Make C floating point literals float (rather than double)

-fsingle-precision-constant flag can be used. It causes floating-point constants to be loaded in single precision even when this is not exact.

Note- This will also use single precision constants in operations on double precision variables.

Use warnings instead: -Wdouble-promotion warns about implicit float to double promotion, as in your example. -Wfloat-conversion will warn about cases where you may still be assigning doubles to floats.

This is a better solution than simply forcing double values to the nearest float value. Your floating-point code is still compliant, and you won't get any nasty surprises if a double value holds a positive value, say, less than FLT_DENORM_MIN (assuming IEEE-754) or greater than FLT_MAX.

You can cast the defined constants to (float) wherever they are used, the optimizer should do its job. This is a portable solution.

#define LIMIT 2.5

if (x < (float)LIMIT) ...

The -Wunsuffixed-float-constants flag could be used too, maybe combined with some of the other options in the accepted answer above. However, this probably won't catch unsuffixed constants in system headers. Would need to use -Wsystem-headers to catch those too. Could generate a lot of warnings...