[Bug gas/22871] New: Encode instructions of 64-bit registers without the REX_W bit

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=22871

            Bug ID: 22871
           Summary: Encode instructions of 64-bit registers without the
                    REX_W bit
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---
            Target: x86-64

When the upper 32 bits of destination registers of instructions are known
to be zero, we can encode them without the REX_W bit.  For example,

    andq $imm31, %r64
    xorq %r64, %r64

can be encoded as:

    andl $imm31, %r32
    xorl %r32, %r32

To make assembler more predictable, this optimization should be done only
when -O is passed to assembler.

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