The Wikibooks x86 Assembly says:
GAS instructions generally have the form mnemonic source, destination. For instance, the following mov instruction:
But it does not say what exceptions to this rule are, and I cannot find a table of instructions. The official AS documentation doesn't seem to have a section that gives syntax for all the instructions (though it's pretty long, and I might have just missed the section). The normal x86 reference that I use for instructions doesn't obviously specify the dialect, though, and it has the arguments in destination, source order.
What's the syntax for mulx in GAS / AT&T, and is there a reference that I can use to easily look up such things in the future?
This wikibook: https://en.wikibooks.org/wiki/X86_Assembly/Arithmetic
doesn't include MULX, but it does show the 3 operand IMUL instruction listed as such:
imul dest, src, aux (Intel syntax)
imul aux, src, dest (GAS Syntax)
so MULX would be:
mulx dest_hi, dest_lo, src1 (Intel syntax)
mulx src1, dest_lo, dest_hi (GAS Syntax)