(SPARC machines) Means to pass the like-named option to GCC; it says to use the Version 8 SPARC instructions, notably integer multiply and divide. The similar -m* GCC options for SPARC also work, actually.

(iX86 machines) GHC tries to “steal” four registers from GCC, for performance reasons; it almost always works. However, when GCC is compiling some modules with four stolen registers, it will crash, probably saying:

Foo.hc:533: fixed or forbidden register was spilled.
This may be due to a compiler bug or to impossible asm
statements or clauses.

Just give some registers back with -monly-N-regs. Try `3' first, then `2'. If `2' doesn't work, please report the bug to us.