### 13.13 Conversions

All conversions between machine modes must be represented by explicit conversion operations. For example, an expression which is the sum of a byte and a full word cannot be written as `(plus:SI (reg:QI 34) (reg:SI 80))` because the `plus` operation requires two operands of the same machine mode. Therefore, the byte-sized operand is enclosed in a conversion operation, as in

         (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))

The conversion operation is not a mere placeholder, because there may be more than one way of converting from a given starting mode to the desired final mode. The conversion operation code says how to do it.

For all conversion operations, x must not be `VOIDmode` because the mode in which to do the conversion would not be known. The conversion must either be done at compile-time or x must be placed into a register.

`(sign_extend:`m x`)`
  Represents the result of sign-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m.

`(zero_extend:`m x`)`
  Represents the result of zero-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m.

`(float_extend:`m x`)`
  Represents the result of extending the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode narrower than m.

`(truncate:`m x`)`
  Represents the result of truncating the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode wider than m.

`(ss_truncate:`m x`)`
  Represents the result of truncating the value x to machine mode m, using signed saturation in the case of overflow. Both m and the mode of x must be fixed-point modes.

`(us_truncate:`m x`)`
  Represents the result of truncating the value x to machine mode m, using unsigned saturation in the case of overflow. Both m and the mode of x must be fixed-point modes.

`(float_truncate:`m x`)`
  Represents the result of truncating the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode wider than m.

`(float:`m x`)`
  Represents the result of converting fixed point value x, regarded as signed, to floating point mode m.

`(unsigned_float:`m x`)`
  Represents the result of converting fixed point value x, regarded as unsigned, to floating point mode m.

`(fix:`m x`)`
  When m is a floating-point mode, represents the result of converting floating point value x (valid for mode m) to an integer, still represented in floating point mode m, by rounding towards zero.

  When m is a fixed-point mode, represents the result of converting floating point value x to mode m, regarded as signed. How rounding is done is not specified, so this operation may be used validly in compiling C code only for integer-valued operands.

`(unsigned_fix:`m x`)`
  Represents the result of converting floating point value x to fixed point mode m, regarded as unsigned. How rounding is done is not specified.

`(fract_convert:`m x`)`
  Represents the result of converting fixed-point value x to fixed-point mode m, signed integer value x to fixed-point mode m, floating-point value x to fixed-point mode m, fixed-point value x to integer mode m regarded as signed, or fixed-point value x to floating-point mode m. When overflows or underflows happen, the results are undefined.

`(sat_fract:`m x`)`
  Represents the result of converting fixed-point value x to fixed-point mode m, signed integer value x to fixed-point mode m, or floating-point value x to fixed-point mode m. When overflows or underflows happen, the results are saturated to the maximum or the minimum.

`(unsigned_fract_convert:`m x`)`
  Represents the result of converting fixed-point value x to integer mode m regarded as unsigned, or unsigned integer value x to fixed-point mode m. When overflows or underflows happen, the results are undefined.

`(unsigned_sat_fract:`m x`)`
  Represents the result of converting unsigned integer value x to fixed-point mode m. When overflows or underflows happen, the results are saturated to the maximum or the minimum.