Re: [PATCH] - Reduce TLB flushing during process migration

[John Richard Moser <nigelenki@comcast.net>]

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David Mosberger wrote:
|>>>>>On Fri, 2 Jul 2004 12:39:05 -0500, Jack Steiner <steiner@sgi.com>
said:
|
| Well, then it's not a true no-op.  The other no-ops are all for
| init-type stuff, so they're not at all performance critical.  Even
| when compiling a non-generic kernel, those no-op functions will be
| called.  This is really a limitation in the current machvec-scheme.  I
| think what we need is a way to explicitly declare a no-op callback,
| such that it can be optimized away completely for platforms that don't
| need it.  Perhaps there could be something along the lines of:
|
| #ifdef CONFIG_IA64_GENERIC
| # define machvec_noop(noop_function)	noop_function
| #else
| # define machvec_noop(noop_function)	/* empty */
| #endif
|

?????

1.  At any -O level, I'd assume the loop optimizer or something along
those lines would see a 1 iteration loop and remove the actual loop
instruction:

MOV CX,1 ; 1 iteration
...
LOOP ; Just DEC CX and falls through

In the pseudocode above, we can clearly see that the MOV and the LOOP
perform no real function.  The optimizer *should* AFAIK notice that a
do{...}while(0) generates useless looping instructions and thus prevent
those instructions from being generated.  In the case of do{}while(0),
this generates 0 instructions.

2.  A function call to a no-op is an unoptimizable and excessive
overhead compared to do{}while(0) in most cases; the no-op would need to
be inline assembly, thus maintained for each arch, AFAIK.

3.  An actual no-op instruction still needs to be iterated past by the
processor.  Usually, no-op is only generated by the compiler for
padding, AFAIK; and even then, there's some kind of jump before the load
of no-ops to get past them.  I don't know if inline assembly is
optimized out; but I would assume not.

4.  I don't think /* empty */; is an error; but the following block has
problems:

if (foo)
~  ;
else
~  do_something();

So, we need something that gets parsed to exactly nothing.

Thus, as far as I can understand, because we use -O2 or -Os, we will
always get exactly 0 instructions from do{}while(0).  This is the best
solution to the problem.
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