topic VEX encoding - 32bit and 64bit in Intel® ISA Extensions

VEX encoding - 32bit and 64bit

Christian_M_2 — Fri, 18 Jan 2013 12:05:10 GMT

AVX instructions came along with the new VEX instruction encoding.

Therefore one should compile with option /arch:AVX (or other depending on compiler). Are there any restrictions concerning 32bit or 64bit compilation?

My real question is: Do I get the same performance of AVX compiled for 32 and 64 bit? Or is the VEX encoding only possible for 64 bit, for example? This would lead to transition penalties (SSE to AVX or vice versa) more often.

I could not find a clear answer on this.

AVX transitions are handled

TimP — Fri, 18 Jan 2013 17:59:13 GMT

AVX transitions are handled the same by 32- and 64-bit compilers. The most likely difference between 32- and 64-bit mode, if you don't specify alignments, is in the better (but still not optimum) default alignments of 64-bit OS. Even if you mix SSE and AVX intrinsics, current Intel compiler should attempt to deal with SSE to AVX transitions automatically. You would have to code in asm if you really want to provoke transition stalls.

Thank you this information is

Christian_M_2 — Sat, 19 Jan 2013 09:40:51 GMT

Thank you this information is very valuable for me!