https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97142
Bug ID: 97142
Summary: __builtin_fmod not optimized on POWER
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: fx at gnu dot org
Target Milestone: ---
I ran some Fortran benchmarks (the "Polyhedron" set) on POWER9, and found one
of them has pathologically bad performance compared with xlf. Profiling shows
that's due to spending most of its time in fmod via a random-number function.
fmod isn't called when compiled with xlf -O5 or when compiling the same on
x86_64. Although it's Fortran, this doesn't appear to be Fortran-specific as
the DMOD intrinsic is turned into __builtin_fmod.
The following is with gcc 10.2, comparing the two targets.
On RHEL7 POWER9 (and the same with -mcpu=native):
$ cat ggl.f90
REAL FUNCTION GGL(Ds)
DOUBLE PRECISION Ds , d2
DATA d2/2147483647.D0/
Ds = DMOD(16807.D0*Ds,d2)
GGL = Ds/d2
END
$ gfortran -O3 -fopt-info-all -c ggl.f90
ggl.f90:4:0: missed: not inlinable: ggl/0 -> __builtin_fmod/2, function body
not available
Unit growth for small function inlining: 12->12 (0%)
Inlined 0 calls, eliminated 0 functions
ggl.f90:6:0: note: ***** Analysis failed with vector mode V2DF
$ nm ggl.o
U fmod
0000000000000000 T ggl_
U .TOC.
On Debian 10 SKX with the same source:
$ gfortran-10 -Ofast -fopt-info-all -c ggl.f90
ggl.f90:4:0: missed: not inlinable: ggl/0 -> __builtin_fmod/2, function body
not available
Unit growth for small function inlining: 12->12 (0%)
Inlined 0 calls, eliminated 0 functions
ggl.f90:6:0: note: ***** Analysis failed with vector mode V2DF
ggl.f90:6:0: note: ***** Skipping vector mode V16QI, which would repeat the
analysis for V2DF
$ nm ggl.o
0000000000000000 r .LC0
0000000000000008 r .LC1
0000000000000010 r .LC2
0000000000000000 T ggl_
