mimalloc: add a hack to fix building for Emscripten

We currently use the dlmalloc allocator as a system allocator, and only
use mimalloc for the temporary heap.

Unfortunately, the Emscripten port of mimalloc assumes that it's used as
a system allocator and depends on the slow emmalloc for low-level access
to memory allocation.

Short of converting to mimalloc as a system allocator as well, our only
other option is to enable emmalloc as a system allocator, but that would
really slow things down everywhere. As a result, we're forced to use an
ugly hack and override the symbol names, so they point to the allocator
in use, rather than hard-coded to emmalloc.

Author: iscgar

CMakeLists.txt

@@ -211,6 +211,18 @@ set(MI_OVERRIDE OFF CACHE BOOL "")
 set(MI_BUILD_SHARED OFF CACHE BOOL "")
 set(MI_BUILD_OBJECT OFF CACHE BOOL "")
 set(MI_BUILD_TESTS OFF CACHE BOOL "")
+if(EMSCRIPTEN)
+    # HACK: We only use mimalloc for the temporary heap, not as a general purpose
+    # allocator. However, the Emscripten port of mimalloc assumes that it is used
+    # as the system allocator, and depends on the slow emmalloc allocator.
+    # We do not want to enable emmalloc, as that would slow down allocations all
+    # over the place, so we must either use mimalloc as the system allocator as
+    # well, or override its definitions to just use the existing system allocator.
+    # The latter option is the least intrusive change from a stability standpoint,
+    # so for now that's what we do here. We can try to convert to mimalloc wholsale
+    # and remove this hack if we find the need to do so in the future.
+    set(MI_EXTRA_CPPDEFS "emmalloc_free=free;emmalloc_memalign=memalign" CACHE INTERNAL "")
+endif()
 add_subdirectory(extlib/mimalloc EXCLUDE_FROM_ALL)
 set(MIMALLOC_INCLUDE_DIR ${CMAKE_SOURCE_DIR}/extlib/mimalloc/include)