5 ms·

As someone who's worked on old Macs and has also done lots of 32 -> 64-bit porting, this is the sort of trick that works wonderfully...until it doesn't. And th

As someone who's worked on old Macs and has also done lots of 32 -> 64-bit porting, this is the sort of trick that works wonderfully...until it doesn't. And then you've got a nightmare on your hands. I'm not saying never do that (ok, maybe I am...) But definitely think long and hard about how long your code will be around before you do it.

astrobe_ 7y ago
I'm curious, when does it fail?
jcelerier 7y ago
> As someone who's worked on old Macs and has also done lots of 32 -> 64-bit porting, this is the sort of trick that works wonderfully...until it doesn't. And then you've got a nightmare on your hands. That's why you hide the trick behind a zero-cost abstraction which checks at compile-time if the platform supports this
- BeeOnRope 7y ago
  Those failures were all about top bits, not low bits though weren't they? What problems did lower bit use cause? You always had to mask it away even on the old boxes, no?
- notacoward 7y ago
  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.

astrobe_ 7y ago

I'm curious, when does it fail?

jcelerier 7y ago

> As someone who's worked on old Macs and has also done lots of 32 -> 64-bit porting, this is the sort of trick that works wonderfully...until it doesn't. And then you've got a nightmare on your hands. That's why you hide the trick behind a zero-cost abstraction which checks at compile-time if the platform supports this

BeeOnRope 7y ago
Those failures were all about top bits, not low bits though weren't they? What problems did lower bit use cause? You always had to mask it away even on the old boxes, no?
notacoward 7y ago
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.