LLVM Tutorial #13: Optimizers Work

In this series I walkthrough the LLVM "Kaleidoscope" Tutorial, where you follow step by step to create your first programming language frontend using LLVM as the backend. Last episode we started work on adding optimizers but had some trouble getting the code to compile. This time I found the fix and we are able to verify that the optimizers are working.

Transcript

The following transcript was automatically generated by an algorithm.

00:00 : hello there welcome to the
00:03 : video series on following along with the
00:06 : llvm tutorial on llvm.org
00:11 : okay so we're at the end oh not the end
00:15 : sorry
00:15 : we're in the middle of chapter four
00:17 : where we're trying to get the
00:19 : optimizer set up and in the last video i
00:21 : had
00:22 : problems getting
00:25 : their code example to compile at first i
00:28 : had troubles getting my code to compile
00:30 : and then i had trouble getting their
00:32 : code to compile
00:36 : and i figured that out and the way i got
00:40 : it to
00:40 : finally compile was i simply took every
00:43 : single
00:44 : static library under their lib directory
00:48 : and appended it to the command line
00:52 : by using this star.a and with that
00:56 : change i was able to get their version
00:58 : of the code to compile
00:59 : so now i'm going to apply this to my
01:02 : code
01:03 : which i did and when i applied it to
01:07 : my code it still didn't compile it had
01:10 : some errors so now i'm going to try to
01:12 : solve those errors by
01:15 : using their code as the working version
01:18 : and
01:18 : my myco as the non-working version we're
01:21 : going to do a bug sandwich
01:23 : on it if you're not familiar with the
01:26 : bug sandwich
01:27 : i documented it at debug school.com
01:32 : as one of the videos there
01:36 : um bug sandwich
01:40 : so if you're interested in learning more
01:42 : about the bug sandwich technique
01:45 : debug school dot com is where you can
01:48 : learn more about it i have
01:49 : like a about 15 minute video that i
01:53 : uh created to explain it
01:57 : um but you but you could also just watch
02:00 : me
02:00 : try to do it um so when i try to build
02:04 : my program i get these six errors
02:09 : i know that not supposed to happen
02:13 : because their version does the same
02:15 : thing and their version compiles
02:17 : so i'm gonna basically the buck sandwich
02:20 : is like i'm gonna compare
02:22 : the good version of the code versus the
02:25 : bad version of the code
02:26 : and at the moment their version is the
02:28 : good version of the code i'm gonna try
02:30 : to figure out what is
02:31 : the difference between them or
02:34 : what's the key difference that makes one
02:37 : of them work
02:38 : and the other one not work okay so the
02:42 : place where
02:45 : the error is is actually this run
02:48 : function i believe
02:52 : yes it says too few arguments to the
02:54 : function call
02:56 : but i think it's matching it against the
02:59 : signature of the past
03:00 : manager which i'm not supposed to use
03:02 : the path manager i'm supposed to use the
03:04 : function path manager
03:07 : i'm going to look at their code to see
03:09 : how
03:11 : their code has it because
03:16 : you know there might be suspicious
03:20 : stuff in there maybe
03:25 : oh it's their code 04 that i want to be
03:28 : looking at
03:30 : i have two different versions of their
03:33 : code
03:36 : okay
03:40 : okay they have this line of code is the
03:42 : same
03:44 : as far as i can tell it has
03:47 : one uh but perhaps i should look at how
03:52 : they defined
03:54 : the static variable oh it should be make
03:56 : unique
03:57 : to a function path manager
04:01 : oh the legacy from colon coding function
04:04 : path manager okay
04:07 : got it i swear i copy that
04:10 : line from them
04:17 : they didn't update this uh if
04:20 : that's if that's the problem
04:24 : they should update that let's see if
04:27 : this fixes it
04:43 : i get more errors than before uh we
04:46 : still have this error
04:48 : interestingly but now i have a few more
04:51 : errors
04:53 : um okay and
04:57 : it doesn't oh
05:02 : probably because i have to match this as
05:08 : well
05:10 : uh let me check the version just in case
05:21 : yes that's how they have it
05:39 : yes that worked all these are warnings
05:42 : and i'm gonna ignore them uh but because
05:46 : there are warnings
05:48 : only and no errors that doesn't mean it
05:50 : worked so i'm going to test it now
05:54 : um okay so with
05:57 : the with these optimization passes
06:00 : supposedly if i put
06:03 : in this code then i should get
06:07 : this simplified output which only
06:09 : performs
06:10 : one ad whereas previously without the
06:13 : optimization manager there was actually
06:16 : two ads
06:17 : here so let's try that
06:25 : and yes there's only one ad now perfect
06:28 : so the optimization worked uh
06:32 : now the tutorial didn't explain that
06:34 : much about what it actually did
06:37 : but um it's basically just uh yeah
06:40 : there's these magic potions you can
06:43 : sprinkle on to your
06:46 : compiler and then it'll if conditions
06:50 : are right
06:50 : it'll magically make your code
06:53 : smaller and faster um
06:58 : yeah so it saved a floating point
07:01 : at instruction on every single execution
07:04 : of this function
07:05 : which is cool if this function gets
07:08 : executed a lot
07:11 : llvm provides a variety of optimizations
07:14 : that can be used in
07:15 : circuit circumstances some documentation
07:18 : about the
07:19 : various passes is available but it isn't
07:22 : very
07:22 : complete another good source of ideas
07:25 : can come from looking at the passes that
07:28 : clang runs to get started
07:30 : the op tool allows you to experiment
07:32 : with passes from the command line so
07:34 : that you can see if they
07:35 : do anything now that we have a
07:38 : reasonable
07:39 : code coming out of our front end let's
07:40 : talk about executing it
07:43 : great um so
07:46 : that completes the first half of chapter
07:49 : four
07:49 : i'm gonna stop this video here and the
07:51 : next one will
07:53 : we start uh we'll do the section called
07:57 : adding a jit compiler