Date: Mon, 7 Sep 2020 19:10:02 -0700 From: Steve Kargl <sgk@troutmask.apl.washington.edu> To: freebsd-toolchain@freebsd.org, freebsd-current@freebsd.org Subject: clang miscompiles OpenLibm on i686-*-freebsd Message-ID: <20200908021002.GA76325@troutmask.apl.washington.edu>
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TL;DR summary: clang is broken for numerical on i686 FreeBSD.
% uname -a
FreeBSD mobile 13.0-CURRENT FreeBSD 13.0-CURRENT #0 r361834M:
Fri Jun 5 08:49:26 PDT 2020 obj/usr/src/i386.i386/sys/MOBILE i386
% which clang
/usr/bin/clang
% clang --version
FreeBSD clang version 10.0.1 (git@github.com:llvm/llvm-project.git llvmorg-10.0.1-rc1-0-gf79cd71e145)
Target: i386-unknown-freebsd13.0
Thread model: posix
InstalledDir: /usr/bin
My testing for exp2f(x) shows
Interval tested for exp2f: [1,8]
ulp <= 0.5: 0.936% 235635 | 0.936% 235635
0.5 < ulp < 0.6: 0.016% 4098 | 0.953% 239733
0.6 < ulp < 0.7: 0.024% 6147 | 0.977% 245880
0.7 < ulp < 0.8: 0.008% 2049 | 0.985% 247929
0.8 < ulp < 0.9: 0.016% 4098 | 1.001% 252027
0.9 < ulp < 1.0: 0.008% 2049 | 1.010% 254076
1.0 < ulp < 1.5: 0.334% 84009 | 1.343% 338085
1.5 < ulp < 2.0: 0.187% 47127 | 1.531% 385212
2.0 < ulp < 3.0: 0.839% 211047 | 2.369% 596259
3.0 < ulp < 0.0: 97.631% 24569565 | 100.000% 25165824
Max ulp: 359512.000000 at 1.96875286e+00
which is bad (ulp means errors in the Unit of Last Place).
By default OpenLibm uses "-march=i686 -O3 -m32" on an i686 target.
The commandline used in compiling s_exp2f.c is
clang -fno-builtin -fno-strict-aliasing -O3 -fPIC -march=i686 -m32
-std=c99 -Wall -I/usr/home/kargl/tmp/olibm
-I/usr/home/kargl/tmp/olibm/include -I/usr/home/kargl/tmp/olibm/i387
-I/usr/home/kargl/tmp/olibm/src -DASSEMBLER -D__BSD_VISIBLE
-Wno-implicit-function-declaration -I/usr/home/kargl/tmp/olibm/ld80
-c src/s_exp2f.c -o src/s_exp2f.c.o
If I use -O1 or -O2 I get the same above results. If I
change to -O0, I get
clang -fno-builtin -fno-strict-aliasing -O0 -fPIC -march=i686 -m32
-std=c99 -Wall -I/usr/home/kargl/tmp/olibm
-I/usr/home/kargl/tmp/olibm/include -I/usr/home/kargl/tmp/olibm/i387
-I/usr/home/kargl/tmp/olibm/src -DASSEMBLER -D__BSD_VISIBLE
-Wno-implicit-function-declaration -I/usr/home/kargl/tmp/olibm/ld80
-c src/s_exp2f.c -o src/s_exp2f.c.o
Interval tested for exp2f: [1,8]
ulp <= 0.5: 0.056% 14072 | 0.056% 14072
0.5 < ulp < 0.6: 0.000% 8 | 0.056% 14080
3.0 < ulp < 0.0: 99.944% 25151744 | 100.000% 25165824
Max ulp: 22729.386719 at 1.00195301e+00
This is better, but still bad. If I remove -m32, I get
clang -fno-builtin -fno-strict-aliasing -O0 -fPIC -march=i686
-std=c99 -Wall -I/usr/home/kargl/tmp/olibm
-I/usr/home/kargl/tmp/olibm/include -I/usr/home/kargl/tmp/olibm/i387
-I/usr/home/kargl/tmp/olibm/src -DASSEMBLER -D__BSD_VISIBLE
-Wno-implicit-function-declaration -I/usr/home/kargl/tmp/olibm/ld80
-c src/s_exp2f.c -o src/s_exp2f.c.o
Interval tested for exp2f: [1,8]
ulp <= 0.5: 99.959% 25155610 | 99.959% 25155610
0.5 < ulp < 0.6: 0.041% 10214 | 100.000% 25165824
Max ulp: 0.500980 at 1.97115958e+00
This is good, but unoptimized. If I use -O1, -O2, or -O3 without
-m32, I get the initial bad results above. If I change -march=i686
to -march=pentiumpro, I get the same results.
Theoretically, OpenLibm would test for SSE capabilities and choose
a better -march, but it doesn't. This, however, is unimportant.
Any port that uses clang (or cc) that does numerical computation
and uses -march=i686 (or pentiumpro) is likely broken.
--
Steve
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