seanjensengrey

# source:http://geocities.com/SiliconValley/heights/7052/opcode.txt
From: mark@omnifest.uwm.edu (Mark Hopkins)
Newsgroups: alt.lang.asm
Subject: A Summary of the 80486 Opcodes and Instructions

(1) The 80x86 is an Octal Machine
   This is a follow-up and revision of an article posted in alt.lang.asm on
7-5-92 concerning the 80x86 instruction encoding.
 
   The only proper way to understand 80x86 coding is to realize that ALL 80x86

rygorous

why doesn't radfft support AVX on PC?

So there's two separate issues here: using instructions added in AVX and using 256-bit wide vectors. The former turns out to be much easier than the latter for our use case.

Problem number 1 was that you positively need to put AVX code in a separate file with different compiler settings (/arch:AVX for VC++, -mavx for GCC/Clang) that make all SSE code emitted also use VEX encoding, and at the time radfft was written there was no way in CDep to set compiler flags for just one file, just for the overall build.

[There's the GCC "target" annotations on individual funcs, which in principle fix this, but I ran into nasty problems with this for several compiler versions, and VC++ has no equivalent, so we're not currently using that and just sticking with different compilation units.]

The other issue is to do with CPU power management.