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Division by INT64_MIN


Hello all,

Apologies in advance if this question is naÃve or misdirected.

tl;dr: Is dividing by INT64_MIN in C undefined behaviour?

In more detail... AIUI when the current target does not have native instructions for a given division or modulo operation, GCC emits calls to software emulation provided by libgcc. For example, a signed 64-bit division on x86 is emitted as a call to __divdi3. If the numerator or denominator are negative, __divdi3 negates them. If either of these values is INT64_MIN, I believe this negation is undefined. Is this correct? If this is the case, then it seems code like "INT64_MIN / INT64_MIN" which should be perfectly legal accidentally causes undefined behaviour via libgcc. In practice, everything seems to work as expected, but it seems to me that the C code of __divdi3 should not be relying on these negations working consistently.

This only occurred to me after being in a situation where I had to implement __divdi3 myself. Afterwards I looked at libgcc's out of curiosity and was surprised to find it was much simpler than my attempt. Any clarification would be appreciated.

Thanks,
Matthew


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