My best introduction to any asm language is just my C compiler putting out asm (gcc -S, I think). I can create small programs to do what I want, and see what the compiler puts out.
https://gcc.godbolt.org is also helpful, it colorizes the assembly output corresponding to the C/C++ source. Furthermore, you can simply change between Intel and AT&T synatx.
The -S flag also works in clang, although the output may differ. For example, last time I checked a comparison check of i < 10 in a for loop would become:
cmpl $9, -4(%rbp)
jle .L3
under GCC ('if i <= 9, loop') and:
cmpl $10, -8(%rbp)
jge .LBB0_4
under clang ('if i >= 10, end the loop'). Neither GCC or clang does an exact conversion, and they both produce different assembly instructions (optimisations disabled in both cases).
If you build a cross-compiler, you can also output assembly for architectures other than your local machine, though this can be quite fiddly (see crosstool-ng for a project which has done most of the work for you).
