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GNU Compiler Collection (GCC) Internals

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8.12 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 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.

(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.


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