MLton runs fine on the AMD64 (aka "x86-64" or "x64") architecture.
When compiling for AMD64, MLton targets the 64-bit ABI.
On AMD64, MLton supports native code generation (-codegen native or -codegen amd64).
When compiling for AMD64, MLton uses -align 8 by default. Using -align 4 may be incompatible with optimized builds of the GnuMP library, which assume 8-byte alignment. (See the thread at http://www.mlton.org/pipermail/mlton/2009-October/030674.html for more details.)