[Bug gas/22871] Encode instructions of 64-bit operand without the REX_W bit

[address@hidden]

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

--- Comment #9 from Linus Torvalds <address@hidden> ---
I already pointed this out in email to hjl, but adding it to the bugzilla too,
in case people want to track it:

There are a few more common cases that can use the REX.W optimization, notably

     movq $imm,%reg

and there it actually allows the whole unsigned 32-bit range, so it changes the
immediate logic a bit.

Right now gas generates a 10-byte instruction for

        movq $0x87654321,%rax

in the form of

   0: 48 b8 21 43 65 87 00 movabs $0x87654321,%rax
   7: 00 00 00

but you can actually do it with just 5 bytes:

   0: b8 21 43 65 87        mov    $0x87654321,%eax

instead. Note that the C7/0 instruction with REX.W wasn't used,
because it sign-extends the 32-bit constant.

But even when the constant *isn't* in the full 32-bit range and you can use
C7/0 to avoid the long constant, gas currently picks a non-optimal instruction:

        movq $0x77654321,%rax

becomes

   0: 48 c7 c0 21 43 65 77 mov    $0x77654321,%rax

which is 7 bytes, even though (again) that 5-byte 32-bit mov would
have sufficed:

   0: b8 21 43 65 77        mov    $0x77654321,%eax

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