gecko/mozglue/crt/Makefile.in

#
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, You can obtain one at http://mozilla.org/MPL/2.0/.

# Roll our own custom logic here for the import library

###############################################################################
#
# Linking Mozilla itself to jemalloc is not particularly difficult.  To do this
# we avoid linking directly to the Microsoft-provided CRT import libraries.
# Instead, we link to our own import library which we generate here.  To
# replace the CRT's malloc/free/other memory management symbols we export
# our own versions out of jemalloc.dll.  We then take the import library that
# the compiler generates for jemalloc.dll and combine it with the MS CRT import
# libraries.  We put our library on the command line first, and the CRT symbols
# are discarded in favor of our versions!
#
# Unfortunately that was too easy.  The CRT import library is not a standard
# import library that contains a list of symbols and whatnot.  It also includes
# object files that are linked into generated programs.  One of these,
# crtdll.obj is (as one might expect) linked into all DLLs that link against
# the CRT.  This file does things like run static C++ constructors when the
# DLL is attached, call DllMain, etc.
#
# In the CRT source all malloc/free calls are made to malloc_crt and free_crt.
# In debug builds these are both defined to malloc_dbg and free_dbg.  In opt
# builds malloc_crt is an actual function, implemented and exposed from the
# CRT.  free_crt is, however, defined to be just plain old free.  This works
# fine inside the CRT where malloc_crt and free operate on the same heap.
# Outside the CRT malloc_crt is in the CRT's heap, but free is in jemalloc's
# heap.  This causes much pain at shutdown :-(
#
# The obvious solution here is to override malloc_crt too.  Unfortunately,
# that doesn't work because the CRT expects to be able to call msize on this
# piece of memory deep inside the CRT, which will fail because it'll call the
# CRT's msize on a pointer in jemalloc's heap.
#
# Our solution to this is quite devious.  We take apart the CRT's import lib
# and remove the problematic object file.  We then poke at the object file's
# symbol table and replace '__imp__free' (which means grab free from some
# other DLL) with '__imp__frex'.  Then we define our own dummy no-op function
# in jemalloc.dll and export it as frex.  Then we put the CRT import lib
# back together with the patched crtdll.obj, glue it to the end of jemalloc's
# import library and link the rest of Mozilla to that.
#
# The result?  A binary that uses jemalloc, doesn't crash, and leaks a tiny
# amount of memory (32 words per DLL in the 2010 CRT) at shutdown.
#
###############################################################################

target:: mozcrt.lib
	$(INSTALL) $(IFLAGS2) mozcrt.lib $(DIST)/lib

# And finally combine that with the jemalloc import library to get an import
# library that has our malloc/free/etc and the CRT's everything else
mozcrt.lib: ../build/mozglue.lib msvc_modified.lib
	lib -OUT:$@ $^

# Put the fixed object file back in
msvc_modified.lib: msvc_removed.lib crtdll_fixed.obj
	lib -OUT:$@ $^

# Fix the object file
crtdll_fixed.obj: crtdll.obj
	$(PYTHON) $(srcdir)/fixcrt.py

# Find the path of crtdll.obj
CRTDLL_FULLPATH=$(subst \,\\,$(shell lib -list msvc_combined.lib | grep crtdll\\.obj))

# Remove the broken object file, only after we have extracted it
msvc_removed.lib: msvc_combined.lib crtdll.obj
	lib -OUT:$@ msvc_combined.lib -REMOVE:$(CRTDLL_FULLPATH)

# Extract the broken object file out of the combined library
crtdll.obj: msvc_combined.lib
	lib -OUT:$@ $^ -EXTRACT:$(CRTDLL_FULLPATH)

# Grab both CRT libraries and combine them into one library to simplify things
msvc_combined.lib:
	lib -OUT:$@ $(WIN32_CRT_LIBS)