[Patch] 1/3 Add new builtin __builtin_flush_icache().
David Daney
ddaney@avtrex.com
Fri Jul 6 06:19:00 GMT 2007
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
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