https://sourceware.org/bugzilla/show_bug.cgi?id=23628
Bug ID: 23628
Summary: Mis-assembling of VSIB offset in EVEX scatter/gather
instructions
Product: binutils
Version: 2.31
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: gas
Assignee: unassigned at sourceware dot org
Reporter: thiago at kde dot org
Target Milestone: ---
Related to bug #16490. This was reported also as a note in bug #18738.
Test:
$ cat test.s
.text
vpgatherqd 12(,%zmm3,1), %ymm1{%k2}
vpgatherqd 12(%rax,%zmm3,1), %ymm1{%k2}
vpgatherqd 512(,%zmm3,1), %ymm1{%k2}
vpgatherqd 512(%rax,%zmm3,1), %ymm1{%k2}
vpgatherqd %xmm2, 12(,%ymm3,1), %xmm1
$ as test.s
$ objdump -dr a.out
a.out: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <.text>:
0: 62 f2 7d 4a 91 0c 1d vpgatherqd 0x3(,%zmm3,1),%ymm1{%k2}
7: 03 00 00 00
b: 62 f2 7d 4a 91 4c 18 vpgatherqd 0xc(%rax,%zmm3,1),%ymm1{%k2}
12: 03
13: 62 f2 7d 4a 91 0c 1d vpgatherqd 0x200(,%zmm3,1),%ymm1{%k2}
1a: 00 02 00 00
1e: 62 f2 7d 4a 91 8c 18 vpgatherqd 0x200(%rax,%zmm3,1),%ymm1{%k2}
25: 00 02 00 00
29: c4 e2 6d 91 0c 1d 0c vpgatherqd %xmm2,0xc(,%ymm3,1),%xmm1
30: 00 00 00
As can be seen on the first instruction, the constant "12" became 0x3. This
problem only appears to exist when the displacement is small enough to fit an
8-bit and when the base register is missing (that is, absolute pointers stored
in the ZMM register).
This is likely caused by the disp8 scaling performed when the disp8 is used
with a base register (see second instruction). It does not happen when the
displacement is larger than 8 bits (third and fourth instructions), nor in the
AVX2 equivalent.
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