sorry, I don't understand, it seems to me that current optimizing compilers are fully compliant with the option of "ignoring the situation completely with unpredictable results". That's how UB exploiting optimizations work: the compiler ignores the possibility that the erroneous case can ever happen and takes into account only all valid states.
Apologies, but "ignoring the situation completely with undpredictable results" is the exact opposite of "assuming the situation cannot occur and acting on that assumption".
If my program attempts to write outside the bounds of declared array, ignoring the situation (that this is UB) is letting that write happen, and letting the chips fall where they might.
How is assuming it cannot/must not happen, and then optimising it away because it did happen "ignoring the situation"??
So a compiler that is allowed to ignore a situation would still be required to generate code for that situation? I don't even know how that would be possible.
1. Arrays ≠ Everything. Please stop with the straw men.
2. "Compiler Advances Double Computing Power Every 50 Years, And The Interval Keeps Growing". (i.e. even Probsting was wildly optimistic when it comes to the benefits of compiler research). The benefits of these shenanigans are much less than even the critics thought, never mind what the advocates claim. They are actually small and shrinking.
It means ignoring the situation and emitting the code that the programmer wrote. The programmer did not write "put this array in registers". The programmer did write "access element 6 of this array".
This isn't hard.
"Keep the spirit of C. The Committee kept as a major goal to preserve the traditional spirit of C. There are many facets of the spirit of C, but the essence is a community sentiment of the underlying principles upon which the C language is based. Some of the facets of the spirit of C can be summarized in phrases like
- Trust the programmer.
- Don't prevent the programmer from doing what needs to be done.
- Keep the language small and simple.
- Provide only one way to do an operation.
- Make it fast, even if it is not guaranteed to be portable.
The last proverb needs a little explanation. The potential for efficient code generation is one of the most important strengths of C. To help ensure that no code explosion occurs for what appears to be a very simple operation, many operations are defined to be how the target machine's hardware does it rather than by a general abstract rule."
> calling scalar replacement of aggregates a weird optimization is very strange.
No it's not, certainly for arrays. It's a little less weird for structs than it is for arrays. But the fact that you consider calling it weird "very strange" is telling, and to me the heart of this disconnect.
The Optimiser-über-alles community feels that the needs of the optimiser far outweigh the explicit instructions of the programmer. Many programmers beg to differ.
At this point it is obvious that you wouldn't accept anything except a trivial translation to ASM. That's a perfectly reasonable thing to want, but you'll also understand that's not what 99.99% of C and C++ programmers want; even assuming (but not conceding) that might have been the original intent of the language, 50 year later user expectations have changed and there is no reason compilers authors should be bound by some questionable dogma.
> Apologies, but "ignoring the situation completely with undpredictable results" is the exact opposite of "assuming the situation cannot occur and acting on that assumption".
Seems to me it's not the opposite but the exact same thing.
"Ignore the situation": emit the code the code for a[6]=3; in the same way you emitted the code for a[2]=2. You've ignored the fact that this is UB.
"Assume the situation cannot occur": don't know, but according to the UB extremists the compiler can now do anything it wants to do, including not emitting any code for this fragment at all (which appears to happen) or formatting your hard disk (which doesn't, last I checked).
Assuming that a[6]=3 does not occur, despite the fact that it is written right there, would also allow putting a into registers, which "ignoring the situation" would not.
