[fasmg] Problem with VPMASKMOVQ AVX2 instruction

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bdlepla

Joined: 05 Sep 2023
Posts: 4

bdlepla 05 Sep 2023, 02:42

using flat assembler version g.k4v8

Snippet I'm assembling:

Code:

        vpmaskmovq      ymm1,ymm0,[rsi]                 
        vpmaskmovq      [rdi],ymm0,ymm1

when I use the following to disassemble my program (on Linux):

Code:

objdump -d test > test.s

This is what is produced for my snippet:

Code:

        401424: c4 e2 fd 8c 0e          vpmaskmovq (%rsi),%ymm0,%ymm1
        401429: c4 e2 f9 8e 0f          vpmaskmovq %xmm1,%xmm0,(%rdi)  ; <-- this should be using ymm0 and ymm1, not xmm0 and xmm1

The issue is that the second call to vpmaskmovq to move from ymm1 with mask ymm0 to memory, is erroneously using xmm1 and xmm0 instead.

Any ideas? Thanks in advance.

05 Sep 2023, 02:42

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20422
Location: In your JS exploiting you and your system

revolution 05 Sep 2023, 04:20

The encoding should be FD

Code:

~ ndisasm -b 64 bdlepla.bin
00000000  C4E2FD8C0E        vpmaskmovq ymm1,ymm0,yword [rsi]
00000005  C4E2F98E0F        vpmaskmovq oword [rdi],xmm0,xmm1
0000000A  C4E2FD8E0F        vpmaskmovq yword [rdi],ymm0,ymm1

05 Sep 2023, 04:20

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20422
Location: In your JS exploiting you and your system

revolution 05 Sep 2023, 04:21

fasm assembles it correctly. fasmg is incorrect.

05 Sep 2023, 04:21

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8356
Location: Kraków, Poland

Tomasz Grysztar 05 Sep 2023, 06:17

It's a mistake in my implementation of AVX2 for fasmg. The same issue is also present in x86-2 encoder (and therefore fasm 2). I'm correcting it in all places.

05 Sep 2023, 06:17

bdlepla

Joined: 05 Sep 2023
Posts: 4

bdlepla 05 Sep 2023, 12:36

Thank you both. I applied the changes locally and all is well.

05 Sep 2023, 12:36

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