Original discussion here: 
http://gcc.gnu.org/ml/gcc-patches/2010-12/msg00751.html

This patch enables vector conversions for ARM NEON architecture.  In its 
current state vectorizer can't handle type conversions in the hottest 
loop of libmp3lame on NEON since its backend doesn't have appropriate 
builtins for type conversion.  For x86_64 and rs6000 architectures that 
also can vectorize conversions the default behavior is retained.  We 
have rewritten condition in vectorizable_conversion() in 
tree-vect-stmts.c for the case of NONE modifier.  Now It first looks in 
convert_optab for suitable operation and then in builtins.

Regtested with arm-qemu ok.
Initially few tests failed (gcc.dg/vect/slp-10.c, gcc.dg/vect/slp-11c.c, 
gcc.dg/vect/slp-33.c, gcc.dg/vect/fast-math-pr35982.c) because now it 
vectorizes more loops than they expected to.  We adjusted 
target-supports.exp so vectorizable conversions and even/odd extractions 
are now supported for NEON.
Ok for trunk?

2011-10-20  Dmitry Plotnikov <dplotnikov@ispras.ru>

gcc/
     * tree-cfg.c (verify_gimple_assign_unary): Allow vector conversions.
     * tree-vect-stmts.c (supportable_convert_operation): New function.
       (vectorizable_conversion): Call it.  Change condition and behavior
       for NONE modifier case.
     * tree-vectorizer.h (supportable_convert_operation): New prototype.
     * tree.h (VECTOR_INTEGER_TYPE_P): New macro.

gcc/config/arm/
     * neon.md (floatv2siv2sf2): New.
       (floatunsv2siv2sf2): New.
       (fix_truncv2sfv2si2): New.
       (fix_truncunsv2sfv2si2): New.
       (floatv4siv4sf2): New.
       (floatunsv4siv4sf2): New.
       (fix_truncv4sfv4si2): New.
       (fix_truncunsv4sfv4si2): New.

gcc/testsuite/
     * gcc.target/arm/vect-vcvt.c: New test.
     * gcc.target/arm/vect-vcvtq.c: New test.

gcc/testsuite/lib/
     * target-supports.exp (check_effective_target_vect_intfloat_cvt): True
       for ARM NEON.
       (check_effective_target_vect_uintfloat_cvt): Likewise.
       (check_effective_target_vect_intfloat_cvt): Likewise.
       (check_effective_target_vect_floatuint_cvt): Likewise.
       (check_effective_target_vect_floatint_cvt): Likewise.
       (check_effective_target_vect_extract_even_odd): Likewise.