https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92276
Bug ID: 92276
Summary: Embedded __attribute__ ((optimize("unroll-loops"))) is
not working together with '__attribute__
((__always_inline__))'
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: Lijian.Zhang at arm dot com
Target Milestone: ---
Dear experts,
I'm trying to use '__attribute__ ((optimize("unroll-loops")))' to apply
automatic loop unrolling to a static-line function with __attribute__
((__always_inline__)).
But the loop is not unrolled from the assembly output. The compiling command is
'gcc -march=armv8-a+crc -O2 -W -Wall -mtune=cortex-a72 unroll.c -S'.
However, if I apply -funroll-loops option to the compiling process, i.e.,
compile with command 'gcc -march=armv8-a+crc -O2 -W -Wall -mtune=cortex-a72
-funroll-loops unroll.c -S'. I can see loop is unrolled from the assembly
output.
And if I compile without -funroll-loops option, and if '__attribute__
((__always_inline__))' is commented out, '__attribute__ ((__always_inline__))'
is also taking effect.
So it seems those two attribute parameters are not working together, which
seems to be unreasonable to me. I want some functions to be inlined and also
the loops inside those functions unrolled automatically, as the loop iteration
number is fixed.
lijian@net-arm-d05-08:~/C/unroll$ gcc --version
gcc (Ubuntu 8.3.0-6ubuntu1~18.04.1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
lijian@net-arm-d05-08:~/C/unroll$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.1 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.1 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/";
SUPPORT_URL="https://help.ubuntu.com/";
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/";
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy";
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
lijian@net-arm-d05-08:~/C/unroll$ gcc -march=armv8-a+crc -O2 -W -Wall
-mtune=cortex-a72 unroll.c -S
lijian@net-arm-d05-08:~/C/unroll$ lscpu
Architecture: aarch64
Byte Order: Little Endian
CPU(s): 64
On-line CPU(s) list: 0-63
Thread(s) per core: 1
Core(s) per socket: 32
Socket(s): 2
NUMA node(s): 4
Vendor ID: ARM
Model: 2
Model name: Cortex-A72
Stepping: r0p2
BogoMIPS: 100.00
L1d cache: 32K
L1i cache: 48K
L2 cache: 1024K
L3 cache: 16384K
NUMA node0 CPU(s): 0-15
NUMA node1 CPU(s): 16-31
NUMA node2 CPU(s): 32-47
NUMA node3 CPU(s): 48-63
Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid
////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <arm_acle.h>
static inline __attribute__ ((__always_inline__))
__attribute__ ((optimize("unroll-loops")))
unsigned int clib_crc32c (unsigned int v, unsigned char * s, int len)
{
for (; len >= 8; len -= 8, s += 8)
v = __crc32cd (v, *((unsigned long *) s));
for (; len >= 4; len -= 4, s += 4)
v = __crc32cw (v, *((unsigned int *) s));
for (; len >= 2; len -= 2, s += 2)
v = __crc32ch (v, *((unsigned short *) s));
for (; len >= 1; len -= 1, s += 1)
v = __crc32cb (v, *((unsigned char *) s));
return v;
}
int main (int argc, char *argv[])
{
unsigned char s[40] = {argc, 0, argc, 0};
unsigned char ss[32] = {argc, 0, argc, 0, argc, 0};
unsigned int v = 0xbeefdead, vv = 0xdeadbeef;
int len = strtol (argv[1], NULL, 10);
for (int i = 0; i < len; i++) {
v = clib_crc32c (v, s, 40);
vv = clib_crc32c (vv, ss, 32);
}
printf ("%8X\n", v);
printf ("%8X\n", vv);
return 0;
}
////////////////////
