3 ms·

One of my favorite system programming tricks is to never believe that a "zero cost abstraction" lives up to the name.

hinkley 7y ago
Sometimes programmer advice needs disclaimers like prescription medicines. † Offer only valid in the contiguous United States. Offer cannot be used in combination with any other offers. See stores for details. ‡ When not in use, return Happy Fun Ball™ to its protective lead case.
jjuhl 7y ago
Modern optimizing C++ compilers (especially with Link Time Optimization enabled) are pretty amazing and can very often actually achieve that abstraction collapsing.. But, of course, always measure.
- notacoward 7y ago
  While it's true that modern compilers are wondrous things, checking whether they're clever enough to optimize away a particular construct - and to do so correctly, and to continue doing so in the next release - still takes time. If the same optimization can be done at a higher level, such that it will apply for any correct (but not necessarily clever) compiler, that's preferable. In my experience that's practically all the time. The best compiler optimizations IMO are the ones that can't be easily done at the source level.
vardump 7y ago
Yeah, don't believe in dogmas, measure. Zero cost abstractions sometimes are anything but.
- hinkley 7y ago
  And sometimes the surprises can be in the other direction. Cache thrashing is a thing. And perf analysis tools have bookkeeping errors.
saagarjha 7y ago
The abstraction necessary to make tagged pointers platform-dependent is quite small and trivially removed by any optimizing compiler.
ajross 7y ago
Yeah, but an even more important system programming trick is to measure what you're doing every time. Performance optimization at this level is never about just trusting tools. If you aren't willing to be reading generated machine code and checking perf counters, you aren't really going to get much benefit. And if you're willing to check your optimizations by reading disassembly, tricks like stuffing tag bits into the bottom of aligned pointer values is pretty routine.

Sometimes programmer advice needs disclaimers like prescription medicines. † Offer only valid in the contiguous United States. Offer cannot be used in combination with any other offers. See stores for details. ‡ When not in use, return Happy Fun Ball™ to its protective lead case.

jjuhl 7y ago

Modern optimizing C++ compilers (especially with Link Time Optimization enabled) are pretty amazing and can very often actually achieve that abstraction collapsing.. But, of course, always measure.

notacoward 7y ago
While it's true that modern compilers are wondrous things, checking whether they're clever enough to optimize away a particular construct - and to do so correctly, and to continue doing so in the next release - still takes time. If the same optimization can be done at a higher level, such that it will apply for any correct (but not necessarily clever) compiler, that's preferable. In my experience that's practically all the time. The best compiler optimizations IMO are the ones that can't be easily done at the source level.

vardump 7y ago

Yeah, don't believe in dogmas, measure. Zero cost abstractions sometimes are anything but.

hinkley 7y ago
And sometimes the surprises can be in the other direction. Cache thrashing is a thing. And perf analysis tools have bookkeeping errors.

saagarjha 7y ago

The abstraction necessary to make tagged pointers platform-dependent is quite small and trivially removed by any optimizing compiler.

ajross 7y ago

Yeah, but an even more important system programming trick is to measure what you're doing every time. Performance optimization at this level is never about just trusting tools. If you aren't willing to be reading generated machine code and checking perf counters, you aren't really going to get much benefit. And if you're willing to check your optimizations by reading disassembly, tricks like stuffing tag bits into the bottom of aligned pointer values is pretty routine.