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Is there any system out there that supports branch 'annotations', of a sort, so that the programmer or the compiler can just tell the CPU what the branch behavi

by ajkjk 9y ago

Is there any system out there that supports branch 'annotations', of a sort, so that the programmer or the compiler can just tell the CPU what the branch behavior is going to be? Like -- it seems kinda silly for the CPU to do so much work to figure out if a loop is going to be repeated frequently, when the code could just explicitly say "fyi, this branch is going to be taken 99 times out of 100". Or, if there's a loop that is always taken 3 times and then passed once, that could be expressed explicitly, with a "predict this branch if i%4 != 0" annotation.

_chris_ 9y ago
Yes, a few ISAs have things like "branch.likely", but the CPU will just ignore it because it will do a better job anyways (the compiler is only as good as the test program used to guide its analysis). GCC allows the programmer to guide "unlikely" branches too. I assume that means GCC moves that code away so it doesn't pollute the I$ and static predictors can predict them correctly. There are some (usually small, embedded) processors that have "loop count" instructions. They are typically stateful and hard to make work in high-performance pipelines.
ryangittins 9y ago
Yes and no. I say yes because C and C++ both have likely() and unlikely() functions (well, technically It's part of a compiler extension rather than the language), which you wrap around the condition inside your if() statement like so: for(i=10000; i<100000; i++) { if(unlikely(isPrime(i))) { // do something special } else { // do something boring } } I say no because most modern compilers simply ignore the functions. Compilers have become so sophisticated over the years that developers trying to help them along or optimize often make things worse, whether that's by getting in the compiler's way or just writing code that's harder to read and debug. I say no (or at least probably not) to your second questions as well. No language I know if implements anything as sophisticated as a specification for regular intervals of branch switching. Many modern compilers have sophisticated branch prediction routines which can detect simple regular intervals like you describe. To develop such a specification would optimize the branch prediction by a tiny margin which would be absolutely dwarfed by the overhead of learning the syntax for the specification, not messing it up, debugging it if you do mess it up, communicating the decision to other team members, and all of the other real-world stuff that gets in the way. Computers are faster than ever and branch prediction algorithms are smarter than ever. Yes, you could help it along in theory but the portion of applications which really require you to do so is dwindling all the time.

by ajkjk 9y ago

_chris_ 9y ago

Yes, a few ISAs have things like "branch.likely", but the CPU will just ignore it because it will do a better job anyways (the compiler is only as good as the test program used to guide its analysis). GCC allows the programmer to guide "unlikely" branches too. I assume that means GCC moves that code away so it doesn't pollute the I$ and static predictors can predict them correctly. There are some (usually small, embedded) processors that have "loop count" instructions. They are typically stateful and hard to make work in high-performance pipelines.

ryangittins 9y ago

Yes and no. I say yes because C and C++ both have likely() and unlikely() functions (well, technically It's part of a compiler extension rather than the language), which you wrap around the condition inside your if() statement like so: for(i=10000; i<100000; i++) { if(unlikely(isPrime(i))) { // do something special } else { // do something boring } } I say no because most modern compilers simply ignore the functions. Compilers have become so sophisticated over the years that developers trying to help them along or optimize often make things worse, whether that's by getting in the compiler's way or just writing code that's harder to read and debug. I say no (or at least probably not) to your second questions as well. No language I know if implements anything as sophisticated as a specification for regular intervals of branch switching. Many modern compilers have sophisticated branch prediction routines which can detect simple regular intervals like you describe. To develop such a specification would optimize the branch prediction by a tiny margin which would be absolutely dwarfed by the overhead of learning the syntax for the specification, not messing it up, debugging it if you do mess it up, communicating the decision to other team members, and all of the other real-world stuff that gets in the way. Computers are faster than ever and branch prediction algorithms are smarter than ever. Yes, you could help it along in theory but the portion of applications which really require you to do so is dwindling all the time.