[CDRIVER-5996] Prevent multi-evaluation of arithmetic macro-function operands #1999
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The prior defn of `mlib_upsize_integer` would necessarily double-evaluate the operand expression when it was of certain types. This new definition makes use of the ternary operator to obtain an appropriate zero value without evaluating the operand.
kevinAlbs
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eramongodb
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Apr 25, 2025
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Although I am in favor of the improvement to avoiding (re)evaluation of the argument, I do not observe it addressing -Wtype-limit warnings as suggested with GCC 13.3.0:
warning: comparison of unsigned expression in ‘< 0’ is always false [-Wtype-limits]
| ((sizeof ((Value)) < sizeof (intmax_t) || (_mlibGetZero(Value) - 1) < _mlibGetZero(Value)) \
| ^
|
It looks there may also be a GCC 14 regression where the |
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Currently,
mlib_upsize_integeruses the expression(0 & V)to obtain a zero-value for the expressionV. This has the unfortunate effect that it must evaluateVin order to obtain this value.There's a better way:
1 ? 0 : V, which will follow the same integer promotion rules as the infix&, but does not evaluate the right-hand operand. This now allowsmlib_upsize_integerand all function-macros based on it to be used safely with operands that are expensive or produce side effects.