How to use AVX/pclmulqdq on Mac OS X

Question

I am trying to compile a program that uses the pclmulqdq instruction present in new Intel processors. I've installed GCC 4.6 using macports but when I compile my program (which uses the intrinsic _mm_clmulepi64_si128), I get

/var/folders/ps/sfjmtgx5771_qbqnh4c9xclr0000gn/T//ccEAWWhd.s:16:no such
instruction: `pclmulqdq $0, %xmm0,%xmm1'

It seems that GCC is able to generate the correct assembly code from the instrinsic, but the assembler does not recognize the instruction.

I've installed binutils using macports, but the problem persists. How do I know which assembler gcc is using? The XCode assembler probably does not support it, but the binutils assembler should.

Answer 1

A simpler solution that fixed this problem for me was adding -Wa,-q to the compiler flags. From the man pages for as (version 1.38):

-q

     Use the clang(1) integrated assembler instead of the GNU based system assembler.

The -Wa part passes it from the compiler driver to the assembler, much like -Wl passes arguments to the linker.

Answer 2

The GNU assembler (GAS) is not supported in Mac OS X.

In order to use AVX, I had to:

• Install GCC using MacPorts;
• Replace the native OS X assembler (/usr/bin/as) by a script which calls the clang assembler.
• Compile the program with the installed GCC (e.g. gcc-mp-4.7)

The strange thing is that while the clang assembler supports AVX, the clang compiler does not recognize the AVX instrinsics, forcing the ugly workaround above.

Answer 3

The built in version of as is outdated. (In OS X 10.8.3)

/usr/libexec/as/x86_64/as -v

Apple Inc version cctools-839, GNU assembler version 1.38

There does not seem to exist a version of gas for OS X. (See: Installing GNU Assembler in OSX)

Using the clang assembler via a script hack (as pointed out by Conrado PLG) is one workaround. However, it does require administrator privileges and overwrites OS X-bundled executables, causing a risk of it being overwritten by a new (yet possibly outdated) version of as bundled with a future version of OS X.

Is there then a better workaround?

As noted on Why does cross gcc invoke native 'as'? it seems to be possible to specify which "as"-executable and flags to use (using "-specs=..."). A cleaner workaround to the problem seems to be to pass the correct "-specs" flags to invoke the clang assembler. This does not require admin privileges and does not risk being broken by an OS X update. The exact details of how to perform this remains to be found out (anyone?).

If this workaround becomes trouble-free and transparent enough, it may be warranted to use those settings as a default (or at least variant) for the macport gcc (so that it supports "-march=native" and the like). There is such as configure.args setting ("--with-as=${prefix}/bin/as", as seen in https://trac.macports.org/browser/trunk/dports/lang/gcc48/Portfile ), which could be replaced.