Re: [patch] spinlocks: remove 'volatile'

Keith Owens
2006-07-09T03:43:42Z

Linus Torvalds (on Sat, 8 Jul 2006 20:29:12 -0700 (PDT)) wrote:
>
>
>On Sun, 9 Jul 2006, Keith Owens wrote:
>>
>> This disagrees with the gcc (4.1.0) docs. info --index-search='Extended Asm' gcc
>>
>> The ordinary output operands must be write-only; GCC will assume
>> that the values in these operands before the instruction are dead and
>> need not be generated. Extended asm supports input-output or
>> read-write operands. Use the constraint character `+' to indicate
>> such an operand and list it with the output operands. You should
>> only use read-write operands when the constraints for the operand (or
>> the operand in which only some of the bits are to be changed) allow a
>> register.
>
>I'm fairly sure the docs are outdated (but may well be correct for older
>gcc versions - as I already discussed elsewhere, that "+" thing was not
>historically useful).
>
>We've been using "+m" for some time in the kernel on several
>architectures.
>
>git aficionados can do
>
> git grep -1 '"+m"' v2.6.17
>
>to see the pre-existing usage of this (most of them go back a lot further,
>although some of them are newer - the <asm-i386/bitops.h> ones were added
>in January.
>
>So if "+m" didn't work, we've been in trouble for at least the last year.

The oldest gcc I can access quickly is 3.2.3 (May 2005) which has
significantly different wording.

The ordinary output operands must be write-only; GCC will assume that
the values in these operands before the instruction are dead and need
not be generated. Extended asm supports input-output or read-write
operands. Use the constraint character `+' to indicate such an
operand and list it with the output operands.

When the constraints for the read-write operand (or the operand in
which only some of the bits are to be changed) allows a register, you
may, as an alternative, logically split its function into two
separate operands, one input operand and one write-only output
operand. The connection between them is expressed by constraints
which say they need to be in the same location when the instruction
executes.

In 3.2.3, the '+' form allows any constraint, only the split notation
requires that the operand be in a register. In 4.1.0, the docs now say
that the '+' form requires a register. I suspect that you are right
and the gcc docs are incorrect, but it does not hurt to check. Would
anybody from the gcc team like to pipe up and give an opinion?