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The compiler isn't changing any programmer dictated behavior. There are no UB sourced 'optimizations' being implicitly disabled and none have been expressly

The compiler isn't __changing any programmer dictated behavior__. There are no UB sourced 'optimizations' being implicitly disabled and none have been expressly enabled. As long as valid code compiles it's on the humans that wrote the code (or that triggered it's writing in some higher level synthesis tool). Expanding on this; I don't want compilers _making_ optimizations based on UB. I want them educating the programmers so that the UB can be eliminated at a human level and correct outcomes are the result, optimized if it makes sense.

UncleMeat 6y ago
But this is a misunderstanding. The “compiler changing behavior” isn’t actually happening in those cases. The program’s behavior never changed. We just have a wrong understanding of the program’s behavior as humans reading the source. If we were to codify and write down all of the expectations we might have about what behavior a source file might represent so that the compiler can match it exactly.... that’d be the standard and we are back where we started.
klodolph 6y ago
So, you want something like, WARNING: p may be NULL (undefined behavior)
- mjevans 6y ago
  That's one way, but I don't want the compiler attempting to out-think whatever wrote the code. If the code has undefined behavior, if the code has a non-obvious transformation that would make it faster (but IE changes the ordering, referenced variables, etc) that should be presented for integration to the source code, NOT silently performed for that single compilation. Never _make_ the optimization for the programmer. It's fine to have it as an error and suggest the optimal variant instead __for human review__. "I don't want compilers _making_ optimizations based on UB. I want them educating the programmers so that the UB can be eliminated at a human level and correct outcomes are the result, optimized if it makes sense."

The compiler isn't __changing any programmer dictated behavior__. There are no UB sourced 'optimizations' being implicitly disabled and none have been expressly

UncleMeat 6y ago

But this is a misunderstanding. The “compiler changing behavior” isn’t actually happening in those cases. The program’s behavior never changed. We just have a wrong understanding of the program’s behavior as humans reading the source. If we were to codify and write down all of the expectations we might have about what behavior a source file might represent so that the compiler can match it exactly.... that’d be the standard and we are back where we started.

klodolph 6y ago

So, you want something like, WARNING: p may be NULL (undefined behavior)

mjevans 6y ago
That's one way, but I don't want the compiler attempting to out-think whatever wrote the code. If the code has undefined behavior, if the code has a non-obvious transformation that would make it faster (but IE changes the ordering, referenced variables, etc) that should be presented for integration to the source code, NOT silently performed for that single compilation. Never _make_ the optimization for the programmer. It's fine to have it as an error and suggest the optimal variant instead __for human review__. "I don't want compilers _making_ optimizations based on UB. I want them educating the programmers so that the UB can be eliminated at a human level and correct outcomes are the result, optimized if it makes sense."