Using Assembly Language in C/C++

Question

I remember reading somewhere that to really optimize & speed up certain section of the code, programmers write that section in Assembly language. My questions are -

Answer 1

It depends. It is (still) being done in some situations, but for the most part, it is not worth it. Modern CPUs are insanely complex, and it is equally complex to write efficient assembly code for them. So most of the time, the assembly you write by hand will end up slower than what the compiler can generate for you.

Assuming a decent compiler released within the last couple of years, you can usually tweak your C/C++ code to gain the same performance benefit as you would using assembly.

A lot of people in the comments and answers here are talking about the "N times speedup" they gained rewriting something in assembly, but that by itself doesn't mean too much. I got a 13 times speedup from rewriting a C function evaluating fluid dynamics equations in C, by applying many of the same optimizations as you would if you were to write it in assembly, by knowing the hardware, and by profiling. At the end, it got close enough to the theoretical peak performance of the CPU that there would be no point in rewriting it in assembly. Usually, it's not the language that's the limiting factor, but the actual code you've written. As long as you're not using "special" instructions that the compiler has difficulty with, it's hard to beat well-written C++ code.

Assembly isn't magically faster. It just takes the compiler out of the loop. That is often a bad thing, unless you really know what you're doing, since the compiler performs a lot of optimizations that are really really painful to do manually. But in rare cases, the compiler just doesn't understand your code, and can't generate efficient assembly for it, and then, it might be useful to write some assembly yourself. Other than driver development or the like (where you need to manipulate the hardware directly), the only place I can think of where writing assembly may be worth it is if you're stuck with a compiler that can't generate efficient SSE code from intrinsics (such as MSVC). Even there, I'd still start out using intrinsics in C++, and profile it and try to tweak it as much as possible, but because the compiler just isn't very good at this, it might eventually be worth it to rewrite that code in assembly.