#llc --version

llon 2.7
  DEBUG build with assertions.
  Built Apr 28 2010 (21:31:06).
  Host: i386-pc-linux-gnu
  Host CPU: prescott

  Registered Targets:
    alpha   - Alpha [experimental]
    arm     - ARM
    bfin    - Analog Devices Blackfin [experimental]
    c       - C backend
    cellspu - STI CBEA Cell SPU [experimental]
    cooper  - PIC16 Cooper [experimental]
    cpp     - C++ backend
    mblaze  - MBlaze
    mips    - Mips
    mipsel  - Mipsel
    msil    - MSIL backend
    msp430  - MSP430 [experimental]
    pic16   - PIC16 14-bit [experimental]
    ppc32   - PowerPC 32
    ppc64   - PowerPC 64
    sparc   - Sparc
    sparcv9 - Sparc V9
    systemz - SystemZ
    thumb   - Thumb
    x86     - 32-bit X86: Pentium-Pro and above
    x86-64  - 64-bit X86: EM64T and AMD64
    xcore   - XCore

#llc -march=arm -mcpu=help fft001.opt.bc
Available CPUs for this target:

  arm1020e      - Select the arm1020e processor.
  arm1020t      - Select the arm1020t processor.
  arm1022e      - Select the arm1022e processor.
  arm10e        - Select the arm10e processor.
  arm10tdmi     - Select the arm10tdmi processor.
  arm1136j-s    - Select the arm1136j-s processor.
  arm1136jf-s   - Select the arm1136jf-s processor.
  arm1156t2-s   - Select the arm1156t2-s processor.
  arm1156t2f-s  - Select the arm1156t2f-s processor.
  arm1176jz-s   - Select the arm1176jz-s processor.
  arm1176jzf-s  - Select the arm1176jzf-s processor.
  arm710t       - Select the arm710t processor.
  arm720t       - Select the arm720t processor.
  arm7tdmi      - Select the arm7tdmi processor.
  arm7tdmi-s    - Select the arm7tdmi-s processor.
  arm8          - Select the arm8 processor.
  arm810        - Select the arm810 processor.
  arm9          - Select the arm9 processor.
  arm920        - Select the arm920 processor.
  arm920t       - Select the arm920t processor.
  arm922t       - Select the arm922t processor.
  arm926ej-s    - Select the arm926ej-s processor.
  arm940t       - Select the arm940t processor.
  arm946e-s     - Select the arm946e-s processor.
  arm966e-s     - Select the arm966e-s processor.
  arm968e-s     - Select the arm968e-s processor.
  arm9e         - Select the arm9e processor.
  arm9tdmi      - Select the arm9tdmi processor.
  cortex-a8     - Select the cortex-a8 processor.
  cortex-a9     - Select the cortex-a9 processor.
  ep9312        - Select the ep9312 processor.
  generic       - Select the generic processor.
  iwmmxt        - Select the iwmmxt processor.
  mpcore        - Select the mpcore processor.
  mpcorenovfp   - Select the mpcorenovfp processor.
  strongarm     - Select the strongarm processor.
  strongarm110  - Select the strongarm110 processor.
  strongarm1100 - Select the strongarm1100 processor.
  strongarm1110 - Select the strongarm1110 processor.
  xscale        - Select the xscale processor.

Available features for this target:

  neon   - Enable NEON instructions.
  thumb2 - Enable Thumb2 instructions.
  v4t    - ARM v4T.
  v5t    - ARM v5T.
  v5te   - ARM v5TE, v5TEj, v5TExp.
  v6     - ARM v6.
  v6t2   - ARM v6t2.
  v7a    - ARM v7A.
  vfp2   - Enable VFP2 instructions.
  vfp3   - Enable VFP3 instructions.

Use +feature to enable a feature, or -feature to disable it.
For example, llc -mcpu=mycpu -mattr=+feature1,-feature2