1# SPIR-V Assembly language syntax
2
3## Overview
4
5The assembly attempts to adhere to the binary form from Section 3 of the SPIR-V
6spec as closely as possible, with one exception aiming at improving the text's
7readability.  The `<result-id>` generated by an instruction is moved to the
8beginning of that instruction and followed by an `=` sign.  This allows us to
9distinguish between variable definitions and uses and locate value definitions
10more easily.
11
12Here is an example:
13
14```
15     OpCapability Shader
16     OpMemoryModel Logical Simple
17     OpEntryPoint GLCompute %3 "main"
18     OpExecutionMode %3 LocalSize 64 64 1
19%1 = OpTypeVoid
20%2 = OpTypeFunction %1
21%3 = OpFunction %1 None %2
22%4 = OpLabel
23     OpReturn
24     OpFunctionEnd
25```
26
27A module is a sequence of instructions, separated by whitespace.
28An instruction is an opcode name followed by operands, separated by
29whitespace.  Typically each instruction is presented on its own line,
30but the assembler does not enforce this rule.
31
32The opcode names and expected operands are described in Section 3 of
33the SPIR-V specification.  An operand is one of:
34* a literal integer: A decimal integer, or a hexadecimal integer.
35  A hexadecimal integer is indicated by a leading `0x` or `0X`.  A hex
36  integer supplied for a signed integer value will be sign-extended.
37  For example, `0xffff` supplied as the literal for an `OpConstant`
38  on a signed 16-bit integer type will be interpreted as the value `-1`.
39* a literal floating point number, in decimal or hexadecimal form.
40  See [below](#floats).
41* a literal string.
42   * A literal string is everything following a double-quote `"` until the
43     following un-escaped double-quote. This includes special characters such
44     as newlines.
45   * A backslash `\` may be used to escape characters in the string. The `\`
46     may be used to escape a double-quote or a `\` but is simply ignored when
47     preceding any other character.
48* a named enumerated value, specific to that operand position.  For example,
49  the `OpMemoryModel` takes a named Addressing Model operand (e.g. `Logical` or
50  `Physical32`), and a named Memory Model operand (e.g. `Simple` or `OpenCL`).
51  Named enumerated values are only meaningful in specific positions, and will
52  otherwise generate an error.
53* a mask expression, consisting of one or more mask enum names separated
54  by `|`.  For example, the expression `NotNaN|NotInf|NSZ` denotes the mask
55  which is the combination of the `NotNaN`, `NotInf`, and `NSZ` flags.
56* an injected immediate integer: `!<integer>`.  See [below](#immediate).
57* an ID, e.g. `%foo`. See [below](#id).
58* the name of an extended instruction.  For example, `sqrt` in an extended
59  instruction such as `%f = OpExtInst %f32 %OpenCLImport sqrt %arg`
60* the name of an opcode for OpSpecConstantOp, but where the `Op` prefix
61  is removed.  For example, the following indicates the use of an integer
62  addition in a specialization constant computation:
63  `%sum = OpSpecConstantOp %i32 IAdd %a %b`
64
65## ID Definitions & Usage
66<a name="id"></a>
67
68An ID _definition_ pertains to the `<result-id>` of an instruction, and ID
69_usage_ is a use of an ID as an input to an instruction.
70
71An ID in the assembly language begins with `%` and must be followed by a name
72consisting of one or more letters, numbers or underscore characters.
73
74For every ID in the assembly program, the assembler generates a unique number
75called the ID's internal number. Then each ID reference translates into its
76internal number in the SPIR-V output. Internal numbers are unique within the
77compilation unit: no two IDs in the same unit will share internal numbers.
78
79The disassembler generates IDs where the name is always a decimal number
80greater than 0.
81
82So the example can be rewritten using more user-friendly names, as follows:
83```
84          OpCapability Shader
85          OpMemoryModel Logical Simple
86          OpEntryPoint GLCompute %main "main"
87          OpExecutionMode %main LocalSize 64 64 1
88  %void = OpTypeVoid
89%fnMain = OpTypeFunction %void
90  %main = OpFunction %void None %fnMain
91%lbMain = OpLabel
92          OpReturn
93          OpFunctionEnd
94```
95
96## Floating point literals
97<a name="floats"></a>
98
99The assembler and disassembler support floating point literals in both
100decimal and hexadecimal form.
101
102The syntax for a floating point literal is the same as floating point
103constants in the C programming language, except:
104* An optional leading minus (`-`) is part of the literal.
105* An optional type specifier suffix is not allowed.
106Infinity and NaN values are expressed in hexadecimal float literals
107by using the maximum representable exponent for the bit width.
108
109For example, in 32-bit floating point, 8 bits are used for the exponent, and the
110exponent bias is 127.  So the maximum representable unbiased exponent is 128.
111Therefore, we represent the infinities and some NaNs as follows:
112
113```
114%float32 = OpTypeFloat 32
115%inf     = OpConstant %float32 0x1p+128
116%neginf  = OpConstant %float32 -0x1p+128
117%aNaN    = OpConstant %float32 0x1.8p+128
118%moreNaN = OpConstant %float32 -0x1.0002p+128
119```
120The assembler preserves all the bits of a NaN value.  For example, the encoding
121of `%aNaN` in the previous example is the same as the word with bits
122`0x7fc00000`, and `%moreNaN` is encoded as `0xff800100`.
123
124The disassembler prints infinite, NaN, and subnormal values in hexadecimal form.
125Zero and normal values are printed in decimal form with enough digits
126to preserve all significand bits.
127
128## Arbitrary Integers
129<a name="immediate"></a>
130
131When writing tests it can be useful to emit an invalid 32 bit word into the
132binary stream at arbitrary positions within the assembly. To specify an
133arbitrary word into the stream the prefix `!` is used, this takes the form
134`!<integer>`. Here is an example.
135
136```
137OpCapability !0x0000FF00
138```
139
140Any token in a valid assembly program may be replaced by `!<integer>` -- even
141tokens that dictate how the rest of the instruction is parsed.  Consider, for
142example, the following assembly program:
143
144```
145%4 = OpConstant %1 123 456 789 OpExecutionMode %2 LocalSize 11 22 33
146OpExecutionMode %3 InputLines
147```
148
149The tokens `OpConstant`, `LocalSize`, and `InputLines` may be replaced by random
150`!<integer>` values, and the assembler will still assemble an output binary with
151three instructions.  It will not necessarily be valid SPIR-V, but it will
152faithfully reflect the input text.
153
154You may wonder how the assembler recognizes the instruction structure (including
155instruction boundaries) in the text with certain crucial tokens replaced by
156arbitrary integers.  If, say, `OpConstant` becomes a `!<integer>` whose value
157differs from the binary representation of `OpConstant` (remember that this
158feature is intended for fine-grain control in SPIR-V testing), the assembler
159generally has no idea what that value stands for.  So how does it know there is
160exactly one `<id>` and three number literals following in that instruction,
161before the next one begins?  And if `LocalSize` is replaced by an arbitrary
162`!<integer>`, how does it know to take the next three tokens (instead of zero or
163one, both of which are possible in the absence of certainty that `LocalSize`
164provided)?  The answer is a simple rule governing the parsing of instructions
165with `!<integer>` in them:
166
167When a token in the assembly program is a `!<integer>`, that integer value is
168emitted into the binary output, and parsing proceeds differently than before:
169each subsequent token not recognized as an OpCode or a `<result-id>` is emitted
170into the binary output without any checking; when a recognizable OpCode or a
171`<result-id>` is eventually encountered, it begins a new instruction and parsing
172returns to normal.  (If a subsequent OpCode is never found, then this alternate
173parsing mode handles all the remaining tokens in the program.)
174
175The assembler processes the tokens encountered in alternate parsing mode as
176follows:
177
178* If the token is a number literal, since context may be lost, the number
179  is interpreted as a 32-bit value and output as a single word.  In order to
180  specify multiple-word literals in alternate-parsing mode, further uses of
181  `!<integer>` tokens may be required.
182  All formats supported by `strtoul()` are accepted.
183* If the token is a string literal, it outputs a sequence of words representing
184  the string as defined in the SPIR-V specification for Literal String.
185* If the token is an ID, it outputs the ID's internal number.
186* If the token is another `!<integer>`, it outputs that integer.
187* Any other token causes the assembler to quit with an error.
188
189Note that this has some interesting consequences, including:
190
191* When an OpCode is replaced by `!<integer>`, the integer value should encode
192  the instruction's word count, as specified in the physical-layout section of
193  the SPIR-V specification.
194
195* Consecutive instructions may have their OpCode replaced by `!<integer>` and
196  still produce valid SPIR-V.  For example, `!262187 %1 %2 "abc" !327739 %1 %3 6
197  %2` will successfully assemble into SPIR-V declaring a constant and a
198  PrivateGlobal variable.
199
200* Enums (such as `DontInline` or `SubgroupMemory`, for instance) are not handled
201  by the alternate parsing mode.  They must be replaced by `!<integer>` for
202  successful assembly.
203
204* The `<result-id>` on the left-hand side of an assignment cannot be a
205  `!<integer>`. The `<result-id>` can be still be manually controlled if desired
206  by expressing the entire instruction as `!<integer>` tokens for its opcode and
207  operands.
208
209* The `=` sign cannot be processed by the alternate parsing mode if the OpCode
210  following it is a `!<integer>`.
211
212* When replacing a named ID with `!<integer>`, it is possible to generate
213  unintentionally valid SPIR-V.  If the integer provided happens to equal a
214  number generated for an existing named ID, it will result in a reference to
215  that named ID being output.  This may be valid SPIR-V, contrary to the
216  presumed intention of the writer.
217
218## Notes
219
220* Some enumerants cannot be used by name, because the target instruction
221in which they are meaningful take an ID reference instead of a literal value.
222For example:
223   * Named enumerated value `CmdExecTime` from section 3.30 Kernel
224     Profiling Info is used in constructing a mask value supplied as
225     an ID for `OpCaptureEventProfilingInfo`.  But no other instruction
226     has enough context to bring the enumerant names from section 3.30
227     into scope.
228   * Similarly, the names in section 3.29 Kernel Enqueue Flags are used to
229     construct a value supplied as an ID to the Flags argument of
230     OpEnqueueKernel.
231   * Similarly for the names in section 3.25 Memory Semantics.
232   * Similarly for the names in section 3.27 Scope.
233* Some enumerants cannot be used by name, because they only name values
234returned by an instruction:
235   * Enumerants from 3.12 Image Channel Order name possible values returned
236     by the `OpImageQueryOrder` instruction.
237   * Enumerants from 3.13 Image Channel Data Type name possible values
238     returned by the `OpImageQueryFormat` instruction.
239