noinline :: foo
while LTO is getting better at detecting "strategically placed debug write()
statements".
