During the last rounds of changes (probably profiling related) a VM_MAKE_TAG() was applied to the mmap() call of ReserveMemory() in GCHeapMac.cpp. This apparently works with 10.5 but not 10.4. Restoring it to -1 resolves the issue.
This issue is currently causing the mac performance run in buildbot to fail as it runs on a 10.4 machine. http://tamarin-builds.mozilla.org/tamarin-central/builders/mac-performance/builds/9/steps/Release/logs/stdio
Created attachment 338320 [details] [diff] [review] fixes mmap usage for 10.4
fixed in 645