Fix potential overflow in FixedDiv, re-add gcc asm implementation #1832
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This reverts commit fac7cf7.
clang generates poor code otherwise: https://stackoverflow.com/questions/16850309/clang-llvm-inline-assembly-multiple-constraints-with-useless-spills-reload
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fabiangreffrath
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Dec 7, 2024
) * Revert "remove GCC variant of `div64_32` (#1818)" This reverts commit fac7cf7. * Remove asm multi-constraint clang generates poor code otherwise: https://stackoverflow.com/questions/16850309/clang-llvm-inline-assembly-multiple-constraints-with-useless-spills-reload * Fix potential FixedDiv overflow when passed INT_MIN as first arg
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If the first argument of
FixedDivisINT_MINits overflow check is effectively bypassed becauseabs(INT_MIN)==INT_MIN. This is the only possible explanation I can see for #1816, even though neither @rfomin nor myself have been able to reproduce this crash.Using an unsigned comparison here fixes the problem and seems like it shouldn't affect demo compatibility - this bug has never been reported in game (see discussion here) so it seems impossible to get
INT_MINas either arg in the course of normal play or else someone would have noticed by now. Modern ports technically already broke compatibility here anyway - in vanilla an overflow will result in a crash, while an implementation using an int64 divide will silently return an incorrect truncated result instead.