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   LukeAlmighty 🇨🇿 (lukealmighty@gameliberty.club)'s status on Monday, 24-Feb-2025 06:34:19 JST LukeAlmighty 🇨🇿

   • SuperDicq

   @SuperDicq
   Thanks, I feel 30 IQ points dumber now.

   I get, that the point was, that compiler cannot optimalize by pushing commands place in code, but I still didn't get 80% of that text.

   • Embed this notice
     Coded Artist (coded_artist@gameliberty.club)'s status on Monday, 24-Feb-2025 07:49:10 JST Coded Artist

     in reply to

     • SuperDicq

     @LukeAlmighty @SuperDicq
     The integer overflow I understand, if a + b is greater than MAX_INT, that's an issue.
     Not sure how this affects prefetching, but then I'm not an expert.
     The rest is Chinese to me.

     LukeAlmighty 🇨🇿 likes this.
   • Embed this notice
     SuperDicq (superdicq@minidisc.tokyo)'s status on Tuesday, 25-Feb-2025 18:19:23 JST SuperDicq

     in reply to

     • Wolf480pl
     • Coded Artist

     @wolf480pl@mstdn.io @coded_artist@gameliberty.club @LukeAlmighty@gameliberty.club Thank you for the explanation, it actually makes a lot of sense. I feel like I understood most of what you said but I couldn't quite put it into words.
     
     Also I recommend you use an instance that is not Mastodon next time to get around the hardcoded 500 character limit so everything can actually be just one post.

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:24 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     Also, a modern compiler like a recent version of LLVM or GCC with optimizations enabled will see right through your arithmetic tricks, and turn them into a temporary variable (that is then held in a CPU register and never reaches main memory).

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:27 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     Also, instruction-level parallelism (ILP) is the property of a compiled program which says how many of its instructions can be executed in parallel if you have an infinitely parallel CPU. If every instruction in your program uses the result from the instruction immediately before it, your ILP is 1, and you cannot take advantage of a CPU that can do multiple things at once.

     8/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:28 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     Modern CPUs are also superscalar (which means they execute multiple instructions in parallel) and out-of-order (which means they can reorder instructions that don't depend on each others' results of side effects).

     A serial dependency between two instructions means they have to be executed in the same order they appear in the code, because one depends on the other.

     6/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:28 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     If you use arithmetic tricks to swap two values, anything that uses any of the two values in the future depends on those swap instructions, which in turn depend on both the instructions that calculated a, and the instructions that calculated b.

     That may prevent CPU from doing things in parallel, or reordering instructions, which again, makes your code slow.

     7/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:29 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     Branch misprediction is when it guesses incorrectly, and it costs a lot of time.

     If a value used inside an if condition is known some time before the execution reaches the if, that might make branch prediction easier for a sufficiently smart CPU.

     But when you obscure which value is which by using arithmetic to swap two variables, the CPU might not be able to see through that, resulting in frequent mispredictions, and making your code slow.

     5/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:30 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq
     So they have to guess which side of an if is going to be executed, start processing instructions from that side of a branch, and if it later turns out they guessed wrong, they need to undo all of that and start again with the correct side of a branch.

     This is called speculative execution.

     Branch prediction is the part where the CPU guesses which way a branch is going to go.

     4/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:31 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq
     The SSA form is a type of internal representation of a program inside a compiler, in which instructions do not have a destination operand. Instead, every instruction creates a new variable with its result, and other instructions can use that as their source operands, but they cannot modify that variable

     This means there are lots of temporary variables, so LLVM must be very good at optimizing them out.
     Which means adding a temp var in your code is cheap
     2/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:31 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     ARM and x86 are CPU architectures, i.e. sets of instructions that a CPU can execute.

     Most CPUs implementing those architectures have a pipeline - that is, they start processing the next instruction before they're done with the previous one, kinda like an assembly line.

     This is great when they know what instruction is going to be executed next. But when there is a branch in the code (eg. an if statement) they don't know ahead of time.

     3/

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:32 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     LLVM is the reusable compiler mid-end - the programming-language-independent part that does optimizations. It is used eg. by clang, which is a C compiler built on top of LLVM.

     LLVM is famous for popularizing the SSA form.

     1/

   • Embed this notice
     Coded Artist (coded_artist@gameliberty.club)'s status on Tuesday, 25-Feb-2025 18:19:33 JST Coded Artist

     in reply to

     • SuperDicq
     • Wolf480pl

     @wolf480pl @LukeAlmighty @SuperDicq I would appreciate that, if you have the time.

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 18:19:34 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @coded_artist @LukeAlmighty @SuperDicq

     Up to "and creating serial dependencies" I understand everything (can explain if you want).

     But the rest is "I know some of these words" for me.

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Tuesday, 25-Feb-2025 19:22:55 JST Wolf480pl

     in reply to

     • SuperDicq
     • Coded Artist

     @SuperDicq @LukeAlmighty @coded_artist
     I actually kinda like splitting things into multiple posts, but if I had a larger character limit, I would split it less / in a more organized way

   • Embed this notice
     SuperDicq (superdicq@minidisc.tokyo)'s status on Tuesday, 25-Feb-2025 19:22:55 JST SuperDicq

     in reply to

     • Wolf480pl
     • Coded Artist

     @wolf480pl@mstdn.io @LukeAlmighty@gameliberty.club @coded_artist@gameliberty.club The 500 character limit of Mastodon is very arbitrary. Other fedi software don't do this. There's no reason to. It isn't the 1980s. Our storage space isn't limited because by goddamn text.

     翠星石 and LukeAlmighty 🇨🇿 like this.
   • Embed this notice
     翠星石 (suiseiseki@freesoftwareextremist.com)'s status on Tuesday, 25-Feb-2025 20:05:43 JST 翠星石

     in reply to

     • SuperDicq
     • Coded Artist
     @coded_artist @LukeAlmighty @SuperDicq I understood every word, but I am extremely disappointed that LLVM was used instead of GCC.