Re: Update doc/extend.texi regarding asm memory clobbers

[Andreas Jaeger <aj at suse dot de>]

Michael Matz <matz@suse.de> writes:

> On Fri, 16 Jan 2004, Andreas Jaeger wrote:
>
>> +@example
>> +@{"m"( (@{ struct @{ char x[10]; @} *p = (void *)ptr ; *p @}) )@}.
>                                                            ^^^
> semicolon missing
>
>> +int foo (int *x)
>> +@{
>> +  int y = bar(x);
>> +  int r;
>> +  asm ("magic stuff accessing (%1)..."
>> +        "=&d" (r) : "a" (y), "m" (*y));
>> +  return r;
>
> 'y' must be a pointer to be dereferencable ;)

Good idea ;-)

> I suggest something like this:
>
> int foo ()
> {
>   int x = 42;
>   int *y = &x;
>   int result;
>   asm ("stuff accessing an 'int' pointed to by '%1'"
>        "=&r" (result) : "r" (y), "m" (*y));
>   return result;
> }
>
> And what could be optimized away is the store to x.

Ok, adjusted.  What do you think of our ;-) patch now?

Cheers,
Andreas

2004-01-16  Andreas Jaeger  <aj@suse.de>
	    Michael Matz  <matz@suse.de>

	* doc/extend.texi (Extended Asm): Clarify memory clobber.

============================================================
Index: gcc/doc/extend.texi
--- gcc/doc/extend.texi	7 Jan 2004 22:24:39 -0000	1.177
+++ gcc/doc/extend.texi	16 Jan 2004 17:04:23 -0000
@@ -4006,13 +4006,35 @@ represents the condition codes as a spec
 condition code is handled differently, and specifying @samp{cc} has no
 effect.  But it is valid no matter what the machine.
 
-If your assembler instruction modifies memory in an unpredictable
+If your assembler instructions access memory in an unpredictable
 fashion, add @samp{memory} to the list of clobbered registers.  This
-will cause GCC to not keep memory values cached in registers across
-the assembler instruction.  You will also want to add the
-@code{volatile} keyword if the memory affected is not listed in the
-inputs or outputs of the @code{asm}, as the @samp{memory} clobber does
-not count as a side-effect of the @code{asm}.
+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 @code{volatile} keyword if the memory
+affected is not listed in the inputs or outputs of the @code{asm}, as
+the @samp{memory} clobber does not count as a side-effect of the
+@code{asm}.  If you known how large the accessed memory is, you can add
+it as input or output but if this is not known, you should add
+@samp{memory}.  As an example, if you access ten bytes of a string, you
+can use a memory input like:
+
+@example
+@{"m"( (@{ struct @{ char x[10]; @} *p = (void *)ptr ; *p; @}) )@}.
+@end example
+
+Note that in the following example the memory input is necessary,
+otherwise GCC might optimize the store to @code{x} away:
+@example
+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;     
+@}
+@end example
 
 You can put multiple assembler instructions together in a single
 @code{asm} template, separated by the characters normally used in assembly

-- 
 Andreas Jaeger, aj@suse.de, http://www.suse.de/~aj
  SuSE Linux AG, Maxfeldstr. 5, 90409 Nürnberg, Germany
   GPG fingerprint = 93A3 365E CE47 B889 DF7F  FED1 389A 563C C272 A126

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