Re: [PATCH] make atomic_t volatile on all architectures
From: Segher Boessenkool
Date: Sun Aug 12 2007 - 06:36:21 EST
Yes, though I would use "=m" on the output list and "m" on the input
list. The reason is that I've seen gcc fall on its face with an ICE
on
s390 due to "+m". The explanation I've got from our compiler people
was
quite esoteric, as far as I remember gcc splits "+m" to an input
operand
and an output operand. Now it can happen that the compiler chooses
two
different registers to access the same memory location. "+m" requires
that the two memory references are identical which causes the ICE if
they are not.
The problem is very nicely described here, last paragraph:
<http://gcc.gnu.org/ml/gcc-patches/2007-07/msg01816.html>
It's not a problem anymore in (very) recent GCC, although
that of course won't help you in the kernel (yet).
So you are saying that gcc 3.x still has this problem ?
Yes. A warning ("read-write constraint does not allow a register")
was added for GCC-3.4, but the fix/workaround is more recent
(4.2 I believe, perhaps it was backported to the 4.1 branch).
I do not know if the current compilers still do this. Has
anyone else seen this happen ?
In recent GCC, it's actually documented:
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.
Note that last line.
I see, thanks for the info.
My pleasure.
Segher
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