Re: [Patch] 1/3 Add new builtin __builtin_flush_icache().

[David Daney <ddaney@avtrex.com>]

Mark Mitchell wrote:
> David Daney wrote:
>
>   
>> Ok, I think I have come around to your way of thinking.  I still have
>> the new predicate __builtin_clear_cache_inline_p() for use in target
>> CLEAR_INSN_CACHE definitions, but I leave __clear_cache unchanged.
>>     
>
> I don't understand the need for the inline_p function.  It seems to me
> there are two scenarios:
>
> 1. The back-end has an inline definition of __builtin___clear_cache.  In
> that case, it does:
>
>   #define CLEAR_INSN_CACHE(BEG, END) __builtin___clear_cache(BEG, END);
>
> 2. The back-end does not have an inline definition of
> __builtin_clear_cache.  In that case, it does:
>
>   #define CLEAR_INSN_CACHE(BEG, END) \
>     /* Something not involving __builtin___clear_cache */
>
> In both cases, the libgcc __clear_cache routine just does:
>
>   CLEAR_INSN_CACHE (beg, end);
>
> And, in both cases, the user can write either __clear_cache (always an
> out-of-line call), or __builtin_clear_cache (may be an out-of-line call,
> or may be inline code).
>
> Is your concern that in case (1) the back-end has to do two things:
> define the builtin and define CLEAR_INSN_CACHE?  That's a little lame,
> but it doesn't seem worse than having to define two builtins.
>
> Am I missing some intermediate case?
>   

The MIPS family has some members that can clear the instruction cache 
with in-line instructions from user space.  Other members of the family 
can only clear the instruction cache from kernel space and must make a 
system call (perhaps via __clear_cache() in libgcc).  There are command 
line switches that select the ISA, and thus for which of these cases we 
are generating code.

When Paolo Bonzini wrote:
 > try to see how __clear_cache could use the definition of the builtin.

I realized that if one were not careful, infinite recursion might result.

The idea behind __builtin_clear_cache_inline_p() is that it allows us to 
test in source code which of these two cases are in effect at compile 
time.  At the time we write a CLEAR_INSN_CACHE macro, we don't know 
which ISA will be targeted when libgcc is built.

When __builtin___clear_cache() expands to a library call to 
__clear_cache() in libgcc, we cannot have __clear_cache() be implemented 
by __builtin___clear_cache() or we would recurse infinitely.  I envision 
that __builtin_clear_cache_inline_p() would be tested in like this:

#define CLEAR_INSN_CACHE(BEG, END)				\
{								\
  extern void _flush_cache (char *b, int l, int f);		\
  if (__builtin_clear_cache_inline_p())				\
    __builtin___clear_cache ((BEG), (END));			\
  else								\
    _flush_cache ((BEG), ((char *)(END) - (char *)(BEG)), 3);	\
}

The same thing could probably be achieved by setting and examining a set of preprocessor symbols, but I think having a single well defined predicate is a cleaner solution.

That said, after corresponding with Richard Sandiford about the MIPS portion of the patch, I think that I will expand the calls to _flush_cache() directly in the back-end rather than call to __clear_cache() in libgcc which would then do the system call.  So for MIPS __builtin_clear_cache_inline_p() would be unconditionally true.  i386 and x86_64 don't need to clear their instruction caches, so for these targets  __builtin_clear_cache_inline_p() would likewise be true unconditionally.

IIRC there are only two targets that currently define CLEAR_INSN_CACHE, they are arm and m86k.  I don't know if __builtin_clear_cache_inline_p() would be useful for them.

I hope that explains what I was thinking.

Not knowing the intimate details of the majority of GCC targets, I do not know if  
__builtin_clear_cache_inline_p() would ever used for anything.  Hypothetically if arm were to add instructions that flushed the cache without making a system call, something like __builtin_clear_cache_inline_p() would be useful in writing the CLEAR_INSN_CACHE macro.

If it is deemed to be useless, I will remove __builtin_clear_cache_inline_p() from the patch as it is unneeded for my current target of interest (MIPS).

David Daney