WANTED: compiler barrier for use within Wine

[David Laight]

On Thu, Nov 01, 2012 at 09:25:53AM +0100, Joerg-Cyril.Hoehle at t-systems.com wrote:
> 
> Background: I want an atomic read of whatever value a single aligned
> int (or DOWRD or whatever) currently appears visible to one core.

A simple read of a volatile data item will do that.
Look at what signal handlers are allowed to change.

New compilers link clang support the gcc asm semantics - they need to
in order to compile a lot of their target code.

The purpose of 'asm volatile ("":::""memory);' is to constrain
the values that the compiler can have cached in registers.

I use it for 3 purposes:
1) To ensure object code accesses memory in the correct order (provided
   the cpu can't reorder the memory accesses itself (which many modern
   cpu will do - at least for cached accesses).
2) To reduce the number 'live' values in a function to stop values
   being spilled to stack.
3) To force the generated code to read values early (sometimes before
   an 'if' when the value is only used in one clause) in order to
   remove cpu stalls waiting for memory and to fill otherwise
   unfillable delay slots.
   This one is very cpu specific - but I am counting every instruction
   in that function.
   (This is a MIPS-like embedded cpu.)

	David

-- 
David Laight: david at l8s.co.uk