120 lines
4.2 KiB
Text
120 lines
4.2 KiB
Text
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@c Copyright (C) 2017-2022 Free Software Foundation, Inc.
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@c This is part of the GAS manual.
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@c For copying conditions, see the file as.texinfo.
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@c man end
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@ifset GENERIC
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@page
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@node WebAssembly-Dependent
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@chapter WebAssembly Dependent Features
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@end ifset
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@ifclear GENERIC
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@node Machine Dependencies
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@chapter WebAssembly Dependent Features
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@end ifclear
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@cindex WebAssembly support
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@menu
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* WebAssembly-Notes:: Notes
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* WebAssembly-Syntax:: Syntax
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* WebAssembly-Floating-Point:: Floating Point
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* WebAssembly-Opcodes:: Opcodes
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* WebAssembly-module-layout:: Module Layout
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@end menu
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@node WebAssembly-Notes
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@section Notes
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@cindex WebAssembly notes
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@cindex notes for WebAssembly
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While WebAssembly provides its own module format for executables, this
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documentation describes how to use @code{@value{AS}} to produce
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intermediate ELF object format files.
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@node WebAssembly-Syntax
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@section Syntax
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@cindex WebAssembly Syntax
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The assembler syntax directly encodes sequences of opcodes as defined
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in the WebAssembly binary encoding specification at
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https://github.com/webassembly/spec/BinaryEncoding.md. Structured
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sexp-style expressions are not supported as input.
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@menu
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* WebAssembly-Chars:: Special Characters
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* WebAssembly-Relocs:: Relocations
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* WebAssembly-Signatures:: Signatures
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@end menu
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@node WebAssembly-Chars
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@subsection Special Characters
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@cindex line comment character, WebAssembly
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@cindex WebAssembly line comment character
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@samp{#} and @samp{;} are the line comment characters. Note that if
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@samp{#} is the first character on a line then it can also be a
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logical line number directive (@pxref{Comments}) or a preprocessor
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control command (@pxref{Preprocessing}).
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@node WebAssembly-Relocs
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@subsection Relocations
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@cindex WebAssembly relocations
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@cindex relocations, WebAssembly
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Special relocations are available by using the @samp{@@@var{plt}},
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@samp{@@@var{got}}, or @samp{@@@var{got}} suffixes after a constant
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expression, which correspond to the R_ASMJS_LEB128_PLT,
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R_ASMJS_LEB128_GOT, and R_ASMJS_LEB128_GOT_CODE relocations,
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respectively.
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The @samp{@@@var{plt}} suffix is followed by a symbol name in braces;
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the symbol value is used to determine the function signature for which
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a PLT stub is generated. Currently, the symbol @emph{name} is parsed
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from its last @samp{F} character to determine the argument count of
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the function, which is also necessary for generating a PLT stub.
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@node WebAssembly-Signatures
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@subsection Signatures
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@cindex WebAssembly signatures
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@cindex signatures, WebAssembly
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Function signatures are specified with the @code{signature}
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pseudo-opcode, followed by a simple function signature imitating a
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C++-mangled function type: @code{F} followed by an optional @code{v},
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then a sequence of @code{i}, @code{l}, @code{f}, and @code{d}
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characters to mark i32, i64, f32, and f64 parameters, respectively;
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followed by a final @code{E} to mark the end of the function
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signature.
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@node WebAssembly-Floating-Point
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@section Floating Point
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@cindex floating point, WebAssembly (@sc{ieee})
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@cindex WebAssembly floating point (@sc{ieee})
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WebAssembly uses little-endian @sc{ieee} floating-point numbers.
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@node WebAssembly-Opcodes
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@section Regular Opcodes
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@cindex opcodes, WebAssembly
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@cindex WebAssembly opcodes
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Ordinary instructions are encoded with the WebAssembly mnemonics as
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listed at:
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@url{https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md}.
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Opcodes are written directly in the order in which they are encoded,
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without going through an intermediate sexp-style expression as in the
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@code{was} format.
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For ``typed'' opcodes (block, if, etc.), the type of the block is
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specified in square brackets following the opcode: @code{if[i]},
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@code{if[]}.
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@node WebAssembly-module-layout
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@section WebAssembly Module Layout
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@cindex module layout, WebAssembly
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@cindex WebAssembly module layout
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@code{@value{AS}} will only produce ELF output, not a valid
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WebAssembly module. It is possible to make @code{@value{AS}} produce
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output in a single ELF section which becomes a valid WebAssembly
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module, but a linker script to do so may be preferable, as it doesn't
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require running the entire module through the assembler at once.
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