| |
Assembly can express very low-level things:
you can access machine-dependent registers and I/O you can control the exact code behavior
in critical sections that might otherwise involve deadlock
between multiple software threads or hardware devices you can break the conventions of your usual compiler,
which might allow some optimizations
(like temporarily breaking rules about memory allocation,
threading, calling conventions, etc) you can build interfaces between code fragments
using incompatible conventions
(e.g. produced by different compilers,
or separated by a low-level interface) you can get access to unusual programming modes of your processor
(e.g. 16 bit mode to interface startup, firmware, or legacy code
on Intel PCs) you can produce reasonably fast code for tight loops
to cope with a bad non-optimizing compiler
(but then, there are free optimizing compilers available!) you can produce hand-optimized code
perfectly tuned for your particular hardware setup,
though not to anyone else's you can write some code for your new language's optimizing compiler
(that is something what very few ones will ever do, and even they not often)
Assembly is a very low-level language
(the lowest above hand-coding the binary instruction patterns).
This means
it is long and tedious to write initially it is quite bug-prone your bugs can be very difficult to chase your code can be fairly difficult to understand and modify,
i.e. to maintain the result is non-portable to other architectures,
existing or upcoming your code will be optimized only for a certain implementation
of a same architecture:
for instance, among Intel-compatible platforms
each CPU design and its variations
(relative latency, through-output, and capacity,
of processing units, caches, RAM, bus, disks,
presence of FPU, MMX, 3DNOW, SIMD extensions, etc)
implies potentially completely different optimization techniques.
CPU designs already include:
Intel 386, 486, Pentium, PPro, PII, PIII, PIV;
Cyrix 5x86, 6x86, M2; AMD K5, K6 (K6-2, K6-III), K7 (Athlon, Duron).
New designs keep popping up, so don't expect either this listing
and your code to be up-to-date. you spend more time on a few details
and can't focus on small and large algorithmic design,
that are known to bring the largest part of the speed up
(e.g. you might spend some time building very fast
list/array manipulation primitives in assembly;
only a hash table would have sped up your program much more;
or, in another context, a binary tree;
or some high-level structure distributed over a cluster of CPUs) a small change in algorithmic design might completely
invalidate all your existing assembly code.
So that either you're ready (and able) to rewrite it all,
or you're tied to a particular algorithmic design On code that ain't too far from what's in standard benchmarks,
commercial optimizing compilers outperform hand-coded assembly
(well, that's less true on the x86 architecture than on RISC architectures,
and perhaps less true for widely available/free compilers;
anyway, for typical C code, GCC is fairly good); And in any case, as says moderator John Levine on
comp.compilers, "compilers make it a lot easier to use complex data structures,
and compilers don't get bored halfway through
and generate reliably pretty good code." They will also correctly propagate code transformations
throughout the whole (huge) program
when optimizing code between procedures and module boundaries.
All in all, you might find that though using assembly is sometimes needed,
and might even be useful in a few cases where it is not, you'll want to:
minimize the use of assembly code encapsulate this code in well-defined interfaces have your assembly code automatically generated
from patterns expressed in a higher-level language
than assembly (e.g. GCC inline assembly macros) have automatic tools translate these programs
into assembly code have this code be optimized if possible All of the above,
i.e. write (an extension to) an optimizing compiler back-end.
Even in cases when assembly is needed (e.g. OS development),
you'll find that not so much of it is,
and that the above principles hold. See the Linux kernel sources concerning this:
as little assembly as needed,
resulting in a fast, reliable, portable, maintainable OS.
Even a successful game like DOOM was almost massively written in C,
with a tiny part only being written in assembly for speed up. |
