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I think pretty much all linear algebra libraries are still Fortran and are unlikely to ever be C because Fortran is legitimately faster than C for this stuff. I

I think pretty much all linear algebra libraries are still Fortran and are unlikely to ever be C because Fortran is legitimately faster than C for this stuff. I don't know if it has anything to do with strong opinions about how values are represented, I think it has more to do with lower overhead of function calling, but that is just repeating what someone told me about Fortran vs C in general, not necessarily applicable to BLAS libraries. Fortran at least used to be very common in heavy scientific computing, but I would bet that relies on GPUs or other accelerators these days.

yosefk 2y ago
The question is how much new numeric code is written in Fortran vs C/C++. My guess is way below 10%, certainly if we measure by usage as opposed to LOC and I would guess by LOC as well. Is Fortran legitimately faster than C with the restrict keyword? Regardless of the function call cost diffs between the two - meaning, even if we assume Fortran is somehow faster here - there's no way numeric production code does enough function calls for this to matter. If Fortran was faster than C at any point in time I can only imagine pointer aliasing issues to have been the culprit and I can't imagine it still being relevant, but I am of course ready to stand corrected.
- eyegor 2y ago
  I don't think fortran is faster by language virtue, but it's certainly easier to scrap together high performance numeric fortran code. And ifort/aocc are amazing at making code that runs well on clusters, which is not a priority for any other domain. Fortran is absolutely still on top for modern numerics research code that involves clusters, talk to anyone who works in simulations at a national lab. If your code is mostly matrix math, modern fortran is very nice to work with. Emphasis on modern because maintaining 70s-80s numeric mathematician cowboy code is a ninth circle of hell. It has a bad rep for that reason.
  - semi-extrinsic 2y ago
    When you speak about code from the 1970s in particular, you need to appreciate the extremely limited language features that were available. They did not even have while-loops in the language until Fortran 77. While-loop functionality used to be implemented using DO-loops mixed with GOTOs. You can't fault the people of that era for using the only tools that were available to them.
- ant6n 2y ago
  At the core, Fortran uses multidimensional numerical arrays, with the shapes being defined being defined in the code. So the compiler knows much more about the data that’s being operated on, which in theory allows better optimization. I thought blas/lapack is still written in Fortran, so most numerical code would still be built on top of Fortran.
- josefx 2y ago
  Restrict probably helps. I can't say much about fortran but C still has warts that can significantly impact its math library. For example almost every math function may set errno, that is a sideeffect that isn't easy to eliminate by the compiler and might bloat the code significantly. For example with gcc a single instruction sqrt turns into a sqrt instruction, followed by several checks to see if it succeeded, followed by a function call to the sqrt library function just to set errno correctly. I just started to disable math-errno completely once I realized that C allows several alternative ways to handle math errors, which basically makes any code relying on it non portable.
Galanwe 2y ago
> I think pretty much all linear algebra libraries are still Fortran and are unlikely to ever be C That is more of an urban legend than reality in 2024. Fact is, although the original BLAS implementation was fortran, it has been at least a decade since every Linux distribution ships either OpenBLAS or ATLAS or the MKL, which are both written in a mix of C and assembly. All modern processor support is only available in these implementations. LAPACK itself is still often built from the Fortran code base, but it's just glue over BLAS, and it gets performance solely from its underlying BLAS. Fortran doesn't bring any value to LAPACK, it's just that rewriting millions of algebraic tricks and heuristics for no performance gain is not enticing.
- deleted 2y ago
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yosefk 2y ago

The question is how much new numeric code is written in Fortran vs C/C++. My guess is way below 10%, certainly if we measure by usage as opposed to LOC and I would guess by LOC as well. Is Fortran legitimately faster than C with the restrict keyword? Regardless of the function call cost diffs between the two - meaning, even if we assume Fortran is somehow faster here - there's no way numeric production code does enough function calls for this to matter. If Fortran was faster than C at any point in time I can only imagine pointer aliasing issues to have been the culprit and I can't imagine it still being relevant, but I am of course ready to stand corrected.

eyegor 2y ago
I don't think fortran is faster by language virtue, but it's certainly easier to scrap together high performance numeric fortran code. And ifort/aocc are amazing at making code that runs well on clusters, which is not a priority for any other domain. Fortran is absolutely still on top for modern numerics research code that involves clusters, talk to anyone who works in simulations at a national lab. If your code is mostly matrix math, modern fortran is very nice to work with. Emphasis on modern because maintaining 70s-80s numeric mathematician cowboy code is a ninth circle of hell. It has a bad rep for that reason.
- semi-extrinsic 2y ago
  When you speak about code from the 1970s in particular, you need to appreciate the extremely limited language features that were available. They did not even have while-loops in the language until Fortran 77. While-loop functionality used to be implemented using DO-loops mixed with GOTOs. You can't fault the people of that era for using the only tools that were available to them.
ant6n 2y ago
At the core, Fortran uses multidimensional numerical arrays, with the shapes being defined being defined in the code. So the compiler knows much more about the data that’s being operated on, which in theory allows better optimization. I thought blas/lapack is still written in Fortran, so most numerical code would still be built on top of Fortran.
josefx 2y ago
Restrict probably helps. I can't say much about fortran but C still has warts that can significantly impact its math library. For example almost every math function may set errno, that is a sideeffect that isn't easy to eliminate by the compiler and might bloat the code significantly. For example with gcc a single instruction sqrt turns into a sqrt instruction, followed by several checks to see if it succeeded, followed by a function call to the sqrt library function just to set errno correctly. I just started to disable math-errno completely once I realized that C allows several alternative ways to handle math errors, which basically makes any code relying on it non portable.

Galanwe 2y ago

> I think pretty much all linear algebra libraries are still Fortran and are unlikely to ever be C That is more of an urban legend than reality in 2024. Fact is, although the original BLAS implementation was fortran, it has been at least a decade since every Linux distribution ships either OpenBLAS or ATLAS or the MKL, which are both written in a mix of C and assembly. All modern processor support is only available in these implementations. LAPACK itself is still often built from the Fortran code base, but it's just glue over BLAS, and it gets performance solely from its underlying BLAS. Fortran doesn't bring any value to LAPACK, it's just that rewriting millions of algebraic tricks and heuristics for no performance gain is not enticing.

deleted 2y ago
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