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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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