[Bug gas/20046] New: x86 feature request: build an instruction including rex and modr/m

[hpa at zytor dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20046

            Bug ID: 20046
           Summary: x86 feature request: build an instruction including
                    rex and modr/m
           Product: binutils
           Version: 2.27 (HEAD)
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: hpa at zytor dot com
  Target Milestone: ---

In the Linux kernel, we constantly bump into the problem that we need to use
new instructions, but that backwards versions of gas which we still need to
handle don't support them.  For plain instructions, it is fine to use .byte,
but it gets very suboptimal when the instructions takes registers or memory
operands.

I'm wondering if something like this might make sense:

    .rex[/w] <r>,<m>
    .modrm <r>,<m>

... which would generate the REX and modr/m (+SIB, displacement etc needed for
the addressing mode) bytes for an instruction with the given r and m operands. 
r might be an integer immediate for cases where the register operand is used as
an opcode extension.

This would allow a gcc inline of the form:

asm(".byte 0xf3 ; .rex/w %0,%1 ; .byte 0x0f, 0x99 ; .modrm %0,%1"
    : "=rm" (...) : "r" (...));

... for some random new instruction with the SDM description F3 0F 99 /r.

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