https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90028
Bug ID: 90028
Summary: On Intel Skylake (-march=native) generated avx512
instruction can be wrong
Product: gcc
Version: 8.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: ferruh.yigit at intel dot com
Target Milestone: ---
Created attachment 46114
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46114&action=edit
19.05-rc1 default gcc build on skylake
gcc version:
gcc (GCC) 8.3.1 20190223 (Red Hat 8.3.1-2)
binutils:
GNU ld version 2.31.1-24.fc29
This is observed in dpdk project (https://git.dpdk.org/dpdk/tree/?h=v19.05-rc1)
on Intel Skylate CPU.
Full build command (removed -I & -D ones):
gcc -Wp,-MD,./.rte_kni.o.d.tmp -m64 -pthread -march=native -W -Wall
-Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations
-Wold-style-definition -Wpointer-arith -Wcast-align -Wnested-externs
-Wcast-qual -Wformat-nonliteral -Wformat-security -Wundef -Wwrite-strings
-Wdeprecated -Werror -Wimplicit-fallthrough=2 -Wno-format-truncation -O3
-fno-strict-aliasing -o rte_kni.o -c
/root/development/dpdk-next-net/lib/librte_kni/rte_kni.c
When related code build with "-mno-avx512f" flag, problem solved. Also clang
(clang version 7.0.1 (Fedora 7.0.1-6.fc29)) output works fine.
Suspected from 'vpgatherqq' instruction usage.
The related .c code is
(https://git.dpdk.org/dpdk/tree/lib/librte_kni/rte_kni.c?h=v19.05-rc1#n546):
"
static void *
va2pa(struct rte_mbuf *m)
{
return (void *)((unsigned long)m -
((unsigned long)m->buf_addr -
(unsigned long)m->buf_iova));
}
unsigned
rte_kni_tx_burst(struct rte_kni *kni, struct rte_mbuf **mbufs, unsigned num)
{
void *phy_mbufs[num];
unsigned int ret;
unsigned int i;
for (i = 0; i < num; i++)
phy_mbufs[i] = va2pa(mbufs[i]);
....
"
'm->buf_addr' & 'm->buf_iova' are next to each other in the struct, so there is
8 bytes difference between their address.
Generated asm code:
avx512 enabled code snippet:
232c: ba ff ff ff ff mov $0xffffffff,%edx
2331: 48 c1 e0 05 shl $0x5,%rax
2335: 31 c9 xor %ecx,%ecx
2337: c5 f9 92 ca kmovb %edx,%k1
233b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
2340: 62 f1 fe 28 6f 0c 0e vmovdqu64 (%rsi,%rcx,1),%ymm1
2347: c5 f9 90 d1 kmovb %k1,%k2
*234b: 62 f2 fd 2a 91 04 0d vpgatherqq 0x1(,%ymm1,1),%ymm0{%k2}
2352: 01 00 00 00
2356: c5 f9 90 d9 kmovb %k1,%k3
235a: c5 fd d4 c1 vpaddq %ymm1,%ymm0,%ymm0
235e: 62 f2 fd 2b 91 14 0d vpgatherqq 0x0(,%ymm1,1),%ymm2{%k3}
2365: 00 00 00 00
2369: c5 fd fb c2 vpsubq %ymm2,%ymm0,%ymm0
236d: 62 d1 fe 28 7f 04 08 vmovdqu64 %ymm0,(%r8,%rcx,1)
same code avx512 disabled (avx2) code snippet:
2332: c5 ed 76 d2 vpcmpeqd %ymm2,%ymm2,%ymm2
2336: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
233d: 00 00 00
2340: c5 fe 6f 0c 0e vmovdqu (%rsi,%rcx,1),%ymm1
2345: c5 fd 6f e2 vmovdqa %ymm2,%ymm4
2349: c4 e2 dd 91 04 0d 08 vpgatherqq %ymm4,0x8(,%ymm1,1),%ymm0
2350: 00 00 00
2353: c5 fd 6f ea vmovdqa %ymm2,%ymm5
2357: c4 e2 d5 91 1c 0d 00 vpgatherqq %ymm5,0x0(,%ymm1,1),%ymm3
235e: 00 00 00
2361: c5 fd d4 c1 vpaddq %ymm1,%ymm0,%ymm0
2365: c5 fd fb c3 vpsubq %ymm3,%ymm0,%ymm0
2369: c4 c1 7e 7f 04 08 vmovdqu %ymm0,(%r8,%rcx,1)
full asm outputs are attached.
In the avx512 one, for 'vpgatherqq', it looks like the offset should be 0x8
instead of 0x1.
