mimalloc: offer a build-time option to enable it

By defining `USE_MIMALLOC`, Git can now be compiled with that
nicely-fast and small allocator.

Note that we have to disable a couple `DEVELOPER` options to build
mimalloc's source code, as it makes heavy use of declarations after
statements, among other things that disagree with Git's conventions.

For example, the `-Wno-array-bounds` flag is needed because in `-O2`
builds, trying to call `NtCurrentTeb()` (which `_mi_thread_id()` does on
Windows) causes the bogus warning about a system header, likely related
to https://sourceforge.net/p/mingw-w64/mailman/message/37674519/ and to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578:

C:/git-sdk-64-minimal/mingw64/include/psdk_inc/intrin-impl.h:838:1:
        error: array subscript 0 is outside array bounds of 'long long unsigned int[0]' [-Werror=array-bounds]
  838 | __buildreadseg(__readgsqword, unsigned __int64, "gs", "q")
      | ^~~~~~~~~~~~~~

Also: The `mimalloc` library uses C11-style atomics, therefore we must
require that standard when compiling with GCC if we want to use
`mimalloc` (instead of requiring "only" C99). This is what we do in the
CMake definition already, therefore this commit does not need to touch
`contrib/buildsystems/`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Makefile

@@ -1942,6 +1942,39 @@ ifdef USE_NED_ALLOCATOR
 	OVERRIDE_STRDUP = YesPlease
 endif
 
+ifdef USE_MIMALLOC
+	MIMALLOC_OBJS = \
+		compat/mimalloc/alloc-aligned.o \
+		compat/mimalloc/alloc.o \
+		compat/mimalloc/arena.o \
+		compat/mimalloc/bitmap.o \
+		compat/mimalloc/heap.o \
+		compat/mimalloc/init.o \
+		compat/mimalloc/options.o \
+		compat/mimalloc/os.o \
+		compat/mimalloc/page.o \
+		compat/mimalloc/random.o \
+		compat/mimalloc/segment.o \
+		compat/mimalloc/segment-cache.o \
+		compat/mimalloc/stats.o
+
+	COMPAT_CFLAGS += -Icompat/mimalloc -DMI_DEBUG=0 -DUSE_MIMALLOC --std=gnu11
+	COMPAT_OBJS += $(MIMALLOC_OBJS)
+
+$(MIMALLOC_OBJS): COMPAT_CFLAGS += -DBANNED_H
+
+ifdef DEVELOPER
+$(MIMALLOC_OBJS): COMPAT_CFLAGS += \
+	-Wno-attributes \
+	-Wno-pedantic \
+	-Wno-unknown-pragmas \
+	-Wno-declaration-after-statement \
+	-Wno-old-style-definition \
+	-Wno-missing-prototypes \
+	-Wno-array-bounds
+endif
+endif
+
 ifdef OVERRIDE_STRDUP
 	COMPAT_CFLAGS += -DOVERRIDE_STRDUP
 	COMPAT_OBJS += compat/strdup.o

config.mak.dev

@@ -22,8 +22,10 @@ endif
 
 ifneq ($(uname_S),FreeBSD)
 ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang7,$(COMPILER_FEATURES))),)
+ifndef USE_MIMALLOC
 DEVELOPER_CFLAGS += -std=gnu99
 endif
+endif
 else
 # FreeBSD cannot limit to C99 because its system headers unconditionally
 # rely on C11 features.

config.mak.uname

@@ -480,7 +480,7 @@ endif
 	CC = compat/vcbuild/scripts/clink.pl
 	AR = compat/vcbuild/scripts/lib.pl
 	CFLAGS =
-	BASIC_CFLAGS = -nologo -I. -Icompat/vcbuild/include -DWIN32 -D_CONSOLE -DHAVE_STRING_H -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE
+	BASIC_CFLAGS = -nologo -I. -Icompat/vcbuild/include -DWIN32 -D_CONSOLE -DHAVE_STRING_H -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE -MP -std:c11
 	COMPAT_OBJS = compat/msvc.o compat/winansi.o \
 		compat/win32/flush.o \
 		compat/win32/path-utils.o \

git-compat-util.h

@@ -371,6 +371,16 @@ char *gitdirname(char *);
 # include <sys/sysinfo.h>
 #endif
 
+#ifdef USE_MIMALLOC
+#include "mimalloc.h"
+#define malloc mi_malloc
+#define calloc mi_calloc
+#define realloc mi_realloc
+#define free mi_free
+#define strdup mi_strdup
+#define strndup mi_strndup
+#endif
+
 /* On most systems <netdb.h> would have given us this, but
  * not on some systems (e.g. z/OS).
  */