[llvm-bugs] [Bug 34733] New: Moffs operands are not encoded

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Sep 26 02:43:14 PDT 2017


            Bug ID: 34733
           Summary: Moffs operands are not encoded
           Product: libraries
           Version: 4.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: kadircetinkaya.06.tr at gmail.com
                CC: llvm-bugs at lists.llvm.org

LLVM is not able to generate instructions with moffs operands, when I try to

mov al, [0x01]

with llvm-mc it generates encoding, [0x8a,0x04,0x25,0x01,0x00,0x00,0x00] which
is an instruction that actually uses full modr/m and sib. Which should decode
into mov al, byte ptr [0x01].

It should've generated  "\xa0\x01\x00\x00\x00\x00\x00\x00\x00". As mentioned in
page 45 of Intel SDM, Instruction Set Reference, July 2017.

" Direct Memory-Offset MOVs
In 64-bit mode, direct memory-offset forms of the MOV instruction are extended
to specify a 64-bit immediate absolute address. This address is called a
moffset. No prefix is needed to specify this 64-bit memory offset. For these
MOV instructions, the size of the memory offset follows the address-size
default (64 bits in 64-bit mode). See Table 2-6.

this assembler manages to do it even tough they are using Keystone and
Capstone. May be it was correct in previous versions, and got broken in a later

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