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Perfect example of the mismatch between (some) programmers' expectations and what language standards and/compilers require/implement. > C has perfecly well def

Perfect example of the mismatch between (some) programmers' expectations and what language standards and/compilers require/implement. > C has perfecly well defined semantics for (eg) null pointer dereference: read or write memory location zero, and consequently probably crash. C does have "perfectly well-defined semantics" for null pointer dereference, but it's undefined behavior. Sure, the null pointer happens to be 0 on the vast majority of architectures most programmers work with, but apparently non-zero null pointers are still used these days (at least in the admgcn LLVM target, from what I understand after a quick glance at the linked page), so it's not even a "all sane modern machines" thing [0]. In any case, I'd love to see some good benchmarks showing the effect of the more "controversial" optimizations. Compiler writers say they're important, people who don't like these optimizations tend to say that they don't make a significant difference. I don't really feel comfortable taking a side until I at least see what these optimizations enable (or fail to enable). I lean towards agreeing with the compiler writers, but perhaps that's because I haven't been bitten by one of these bugs yet... [0]: https://reviews.llvm.org/D26196

keldaris 8y ago
The compiler writers are right in the sense that for every one of those optimizations you can have a microbenchmark that will benefit hugely. The opponents are right too, in the sense that for most codebases the more niche optimizations don't matter at all. The right answer, as always, is to take a particular case you actually care about and benchmark yourself. There are no benchmarks that would be generally representative, there is no meaningful "average case". Personally, I mostly use C/C++ for fairly high performance numerical code and happen to benefit greatly from all the "unsafe" stuff, including optimizations only enabled by compilers taking advantage of undefined behavior. I'm therefore naturally inclined to strongly oppose any attempts to eliminate undefined behavior from the language standards. At the same time, however, I fully recognize that most people would probably benefit from a safer set of compiler defaults (as well as actually reading the compiler manuals once in a while) or even using languages with more restrictive semantics. Ultimately, there is no free lunch and performance doesn't come without its costs.

keldaris 8y ago

The compiler writers are right in the sense that for every one of those optimizations you can have a microbenchmark that will benefit hugely. The opponents are right too, in the sense that for most codebases the more niche optimizations don't matter at all. The right answer, as always, is to take a particular case you actually care about and benchmark yourself. There are no benchmarks that would be generally representative, there is no meaningful "average case". Personally, I mostly use C/C++ for fairly high performance numerical code and happen to benefit greatly from all the "unsafe" stuff, including optimizations only enabled by compilers taking advantage of undefined behavior. I'm therefore naturally inclined to strongly oppose any attempts to eliminate undefined behavior from the language standards. At the same time, however, I fully recognize that most people would probably benefit from a safer set of compiler defaults (as well as actually reading the compiler manuals once in a while) or even using languages with more restrictive semantics. Ultimately, there is no free lunch and performance doesn't come without its costs.