WIP/NFC: create GC interface for third-party GCs (JuliaLang#55256)

Prelude to the MMTk integration.

Creates an interface header specifying the functions that a third-party
GC must implement to plug into Julia, and also splits the stock Julia GC
implementation into a separate file `gc-stock.c`.

In the future, we'll include a few pre-processor guards to ensure some
of these files are not compiled if MMTk is enabled.

A WIP design document describing the rationale behind the interface and
the plan for MMTk integration should be in this [design
document](https://docs.google.com/document/d/1v0jtSrIpdEDNOxj5S9g1jPqSpuAkNWhr_T8ToFC9RLI/edit)
(feedback is welcome at this stage!).

## TODO
- [x] Ensure functions in `gc-interface.h` are only declared in this
header.
- [x] Add allocation fast-path & codegen write-barriers into the
interface.

Author: d-netto

base/timing.jl

@@ -1,6 +1,6 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-# This type must be kept in sync with the C struct in src/gc.h
+# This type must be kept in sync with the C struct in src/gc-interface.h
 struct GC_Num
     allocd          ::Int64 # GC internal
     deferred_alloc  ::Int64 # GC internal
@@ -47,7 +47,7 @@ gc_total_bytes(gc_num::GC_Num) =
     gc_num.allocd + gc_num.deferred_alloc + gc_num.total_allocd
 
 function GC_Diff(new::GC_Num, old::GC_Num)
-    # logic from `src/gc.c:jl_gc_total_bytes`
+    # logic from `jl_gc_total_bytes`
     old_allocd = gc_total_bytes(old)
     new_allocd = gc_total_bytes(new)
     return GC_Diff(new_allocd       - old_allocd,

doc/src/devdocs/object.md

@@ -92,7 +92,7 @@ The corresponding global `jl_datatype_t` objects are created by [`jl_init_types`
 
 The garbage collector uses several bits from the metadata portion of the `jl_typetag_t` to track
 each object in the system. Further details about this algorithm can be found in the comments of
-the [garbage collector implementation in `gc.c`](https://github.com/JuliaLang/julia/blob/master/src/gc.c).
+the [garbage collector implementation in `gc-stock.c`](https://github.com/JuliaLang/julia/blob/master/src/gc-stock.c).
 
 ## Object allocation
 
@@ -179,7 +179,7 @@ jl_value_t *newstruct(jl_value_t *type);
 jl_value_t *newobj(jl_value_t *type, size_t nfields);
 ```
 
-And at the lowest level, memory is getting allocated by a call to the garbage collector (in `gc.c`),
+And at the lowest level, memory is getting allocated by a call to the garbage collector (in `gc-stock.c`),
 then tagged with its type:
 
 ```c

src/Makefile

@@ -44,7 +44,7 @@ SRCS := \
 	jltypes gf typemap smallintset ast builtins module interpreter symbol \
 	dlload sys init task array genericmemory staticdata toplevel jl_uv datatype \
 	simplevector runtime_intrinsics precompile jloptions mtarraylist \
-	threading scheduler stackwalk gc gc-debug gc-pages gc-stacks gc-alloc-profiler gc-page-profiler method \
+	threading scheduler stackwalk gc-common gc-stock gc-debug gc-pages gc-stacks gc-alloc-profiler gc-page-profiler method \
 	jlapi signal-handling safepoint timing subtype rtutils gc-heap-snapshot \
 	crc32c APInt-C processor ircode opaque_closure codegen-stubs coverage runtime_ccall engine
 
@@ -103,7 +103,7 @@ ifeq ($(USE_SYSTEM_LIBUV),0)
 UV_HEADERS += uv.h
 UV_HEADERS += uv/*.h
 endif
-PUBLIC_HEADERS := $(BUILDDIR)/julia_version.h $(wildcard $(SRCDIR)/support/*.h) $(addprefix $(SRCDIR)/,work-stealing-queue.h gc-tls.h julia.h julia_assert.h julia_threads.h julia_fasttls.h julia_locks.h julia_atomics.h jloptions.h)
+PUBLIC_HEADERS := $(BUILDDIR)/julia_version.h $(wildcard $(SRCDIR)/support/*.h) $(addprefix $(SRCDIR)/,work-stealing-queue.h gc-interface.h gc-tls.h julia.h julia_assert.h julia_threads.h julia_fasttls.h julia_locks.h julia_atomics.h jloptions.h)
 ifeq ($(OS),WINNT)
 PUBLIC_HEADERS += $(addprefix $(SRCDIR)/,win32_ucontext.h)
 endif
@@ -316,11 +316,11 @@ $(BUILDDIR)/codegen.o $(BUILDDIR)/codegen.dbg.obj: $(addprefix $(SRCDIR)/,\
 $(BUILDDIR)/datatype.o $(BUILDDIR)/datatype.dbg.obj: $(SRCDIR)/support/htable.h $(SRCDIR)/support/htable.inc
 $(BUILDDIR)/debuginfo.o $(BUILDDIR)/debuginfo.dbg.obj: $(addprefix $(SRCDIR)/,debuginfo.h processor.h jitlayers.h debug-registry.h)
 $(BUILDDIR)/disasm.o $(BUILDDIR)/disasm.dbg.obj: $(SRCDIR)/debuginfo.h $(SRCDIR)/processor.h
-$(BUILDDIR)/gc-debug.o $(BUILDDIR)/gc-debug.dbg.obj: $(SRCDIR)/gc.h
-$(BUILDDIR)/gc-pages.o $(BUILDDIR)/gc-pages.dbg.obj: $(SRCDIR)/gc.h
-$(BUILDDIR)/gc.o $(BUILDDIR)/gc.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-heap-snapshot.h $(SRCDIR)/gc-alloc-profiler.h $(SRCDIR)/gc-page-profiler.h
-$(BUILDDIR)/gc-heap-snapshot.o $(BUILDDIR)/gc-heap-snapshot.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-heap-snapshot.h
-$(BUILDDIR)/gc-alloc-profiler.o $(BUILDDIR)/gc-alloc-profiler.dbg.obj: $(SRCDIR)/gc.h $(SRCDIR)/gc-alloc-profiler.h
+$(BUILDDIR)/gc-debug.o $(BUILDDIR)/gc-debug.dbg.obj: $(SRCDIR)/gc-common.h $(SRCDIR)/gc-stock.h
+$(BUILDDIR)/gc-pages.o $(BUILDDIR)/gc-pages.dbg.obj: $(SRCDIR)/gc-common.h $(SRCDIR)/gc-stock.h
+$(BUILDDIR)/gc-stock.o $(BUILDDIR)/gc.dbg.obj: $(SRCDIR)/gc-common.h $(SRCDIR)/gc-stock.h $(SRCDIR)/gc-heap-snapshot.h $(SRCDIR)/gc-alloc-profiler.h $(SRCDIR)/gc-page-profiler.h
+$(BUILDDIR)/gc-heap-snapshot.o $(BUILDDIR)/gc-heap-snapshot.dbg.obj: $(SRCDIR)/gc-heap-snapshot.h
+$(BUILDDIR)/gc-alloc-profiler.o $(BUILDDIR)/gc-alloc-profiler.dbg.obj: $(SRCDIR)/gc-alloc-profiler.h
 $(BUILDDIR)/gc-page-profiler.o $(BUILDDIR)/gc-page-profiler.dbg.obj: $(SRCDIR)/gc-page-profiler.h
 $(BUILDDIR)/init.o $(BUILDDIR)/init.dbg.obj: $(SRCDIR)/builtin_proto.h
 $(BUILDDIR)/interpreter.o $(BUILDDIR)/interpreter.dbg.obj: $(SRCDIR)/builtin_proto.h
@@ -331,10 +331,10 @@ $(BUILDDIR)/llvm-alloc-helpers.o $(BUILDDIR)/llvm-alloc-helpers.dbg.obj: $(SRCDI
 $(BUILDDIR)/llvm-alloc-opt.o $(BUILDDIR)/llvm-alloc-opt.dbg.obj: $(SRCDIR)/llvm-codegen-shared.h $(SRCDIR)/llvm-pass-helpers.h $(SRCDIR)/llvm-alloc-helpers.h
 $(BUILDDIR)/llvm-cpufeatures.o $(BUILDDIR)/llvm-cpufeatures.dbg.obj: $(SRCDIR)/jitlayers.h
 $(BUILDDIR)/llvm-demote-float16.o $(BUILDDIR)/llvm-demote-float16.dbg.obj: $(SRCDIR)/jitlayers.h
-$(BUILDDIR)/llvm-final-gc-lowering.o $(BUILDDIR)/llvm-final-gc-lowering.dbg.obj: $(SRCDIR)/llvm-pass-helpers.h $(SRCDIR)/llvm-codegen-shared.h
+$(BUILDDIR)/llvm-final-gc-lowering.o $(BUILDDIR)/llvm-final-gc-lowering.dbg.obj: $(SRCDIR)/llvm-gc-interface-passes.h
 $(BUILDDIR)/llvm-gc-invariant-verifier.o $(BUILDDIR)/llvm-gc-invariant-verifier.dbg.obj: $(SRCDIR)/llvm-codegen-shared.h
 $(BUILDDIR)/llvm-julia-licm.o $(BUILDDIR)/llvm-julia-licm.dbg.obj: $(SRCDIR)/llvm-codegen-shared.h $(SRCDIR)/llvm-alloc-helpers.h $(SRCDIR)/llvm-pass-helpers.h
-$(BUILDDIR)/llvm-late-gc-lowering.o $(BUILDDIR)/llvm-late-gc-lowering.dbg.obj: $(SRCDIR)/llvm-pass-helpers.h $(SRCDIR)/llvm-codegen-shared.h
+$(BUILDDIR)/llvm-late-gc-lowering.o $(BUILDDIR)/llvm-late-gc-lowering.dbg.obj: $(SRCDIR)/llvm-gc-interface-passes.h
 $(BUILDDIR)/llvm-lower-handlers.o $(BUILDDIR)/llvm-lower-handlers.dbg.obj: $(SRCDIR)/llvm-codegen-shared.h
 $(BUILDDIR)/llvm-multiversioning.o $(BUILDDIR)/llvm-multiversioning.dbg.obj: $(SRCDIR)/llvm-codegen-shared.h $(SRCDIR)/processor.h
 $(BUILDDIR)/llvm-pass-helpers.o $(BUILDDIR)/llvm-pass-helpers.dbg.obj: $(SRCDIR)/llvm-pass-helpers.h $(SRCDIR)/llvm-codegen-shared.h
@@ -348,7 +348,7 @@ $(BUILDDIR)/toplevel.o $(BUILDDIR)/toplevel.dbg.obj: $(SRCDIR)/builtin_proto.h
 $(BUILDDIR)/ircode.o $(BUILDDIR)/ircode.dbg.obj: $(SRCDIR)/serialize.h $(SRCDIR)/common_symbols1.inc $(SRCDIR)/common_symbols2.inc
 $(BUILDDIR)/pipeline.o $(BUILDDIR)/pipeline.dbg.obj: $(SRCDIR)/passes.h $(SRCDIR)/jitlayers.h
 
-$(addprefix $(BUILDDIR)/,threading.o threading.dbg.obj gc.o gc.dbg.obj init.c init.dbg.obj task.o task.dbg.obj): $(addprefix $(SRCDIR)/,threading.h)
+$(addprefix $(BUILDDIR)/,threading.o threading.dbg.obj gc-common.o gc-stock.o gc.dbg.obj init.c init.dbg.obj task.o task.dbg.obj): $(addprefix $(SRCDIR)/,threading.h)
 $(addprefix $(BUILDDIR)/,APInt-C.o APInt-C.dbg.obj runtime_intrinsics.o runtime_intrinsics.dbg.obj): $(SRCDIR)/APInt-C.h
 
 # archive library file rules

src/gc-alloc-profiler.cpp

@@ -3,7 +3,6 @@
 #include "gc-alloc-profiler.h"
 
 #include "julia_internal.h"
-#include "gc.h"
 
 #include "llvm/ADT/SmallVector.h"