Re: introduce overridable clear_cache emitter

[Jeff Law <law@redhat.com>]

On 11/10/20 7:35 PM, Alexandre Oliva wrote:
> This patch introduces maybe_emit_call_builtin___clear_cache for the
> builtin expander machinery and the trampoline initializers to use to
> clear the instruction cache, removing a source of inconsistencies and
> subtle errors in low-level machinery.
>
> I've adjusted all trampoline_init implementations that used to issue
> explicit calls to __clear_cache or similar to use this new primitive.
>
>
> Specifically on vxworks targets, we needed to drop the __clear_cache
> symbol in libgcc, for reasons related with linking that I didn't need
> to understand, and we wanted to call cacheTextUpdate directly, despite
> the different calling conventions: the second argument is a length
> rather than the end address.
>
> So I introduced a target hook to enable target OS-level overriding of
> builtin __clear_cache call emission, retaining nearly (*) the same
> logic to govern the decision on whether to emit a call (or nothing, or
> a machine-dependent insn) but enabling a call to a target
> system-defined function with different calling conventions to be
> issued, without having to modify .md files of the various
> architectures supported by the target system to introduce or modify
> clear_cache insns.
>
> (*) I write "nearly" mainly because, when not optimizing, we'd issue a
> call regardless, but since the call may now be overridden, I added it
> to the set of builtins that are not directly turned into calls when
> not optimizing, following the normal expansion path instead.  It
> wouldn't be hard to skip the emission of cache-clearing insns when not
> optimizing, but it didn't seem very important, especially for the new
> uses from trampoline init.
>
>     Another difference that might be relevant is that now we expand
> the begin and end arguments unconditionally.  This might make a
> difference if they have side effects.  That's prettty much impossible
> at expand time, but I thought I'd mention it.
>
>
> I have NOT modified targets that did not issue cache-clearing calls in
> trampoline init to use the new clear_cache-calling infrastructure even
> if it would expand to nothing.  I have considered doing so, to have
> __builtin___clear_cache and trampoline init call cacheTextUpdate on
> all vxworks targets, but decided not to, since on targets that don't
> do any cache clearing, cacheTextUpdate ought to be a no-op, even
> though rs6000 seems to use icbi and dcbf instructions in the function
> called to initialize a trampoline, but AFAICT not in the __clear_cache
> builtin.  Hopefully target maintainers will have a look and take
> advantage of this new piece of infrastructure to remove such
> (apparent?) inconsistencies.  Not rs6000 and other that call asm-coded
> trampoline setup instructions, for sure, but they might wish to
> introduce a CLEAR_INSN_CACHE macro or a clear_cache expander if they
> don't have one.
>
>
> This was regstrapped on x86_64-linux-gnu, and a trivial backport was
> tested on multiple vxworks targets.  Ok to install?
>
>
> for  gcc/ChangeLog
>
> 	* builtins.c (default_emit_call_builtin___clear_cache): New.
> 	(maybe_emit_call_builtin___clear_cache): New.
> 	(expand_builtin___clear_cache): Split into the above.
> 	(expand_builtin): Do not issue clear_cache call any more.
> 	* builtins.h (maybe_emit_call_builtin___clear_cache): Declare.
> 	* config/aarch64/aarch64.c (aarch64_trampoline_init): Use
> 	maybe_emit_call_builtin___clear_cache.
> 	* config/arc/arc.c (arc_trampoline_init): Likewise.
> 	* config/arm/arm.c (arm_trampoline_init): Likewise.
> 	* config/c6x/c6x.c (c6x_initialize_trampoline): Likewise.
> 	* config/csky/csky.c (csky_trampoline_init): Likewise.
> 	* config/m68k/linux.h (FInALIZE_TRAMPOLINE): Likewise.
> 	* config/tilegx/tilegx.c (tilegx_trampoline_init): Likewise.
> 	* config/tilepro/tilepro.c (tilepro_trampoline_init): Ditto.
> 	* config/vxworks.c: Include rtl.h, memmodel.h, and optabs.h.
> 	(vxworks_emit_call_builtin___clear_cache): New.
> 	* config/vxworks.h (CLEAR_INSN_CACHE): Drop.
> 	(TARGET_EMIT_CALL_BUILTIN___CLEAR_CACHE): Define.
> 	* target.def (trampoline_init): In the documentation, refer to
> 	maybe_emit_call_builtin___clear_cache.
> 	(emit_call_builtin___clear_cache): New.
> 	* doc/tm.texi.in: Add new hook point.
> 	(CLEAR_CACHE_INSN): Remove duplicate 'both'.
> 	* doc/tm.texi: Rebuilt.
> 	* targhooks.h (default_meit_call_builtin___clear_cache):
> 	Declare.
> 	* tree.h (BUILTIN_ASM_NAME_PTR): New.
>
> for  libgcc/ChangeLog
>
> 	* config/t-vxworks (LIB2ADD): Drop.
> 	* config/t-vxworks7 (LIB2ADD): Likewise.
> 	* config/vxcache.c: Remove.
OK
jeff