mirror of
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a2c5833233
The result of running etc/update-copyright.py --this-year, fixing all the files whose mode is changed by the script, plus a build with --enable-maintainer-mode --enable-cgen-maint=yes, then checking out */po/*.pot which we don't update frequently. The copy of cgen was with commit d1dd5fcc38ead reverted as that commit breaks building of bfp opcodes files.
120 lines
4.2 KiB
Plaintext
120 lines
4.2 KiB
Plaintext
@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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