[saagarjha]

Would it be impossible to implement mmap and sbrk in the runtime?

[kllrnohj]

mmap is arguably the only thing it should have had in the first place. brk/sbrk is fairly dead.

[saagarjha]

  $ strace /bin/ls 2>&1 | grep brk
  brk(NULL)                               = 0x55fb032b3000
  brk(NULL)                               = 0x55fb032b3000
  brk(0x55fb032d4000)                     = 0x55fb032d4000

[wahern]

  $ ktrace /bin/ls >/dev/null
  $ kdump | grep brk | wc -l     
         0
  $ kdump | grep mmap | wc -l
       130

glibc, musl, and jemalloc got the memo, they just couldn't be bothered to stop using brk by default. They're all capable of using mmap exclusively, however, because whether or not brk is actually dead, there's long been consensus that mmap is the better abstraction on balance, particularly given the benefit of ASLR. So there's little reason to support brk instead of mmap for compatibility. Presumably it was chosen for the convenience of WebAssembly implementations--contiguous, flat memory is a great simplifier of VM, JIT, and even traditional AoT architectures.