Hi all,
I ran into a problem today involving an incorrectly decoded AVX
instruction. Hoping someone can tell me of a workaround or a quick fix
other than disabling AVX (unless that's the only solution).
Here's a basic testcase.
int main() {
float x;
__asm__("vxorps %0, %0, %0" : "=x"(x) : "x"(x) :);
return x == 0;
}
Compiled with gcc 5.4:
gcc -static -O3 -o test -mavx test.c
Produces the following assembly:
00000000004005f0 <main>:
4005f0: 66 0f ef c9 pxor xmm1,xmm1
4005f4: 31 c0 xor eax,eax
4005f6: 66 0f ef c0 pxor xmm0,xmm0
4005fa: ba 00 00 00 00 mov edx,0x0
4005ff: c5 f8 57 c0 vxorps xmm0,xmm0,xmm0
400603: 0f 2e c1 ucomiss xmm0,xmm1
400606: 0f 9b c0 setnp al
400609: 0f 45 c2 cmovne eax,edx
40060c: c3 ret
40060d: 0f 1f 00 nop DWORD PTR [rax]
gem5 decodes the instruction as XORPS_XMM_M, instead of (something more
like) VXORPS_XMM_XMM_XMM. This leads to a segfault in the simulated program
when it attempts to access an unmapped address
Here is the relevant section of the debug trace.
416293000: system.cpu: Fetch
416293000: system.cpu: Translating address 0x4005f0
416293000: system.cpu: Sending fetch for addr 0x4005f0(pa: 0x5f0)
416293000: system.cpu: -- pkt addr: 0x5f0
416348000: system.cpu.icache_port: Received fetch response 0x5f0
416348000: system.cpu: Complete ICache Fetch for addr 0x5f0
416348000: global: Getting more bytes.
416348000: global: Setting origPC to 0x4005f0
416348000: global: Found VEX two-byte prefix 0xc5.
416348000: global: Found VEX opcode 0x57.
416348000: global: Found modrm byte 0x57.
416348000: global: Collecting 1 byte displacement, got 1 bytes.
416348000: global: Collected displacement 0xffffffffffffffc9.
416348000: global: Calculating the instruction size: basePC: 0x4005f0
offset: 0x4 origPC: 0x4005f0 size: 4
416348000: global: XORPS_XMM_M : ldfp: The address is 0xffffffffffffffca
416348000: system.cpu: Fault occured, scheduling fetch event
panic: Tried to read unmapped address 0xffffffffffffffca.
Toolchain information
=====================
samxi $ gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.5) 5.4.0 20160609
samxi $ as --version
GNU assembler (GNU Binutils for Ubuntu) 2.26.1
gem5 revision: e79c4c6f033581f84072ddb45d2ec9543c31af55
Thanks,
Sam
--
Thanks,
Sam Xi
Harvard University
Computer Science, Ph.D. Candidate
http://www.samxi.org
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