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syntax.md: Describe hex floats and their use.

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David Neto 2015-11-10 14:46:35 -05:00
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@ -36,7 +36,8 @@ the SPIR-V specification. An operand is one of:
integer supplied for a signed integer value will be sign-extended.
For example, `0xffff` supplied as the literal for an `OpConstant`
on a signed 16-bit integer type will be interpreted as the value `-1`.
* a literal floating point number.
* a literal floating point number, in decimal or hexadecimal form.
See [below](#floats).
* a literal string.
* A literal string is everything following a double-quote `"` until the
following un-escaped double-quote. This includes special characters such
@ -86,6 +87,38 @@ So the example can be rewritten using more user-friendly names, as follows:
OpFunctionEnd
```
## Floating point literals
<a name="floats"></a>
The assembler and disassembler support floating point literals in both
decimal and hexadecimal form.
The syntax for a floating point literal is the same as floating point
constants in the C programming language, except:
* An optional leading minus (`-`) is part of the literal.
* An optional type specifier suffix is not allowed.
Infinity and NaN values are expressed in hexadecimal float literals
by using the maximum representable exponent for the bit width.
For example, in 32-bit floating point, 8 bits are used for the exponent, and the
exponent bias is 127. So the maximum representable unbiased exponent is 128.
Therefore, we represent the infinities and a some NaNs as follows:
```
%float32 = OpTypeFloat 32
%inf = OpConstant %float32 0x1p+128
%neginf = OpConstant %float32 -0x1p+128
%aNaN = OpConstant %float32 0x1.8p+128
%moreNaN = OpConstant %float32 -0x1.0002p+128
```
The assembler preserves all the bits of a NaN value. For example, the encoding
of `%aNaN` in the previous example is the same as `!0x7fc00000`, and `%moreNaN`
is encoded as `!0xff800100`.
The disassembler prints infinite, NaN, and subnormal values in hexadecimal form.
Zero and normal values are printed in decimal form with enough digits
to preserve all significand bits.
## Arbitrary Integers
<a name="immediate"></a>