5 ms·

A Foray into Go Assembly Programming

143 points by sgmansfield 9y ago

dlsniper 9y ago
Excellent read about Go Assembly and how to use it. I'd love to see more on this, especially since assembly isn't something that often used, but after reading your post I can appreciate the overhead of having to maintain it in the future. Thank you for the time put in the post about this.
andreiw 9y ago
Wait, so the Go assembler allows generating invalid opcodes? Any way of using a more normal assembler with a non-GNU Go compiler?
- sgmansfield 9y ago
  This was probably not explained well. The compilation failed with an error saying that I was basically trying to do something that was not allowed. For your second question: I don't believe there is a way to use a separate assembler.
rcarmo 9y ago
FYI, blog layout breaks down completely when trying to zoom text on an iPad, making it impossible to read.
- sgmansfield 9y ago
  Thanks for letting me know, I'll see if I can fix it. If I could trouble you to do so, can you provide a screenshot?
  - shurcooL 9y ago
    I'm not the OP, but I have an iPad too, and I can provide screenshots. http://instantshare.virtivia.com:27080/1eikl92n5jtyc.PNG http://instantshare.virtivia.com:27080/1eikl92n5jtyc.PNG - default view http://instantshare.virtivia.com:27080/o1wn6nbthvuu.PNG http://instantshare.virtivia.com:27080/o1wn6nbthvuu.PNG - after zooming in unsuccessfully, OR just scrolling to the right http://instantshare.virtivia.com:27080/eiw3z2u1n52h.PNG http://instantshare.virtivia.com:27080/eiw3z2u1n52h.PNG - fully zoomed out http://instantshare.virtivia.com:27080/1jsmbx8km815h.PNG http://instantshare.virtivia.com:27080/1jsmbx8km815h.PNG - the culprit causing this, a long URL which isn't being wrapped by your CSS rules To be honest, the same issue happens on desktop browsers, it's just not as noticeable because the default zoom kinda matches what you'd expect. But you can still scroll to the side, in a desktop browser. See: http://instantshare.virtivia.com:27080/b69zdjyzbvfo.png http://instantshare.virtivia.com:27080/b69zdjyzbvfo.png I think the CSS fix can be something like this: word-wrap: break-word; http://instantshare.virtivia.com:27080/1l8rbeisu9a2a.png http://instantshare.virtivia.com:27080/1l8rbeisu9a2a.png However, different browsers behave differently™, so you may need to also add: word-wrap: break-word; white-space: pre-wrap; To make it work in Firefox (but you might need to adjust other things, also play with word-break CSS property, and test in all browsers). I've dealt with this previously at https://github.com/shurcooL/play/issues/4 https://github.com/shurcooL/play/issues/4 and other places that deal with displaying user generated content.
bkeroack 9y ago
Here's a good talk about the Go assembler backend: https://www.youtube.com/watch?v=KINIAgRpkDA https://www.youtube.com/watch?v=KINIAgRpkDA Spoiler: they're nearly to the point (maybe now they've achieved it fully) of autogenerating a new language backend from ISA documentation PDFs! Bring that up next time someone complains about Go not using LLVM.
- shouldbworking 9y ago
  Does this make targeting new ISA's trivial?
  - AstralStorm 9y ago
    Not really, writing a good enough formal ISA spec is quite hard. More so if the architecture is quirky.
- gcatlin 9y ago
  Slides: https://talks.golang.org/2016/asm.slide https://talks.golang.org/2016/asm.slide
Marat_Dukhan 9y ago
PeachPy, an x86-64 assembler in Python, is often used to target Golang assembler without dealing with its weird syntax. PeachPy uses Intel assembly syntax, but can generate assembly listing for Golang toolchain as its output. Damian Gryski wrote a tutorial about using PeachPy for Golang assembly: https://blog.gopheracademy.com/advent-2016/peachpy/ https://blog.gopheracademy.com/advent-2016/peachpy/ Disclosure: I'm the author of PeachPy
Animats 9y ago
If writing assembly code to run in the Go environment, and you're changing pointers, you have to write garbage-collector aware code.[1][2] Go has concurrent garbage collection, which means there are some unusual constraints on code that manipulates pointers. This is an area where proofs of correctness are considered necessary, because intuition fails. See [3] [1] https://blog.golang.org/go15gc https://blog.golang.org/go15gc [2] https://golang.org/src/runtime/mbarrier.go https://golang.org/src/runtime/mbarrier.go [3] https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md https://github.com/golang/proposal/blob/master/design/17503-...
libeclipse 9y ago
> In this case, a divide by 3 is equivalent to multiplying by 0x55555556 and then taking the top half of the output. What the fuck. That's amazing!
- vardump 9y ago
  In that case, fixed point arithmetic might blow your mind. :) https://en.wikipedia.org/wiki/Fixed-point_arithmetic https://en.wikipedia.org/wiki/Fixed-point_arithmetic
- chris_overseas 9y ago
  In more general terms, take a look at Montgomery multiplication[1] which is where this trick originates. It's a common technique used by compilers to optimise away divisions. [1] https://en.wikipedia.org/wiki/Montgomery_modular_multiplication https://en.wikipedia.org/wiki/Montgomery_modular_multiplicat...
webhat 9y ago
"I literally copy-paste the middot every time. Who has a middot key?" I did too, until I found. http://middot.net/ http://middot.net/
- zeveb 9y ago
  As that site indicates, in X.org with the defaults, '·' is easily typed with 'Compose . -', 'Compose . ^' or 'Compose ^ .' (this should also work from the console in Debian, at least, which uses xkb bindings throughout). I love the Compose key, and don't think I'll ever use a desktop without it.
Karliss 9y ago
Tried writing a version with no divisions. At least in my tests it was ~5x faster. Correctness not guaranteed. https://gist.github.com/karliss/6cc343f7359c2de9166f1551f88482d9 https://gist.github.com/karliss/6cc343f7359c2de9166f1551f884...
kristianp 9y ago
Couldn't he have compiled his go code with assembly output and edited the resulting file? Wouldn't that have saved a lot of struggle with the format? go tool compile -S -S file.go
- sgmansfield 9y ago
  Sure, but that ruins the fun :)

143 points by sgmansfield 9y ago

Excellent read about Go Assembly and how to use it. I'd love to see more on this, especially since assembly isn't something that often used, but after reading your post I can appreciate the overhead of having to maintain it in the future. Thank you for the time put in the post about this.

andreiw 9y ago

Wait, so the Go assembler allows generating invalid opcodes? Any way of using a more normal assembler with a non-GNU Go compiler?

sgmansfield 9y ago
This was probably not explained well. The compilation failed with an error saying that I was basically trying to do something that was not allowed. For your second question: I don't believe there is a way to use a separate assembler.

rcarmo 9y ago

FYI, blog layout breaks down completely when trying to zoom text on an iPad, making it impossible to read.

sgmansfield 9y ago
Thanks for letting me know, I'll see if I can fix it. If I could trouble you to do so, can you provide a screenshot?
- shurcooL 9y ago
  I'm not the OP, but I have an iPad too, and I can provide screenshots. http://instantshare.virtivia.com:27080/1eikl92n5jtyc.PNG http://instantshare.virtivia.com:27080/1eikl92n5jtyc.PNG - default view http://instantshare.virtivia.com:27080/o1wn6nbthvuu.PNG http://instantshare.virtivia.com:27080/o1wn6nbthvuu.PNG - after zooming in unsuccessfully, OR just scrolling to the right http://instantshare.virtivia.com:27080/eiw3z2u1n52h.PNG http://instantshare.virtivia.com:27080/eiw3z2u1n52h.PNG - fully zoomed out http://instantshare.virtivia.com:27080/1jsmbx8km815h.PNG http://instantshare.virtivia.com:27080/1jsmbx8km815h.PNG - the culprit causing this, a long URL which isn't being wrapped by your CSS rules To be honest, the same issue happens on desktop browsers, it's just not as noticeable because the default zoom kinda matches what you'd expect. But you can still scroll to the side, in a desktop browser. See: http://instantshare.virtivia.com:27080/b69zdjyzbvfo.png http://instantshare.virtivia.com:27080/b69zdjyzbvfo.png I think the CSS fix can be something like this: word-wrap: break-word; http://instantshare.virtivia.com:27080/1l8rbeisu9a2a.png http://instantshare.virtivia.com:27080/1l8rbeisu9a2a.png However, different browsers behave differently™, so you may need to also add: word-wrap: break-word; white-space: pre-wrap; To make it work in Firefox (but you might need to adjust other things, also play with word-break CSS property, and test in all browsers). I've dealt with this previously at https://github.com/shurcooL/play/issues/4 https://github.com/shurcooL/play/issues/4 and other places that deal with displaying user generated content.

bkeroack 9y ago

Here's a good talk about the Go assembler backend: https://www.youtube.com/watch?v=KINIAgRpkDA https://www.youtube.com/watch?v=KINIAgRpkDA Spoiler: they're nearly to the point (maybe now they've achieved it fully) of autogenerating a new language backend from ISA documentation PDFs! Bring that up next time someone complains about Go not using LLVM.

shouldbworking 9y ago
Does this make targeting new ISA's trivial?
- AstralStorm 9y ago
  Not really, writing a good enough formal ISA spec is quite hard. More so if the architecture is quirky.
gcatlin 9y ago
Slides: https://talks.golang.org/2016/asm.slide https://talks.golang.org/2016/asm.slide

Marat_Dukhan 9y ago

PeachPy, an x86-64 assembler in Python, is often used to target Golang assembler without dealing with its weird syntax. PeachPy uses Intel assembly syntax, but can generate assembly listing for Golang toolchain as its output. Damian Gryski wrote a tutorial about using PeachPy for Golang assembly: https://blog.gopheracademy.com/advent-2016/peachpy/ https://blog.gopheracademy.com/advent-2016/peachpy/ Disclosure: I'm the author of PeachPy

Animats 9y ago

If writing assembly code to run in the Go environment, and you're changing pointers, you have to write garbage-collector aware code.[1][2] Go has concurrent garbage collection, which means there are some unusual constraints on code that manipulates pointers. This is an area where proofs of correctness are considered necessary, because intuition fails. See [3] [1] https://blog.golang.org/go15gc https://blog.golang.org/go15gc [2] https://golang.org/src/runtime/mbarrier.go https://golang.org/src/runtime/mbarrier.go [3] https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md https://github.com/golang/proposal/blob/master/design/17503-...

libeclipse 9y ago

> In this case, a divide by 3 is equivalent to multiplying by 0x55555556 and then taking the top half of the output. What the fuck. That's amazing!

vardump 9y ago
In that case, fixed point arithmetic might blow your mind. :) https://en.wikipedia.org/wiki/Fixed-point_arithmetic https://en.wikipedia.org/wiki/Fixed-point_arithmetic
chris_overseas 9y ago
In more general terms, take a look at Montgomery multiplication[1] which is where this trick originates. It's a common technique used by compilers to optimise away divisions. [1] https://en.wikipedia.org/wiki/Montgomery_modular_multiplication https://en.wikipedia.org/wiki/Montgomery_modular_multiplicat...

webhat 9y ago

"I literally copy-paste the middot every time. Who has a middot key?" I did too, until I found. http://middot.net/ http://middot.net/

zeveb 9y ago
As that site indicates, in X.org with the defaults, '·' is easily typed with 'Compose . -', 'Compose . ^' or 'Compose ^ .' (this should also work from the console in Debian, at least, which uses xkb bindings throughout). I love the Compose key, and don't think I'll ever use a desktop without it.

Karliss 9y ago

Tried writing a version with no divisions. At least in my tests it was ~5x faster. Correctness not guaranteed. https://gist.github.com/karliss/6cc343f7359c2de9166f1551f88482d9 https://gist.github.com/karliss/6cc343f7359c2de9166f1551f884...

kristianp 9y ago

Couldn't he have compiled his go code with assembly output and edited the resulting file? Wouldn't that have saved a lot of struggle with the format? go tool compile -S -S file.go

sgmansfield 9y ago
Sure, but that ruins the fun :)