| Summary: | __builtin_cpu_supports() doesn't work on powerpc | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Mathieu Malaterre <malat> |
| Component: | target | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | CLOSED INVALID | ||
| Severity: | normal | CC: | bergner, bill.schmidt, dje, kilobyte |
| Priority: | P3 | ||
| Version: | 6.1.1 | ||
| Target Milestone: | --- | ||
| URL: | https://bugs.debian.org/826987 | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
| Attachments: | demo code | ||
|
Description
Mathieu Malaterre
2016-07-06 06:44:16 UTC
The documentation doesn't mention this (and I should probably add it), but the __builtin_cpu_is() and __builtin_cpu_supports() builtins rely on support from glibc (glibc 2.23 and later to be exact). If you build against an oldish glibc (2.22 and earlier), the builtins will always return 0. You can verify whether your gcc was built with a new enough glibc by looking at the assembly output of a call to one of the builtins. For example:
bergner@genoa:~/gcc/BUGS/CPU_IS$ cat cpu.c
int
foo (void)
{
return __builtin_cpu_is ("power8");
}
bergner@genoa:~/gcc/BUGS/CPU_IS$ /home/bergner/gcc/build/gcc-fsf-mainline-debug-2/gcc/xgcc -B/home/bergner/gcc/build/gcc-fsf-mainline-debug-2/gcc -O2 -S cpu.c
bergner@genoa:~/gcc/BUGS/CPU_IS$ cat cpu.s
...
foo:
li 3,0 <- No support from glibc.
blr
If your gcc/glibc both support the builtins, then you'd see a load off of r13 and some logical instructions on the result, like so:
foo:
lwz 3,-28764(13)
xori 3,3,0x2d
cntlzw 3,3
srwi 3,3,5
blr
Another check is to see whether the libc.a and/or ld.so export the symbol __parse_hwcap_and_convert_at_platform. If it doesn't, then your glibc is too old:
linux%: objdump -x /usr/lib/debug/lib/powerpc64le-linux-gnu/ld-2.23.so | grep __parse_hwcap_and_convert_at_platform
00000000000289d0 g F .text 0000000000000348 0x60 __parse_hwcap_and_convert_at_platform@@GLIBC_2.23
My guess if this is your problem. If not, I can help you diagnose further to see what the problem is.
You can read my post here on why we need glibc for these builtins:
https://gcc.gnu.org/ml/gcc-patches/2016-01/msg01012.html
FYI, this is the output of your demo code on my POWER8 LE Ubuntu 16.04 system: bergner@ampere:~$ gcc demo.c bergner@ampere:~$ ./a.out 1 -- 1 2 3 4 6 9 10 11 14 15 16 17 19 20 23 24 25 27 29 31 33 34 36 37 Right. I totally missed that from your post. Sorry for the noise, this works for me now (was using glibc 2.22). For reference on POWER7, I get: 2 -- 1 2 3 6 9 10 14 17 20 23 24 25 27 29 30 31 34 37 Closing per above. |