Re: Behaviour of i386 add instruction changes based on operand

["Jan Beulich" <jbeulich at novell dot com>]

>>> Ian Lance Taylor <iant@google.com> 07.12.06 22:44 >>>
>Assemble this i386 source:
>
>	add $1, 0(%edx)
>	add $128, 0(%edx)
>
>I get this:
>
>foo.s: Assembler messages:
>foo.s:2: Error: no instruction mnemonic suffix given and no register operands; can't size instruction
>
>The instructions do generate different opcodes (0x80 vs. 0x83).  When
>adding a constant to memory, if the operand is a sign extended 8-bit
>value, we default to addl.  If the operand is something else, we have
>no default.  That seems inconsistent.  Is this a bug or a feature?

Half and half - the opcode selection I guess is meant to be a feature (as
it's really clear that in the first case the shorter instruction can be used),
but the defaulting to addl (or better, to the default operand size) I always
considered a bug, where at least a warning should be given. Quite a while
ago I actually made this an error in Intel mode, but I refrained from doing
so for AT&T due to the vague status of that code (given there's no official
specification for this).

Jan