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SubjectRE: [tip:x86/asm] x86/entry/64: Add two more instruction suffixes
Date
From: Denys Vlasenko
> Sent: 03 July 2018 12:59
>
> On 07/03/2018 10:46 AM, David Laight wrote:
> > From: Jan Beulich
> >> Sent: 03 July 2018 09:36
> > ...
> >> As said there, omitting suffixes from instructions in AT&T mode is bad
> >> practice when operand size cannot be determined by the assembler from
> >> register operands, and is likely going to be warned about by upstream
> >> gas in the future (mine does already).
> > ...
> >> - bt $9, EFLAGS(%rsp) /* interrupts off? */
> >> + btl $9, EFLAGS(%rsp) /* interrupts off? */
> >
> > Hmmm....
> > Does the operand size make any difference at all for the bit instructions?
> > I'm pretty sure that the cpus (386 onwards) have always done aligned 32bit
> > transfers (the docs never actually said aligned).
> > I can't remember whether 64bit mode allows immediates above 31.
>
> Immediates up to 63 are allowed in 64 bit mode (IOW: for REX-prefixed form)
> (run-tested).
>
> Keep in mind that this instruction is "special" with register bit offset:
>
> Register/memory form (BT REG,[MEM]) does not limit or mask the value of bit offset
> in REG, the instruction uses bit REG%8 in byte at address [MEM+REG/8].
>
> This works correctly even for negative values: REG = -1 will access
> the most significant bit in the byte immediately before MEM.
>
> Thus, for accesses of standard RAM locations (not memory-mapped IO and such),
> the "operand size" concept for this instruction (and BTC, BTR, BTS)
> does not make much sense: it accesses one bit. The width of actual memory
> access is irrelevant.
>
> I'd say assembler should just use the "natural" width for current mode
> (16 or 32-bit), and warn when code tries to use immediate operand which
> will be truncated and thus needs a wider operand size.
...


Indeed.

Truncating the immediate value really ought to be an error.
In 64bit mode the 64bit form should be used for immediates [32..63).
The 16bit form could safely be selected for immediates [0..15).

Requiring the width suffix is just stupid.

David


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