A guide to inline assembly code in GCC

Posted May 3, 2016 18:48 UTC (Tue) by khim (subscriber, #9252)
In reply to: A guide to inline assembly code in GCC by adobriyan
Parent article: A guide to inline assembly code in GCC

Well, compiler generates correct code somehow.

Well, yeah.

This "somehow" should be enough for managing register interdependencies and stuff.

Sure. But it's not called "somehow". It's called… drumroll… constraints. All the things which you could use are quite literally described in the same language you are using to describe your inline assembler. Here is description of x86 CPU. And here is an arm.

Compiler's CPU model does not have instructions like rdtsc or cpuid thus to generate correct code compiler most obtain that information… "somehow".

If you dig out through one level of macros the "low" and "high" are "unsigned long" but they were "unsigned int" earlier and gcc generated dummy MOV instruction. Not much of a loss, of course, but knowledge about RDTSC clearing the upper half of a register will make this code efficient automatically. It would look something like:
#ifdef CONFIG_X86_64 uint64_t rdx, rax; asm ((rdx, rax) = rdtsc();) return (rdx <<32) | rax; #else uint32_t edx, eax; asm ((edx, eax) = rdtsc();) return ((uint64_t)edx <<32) | eax; #endif

Sure. But that's not called "assembler" at this point. These are intrinsic. There are many of them and when they work - you should use them. But if they don't… then they don't—compiler must be teached to use the capabilities it knows nothing about.

If the constraints which so irritate you would have been were invented for the sole purpose of writing asm—it would have been strange. But the fact that you must use the same method which is used to teach compiler about CPU in the first place does not really looks surprising to me. I mean: what else would you use?