Re: Optimization breaks inline asm code w/ptrs

[Michael Matz <matz at suse dot de>]

Hi,

On Mon, 14 Aug 2017, Alan Modra wrote:

> I've opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81845 to track 
> the lack of documentation.

You mean like in this paragraph discussing memory clobbers and uses in 
extended asms that we have since 2004? :

----------------
 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:

     {"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.

 Note that in the following example the memory input is necessary,
otherwise GCC might optimize the store to `x' away:
     int foo ()
     {
       int x = 42;
       int *y = &x;
       int result;
       asm ("magic stuff accessing an 'int' pointed to by '%1'"
             "=&d" (r) : "a" (y), "m" (*y));
       return result;
     }
----------------


Ciao,
Michael.