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Ask SN: When Is Assembly Worth It?

posted by Cactus on Saturday March 08 2014, @03:30AM   Printer-friendly
from the don't-tell-me-upgrade-PCs dept.

Subsentient writes:

"I'm a C programmer and Linux enthusiast. For some time, I've had it on my agenda to build the new version of my i586/Pentium 1 compatible distro, since I have a lot of machines that aren't i686 that are still pretty useful.

Let me tell you, since I started working on this, I've been in hell these last few days! The Pentium Pro was the first chip to support CMOV (Conditional move), and although that was many years ago, lots of chips were still manufactured that didn't support this (or had it broken), including many semi-modern VIA chips, and the old AMD K6.

Just about every package that has to deal with multimedia has lots of inline assembler, and most of it contains CMOV. Most packages let you disable it, either with a switch like ./configure --disable-asm or by tricking it into thinking your chip doesn't support it, but some of them (like MPlayer, libvpx/vp9) do NOT. This means, that although my machines are otherwise full blown, good, honest x86-32 chips, I cannot use that software at all, because it always builds in bad instructions, thanks to these huge amounts of inline assembly!

Of course, then there's the fact that these packages, that could otherwise possibly build and work on all types of chips, are now limited to what's usually the ARM/PPC/x86 triumvirate (sorry, no SPARC Linux!), and the small issue that inline assembly is not actually supported by the C standard.

Is assembly worth it for the handicaps and trouble that it brings? Personally I am a language lawyer/standard Nazi, so inline ASM doesn't sit well with me for additional reasons."

 
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  • (Score: 1) by tftp on Saturday March 08 2014, @05:21PM

    by tftp (806) on Saturday March 08 2014, @05:21PM (#13308) Homepage

    Perhaps I was a bit harsh claiming undocumentality of assembly code. However, the problem is that your execution environment changes as you move from one area of the code to another. You use registers, or parts of registers, for one function here, and then for another function there. There is no consistency; you can't have as story here. You can have a single bit flag in R3 bit 0 in one place; but then you optimize the code and notice that 100 lines down R3 comes loaded with something that you need, so in that place you have a similar flag in R2 bit 3. The compiler can't care less about such things; a bit is a bit. A human, however, is very likely to lose track of things. Coupled with lack of verification of the code, aside from mere syntax checks, a program in assembly is not an easy thing to work on. Small, tight loops are often OK, as they can be seen inserted into an otherwise C code (for GCC.) But a whole 'wc' in assembly is pure masochism. One could do it for a very clear, specific and justified purpose, like when coding something for a 4-bit MCU that runs a doll and costs 7.02 cents.