fixup! mimalloc: offer a build-time option to enable it

Re-running the `git repack -adfq` benchmark from
6a29c2d ("mingw: use mimalloc",
2019-06-24) against the platform's *current* default allocator (so
without `nedmalloc` in the picture at all) shows mimalloc is no longer
faster than the system allocator on any of Windows, macOS, or Linux,
neither for the original ~30-second `linux v2.6.20` workload nor for a
4x larger `linux v3.0` workload where each individual run takes ~2
minutes (and the noise floor on Linux is below 0.3% of the mean, so
even small differences would be visible if any existed).

`mimalloc` was originally chosen over nedmalloc, not over the system
allocator. Six years on, with nedmalloc now being dropped from the
codebase entirely, the allocator that mimalloc has to beat is whatever
the OS ships by default; modern Windows segment-heap, glibc malloc, and
the macOS libsystem allocator have all closed the gap, and there is no
longer a measurable benefit to keep maintaining a custom allocator.

The actual benchmark methodology, the per-platform numbers, and links
to the workflow runs that produced them are spelled out in the PR
description rather than repeated across each fixup.

The `fixup!` subject is so that the next rebase against an upstream
Git that already lacks this commit will autosquash this revert into
the original (which becomes empty and is dropped), leaving the tree
free of `mimalloc`.

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

Author: dscho

Makefile

@@ -1519,7 +1519,6 @@ BUILTIN_OBJS += builtin/write-tree.o
 # upstream unnecessarily (making merging in future changes easier).
 THIRD_PARTY_SOURCES += compat/inet_ntop.c
 THIRD_PARTY_SOURCES += compat/inet_pton.c
-THIRD_PARTY_SOURCES += compat/mimalloc/%
 THIRD_PARTY_SOURCES += compat/nedmalloc/%
 THIRD_PARTY_SOURCES += compat/obstack.%
 THIRD_PARTY_SOURCES += compat/poll/%
@@ -2297,47 +2296,6 @@ 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/libc.o \
-		compat/mimalloc/options.o \
-		compat/mimalloc/os.o \
-		compat/mimalloc/page.o \
-		compat/mimalloc/random.o \
-		compat/mimalloc/prim/prim.o \
-		compat/mimalloc/segment.o \
-		compat/mimalloc/segment-map.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
-
-$(MIMALLOC_OBJS): COMPAT_CFLAGS += \
-	-DMI_WIN_USE_FLS \
-	-Wno-attributes \
-	-Wno-unknown-pragmas \
-	-Wno-unused-function \
-	-Wno-array-bounds
-
-ifdef DEVELOPER
-$(MIMALLOC_OBJS): COMPAT_CFLAGS += \
-	-Wno-pedantic \
-	-Wno-declaration-after-statement \
-	-Wno-old-style-definition \
-	-Wno-missing-prototypes \
-	-Wno-implicit-function-declaration
-endif
-endif
-
-
 ifdef OVERRIDE_STRDUP
 	COMPAT_CFLAGS += -DOVERRIDE_STRDUP
 	COMPAT_OBJS += compat/strdup.o

compat/.gitattributes

@@ -1,2 +1 @@
 /zlib-uncompress2.c	whitespace=-indent-with-non-tab,-trailing-space
-/mimalloc/**/*	whitespace=-trailing-space

compat/posix.h

@@ -176,16 +176,6 @@ typedef unsigned long uintptr_t;
 #define _ALL_SOURCE 1
 #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
-
 #ifdef MKDIR_WO_TRAILING_SLASH
 #define mkdir(a,b) compat_mkdir_wo_trailing_slash((a),(b))
 int compat_mkdir_wo_trailing_slash(const char*, mode_t);

config.mak.dev

@@ -22,10 +22,8 @@ endif
 
 ifneq ($(uname_S),FreeBSD)
 ifneq ($(filter gcc6,$(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

@@ -515,7 +515,7 @@ 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 -MP -std:c11
+	BASIC_CFLAGS = -nologo -I. -Icompat/vcbuild/include -DWIN32 -D_CONSOLE -DHAVE_STRING_H -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE
 	COMPAT_OBJS = compat/msvc.o compat/winansi.o \
 		compat/win32/flush.o \
 		compat/win32/path-utils.o \