Re: A question about "memory" clobbers in asm

[Jamie Lokier <jamie at shareable dot org>]

Nathan Sidwell wrote:
> my understanding is
> 
> * volatile asm says something to the effect 'this changes state that you
> (the compiler) don't know about' -- such as writing to an IO port.
> 
> * memory clobber says 'this asm will change memory, so don't cache anything
> across this asm'.

You and Ian agree, and that makes sense to me.

So I think the semantics are fine, but the documentation is unclear.

You have both explained the semantics in clear and concise language, so
my only issue is with the documentation.

Specifically, "You will also want to add the `volatile' keyword if the
memory affected is not listed in the inpus or outputs of the `asm'" is
misleading.

How about changing:

   If your assembler instructions access memory in an unpredictable
   fashion, add `memory' to the list of clobbered registers.  This will
   cause GCC to not keep memory values cached in registers across the
   assembler instruction and not optimize stores or loads to that memory.
   You will also want to add the `volatile' keyword if the memory affected
   is not listed in the inputs or outputs of the `asm', as the `memory'
   clobber does not count as a side-effect of the `asm'.  If you know how
   large the accessed memory is, you can add it as input or output but if
   this is not known, you should add `memory'.  As an example, if you
   access ten bytes of a string, you can use a memory input like:

to:

   If your assembler instruction loads from memory which is not listed
   in the inputs of the `asm', or stores to memory which is not listed
   in the outputs, add `memory' to the list of clobbered registers.
   This will cause GCC to not keep memory values cached in registers
   across the assembler instruction and not optimize stores or loads
   to that memory.

and later add:

   When to clobber `memory' and when to use `volatile'
   ...................................................

   Clobbering `memory' is independent of `volatile', although it is
   usually the case that if you clobber `memory' then you want
   `volatile' as well.

   Putting `memory' in the list of clobbered registers tells the
   compiler to not keep memory values cached acress the `asm'
   instruction, and not make other assumptions about the contents of
   memory.  (This is sometimes called a compiler memory barrier).  The
   compiler may still cache local variables whose addresses have not
   been taken.  Unless `volatile' is also specified, the `asm' can
   still be deleted, moved, or two combined if they're a common
   subexpression.

   Writing the `volatile' keyword after `asm' tells the compiler not
   to delete or significantly move the `asm' instruction, or combine
   two of them if they're a common subexpression.  Unless `memory' is
   clobbered as well, the compiler is permitted to cache memory values
   across an `asm volatile'.

How about that?

-- Jamie