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by imtringued
684 days ago
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That's not how compiler developers interpret undefined behaviour. Undefined behaviour is closer to an 'U', 'X' or "don't care" in VHDL. These things don't exist in real hardware and only in simulation, therefore the synthesis tool simply assumes that they never happen and optimizes according to that. However, C does not have a simulation environment and UB propagation is not a thing. It will simply do weird shit, like run an infinite loop, because you forgot to write the return keyword, when you changed a function from void. |
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"undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements
NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
EXAMPLE An example of undefined behavior is the behavior on integer overflow." (The C99 standard, section 3.4.3)
translates into "whatever your CPU does" because while there is not requirement imposed, in general the compiler does make it work "in a manner characteristic of the environment".
I believe that memory accesses outside of array bounds, signed integer overflow, null pointer dereference are all examples of "undefined behaviour", which in practice all boil down to what the CPU does in those cases. I.e. commonly memory access outside of array bounds returns whatever is at that address as long as address is valid because there are no checks and that's what the CPU does when asked to load from address. Integer overflow? If a result of adding/subtracting, commonly it wraps around because that's how the CPU behaves, etc.
And I believe this is all on purpose. C is an abstraction over assembly and I believe that people who were used to their CPU's behaviour wanted to keep it that way in C, and also compilers were simple.