Document -mprefer-avx128

	PR target/54785
	* doc/invoke.texi: Document -mprefer-avx128.

From-SVN: r192007

gcc/ChangeLog

+2012-10-02  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR target/54785
+	* doc/invoke.texi: Document -mprefer-avx128.
+
 2012-10-02  Andrew Pinski  <apinski@cavium.com>
 
 	* simplify-rtx.c (simplify_unary_operation_1 <case TRUNCATE>):

gcc/doc/invoke.texi

@@ -630,7 +630,7 @@ Objective-C and Objective-C++ Dialects}.
 -mincoming-stack-boundary=@var{num} @gol
 -mcld -mcx16 -msahf -mmovbe -mcrc32 @gol
 -mrecip -mrecip=@var{opt} @gol
--mvzeroupper @gol
+-mvzeroupper -mprefer-avx128 @gol
 -mmmx  -msse  -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx @gol
 -mavx2 -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma @gol
 -msse4a -m3dnow -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -mlzcnt @gol
@@ -13926,6 +13926,11 @@ before a transfer of control flow out of the function to minimize
 the AVX to SSE transition penalty as well as remove unnecessary @code{zeroupper}
 intrinsics.
 
+@item -mprefer-avx128
+@opindex mprefer-avx128
+This option instructs GCC to use 128-bit AVX instructions instead of
+256-bit AVX instructions in the auto-vectorizer.
+
 @item -mcx16
 @opindex mcx16
 This option enables GCC to generate @code{CMPXCHG16B} instructions.