Some flags only make sense for particular target platforms.
(x86 only, added in GHC 7.0.1) Use the SSE2 registers and instruction set to implement floating point operations when using the native code generator. This gives a substantial performance improvement for floating point, but the resulting compiled code will only run on processors that support SSE2 (Intel Pentium 4 and later, or AMD Athlon 64 and later).
SSE2 is unconditionally used on x86-64 platforms.
(x86 only) 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.