80 lines
3.2 KiB
Markdown
80 lines
3.2 KiB
Markdown
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# Preprocessing: include, define and selection
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Layer 1, the common language. Verified against the preprocessor inside
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`go tool asm` of Go 1.27.1 (`cmd/asm/internal/lex`), whose directives are
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`#define`, `#undef`, `#include`, `#ifdef`, `#ifndef`, `#else`, `#endif` and
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`#line`, and against gasm's implementation, which is differentially tested
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against the toolchain's.
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Input runs through a simplified C preprocessor before the parser sees it.
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The set is deliberately small: there is no `#if` with constant expressions
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and no token pasting with `##`. `#line` is honoured, so it changes the
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positions the assembler reports and records.
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## #include
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```text
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#include "textflag.h"
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#include "go_asm.h"
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#include "defs_linux_amd64.h"
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```
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The search path, in order: the directory of the including file, then the
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directories given by repeatable `-I` flags. The assembler seeds no default
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of its own: a bare `go tool asm` invocation finds none of the standard
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headers, and it is the `go` build system that passes `$GOROOT/pkg/include`
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among the `-I` directories when it drives the build. That directory ships
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`textflag.h`, `funcdata.h` and the per architecture register headers.
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Includes nest; a file included twice through different paths is processed
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twice, which is why headers guard their defines.
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## #define and #undef
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```text
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#define bufSize 1024
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#define MOVD(d, s) MOVQ s, d
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#undef bufSize
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```
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- An object macro replaces its name with its token sequence at the point of
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use.
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- A parameterised macro takes its arguments in parentheses and substitutes
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them into the body. Macro parameters compose with the rest of the
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language: an argument used with an element suffix, as in `A.S4` on the
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vector forms, substitutes correctly.
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- Redefinition is an error; `#undef` first, or pick a new name.
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- The `-D name[=value]` flag predefines an object macro from the command
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line, repeatable, exactly as `#define` would; a `-D` without a value
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defines the name as `1`.
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- Expansion happens when the name is used, so a macro may expand to
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instructions, operands or fragments of either, and a macro body may use
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macros defined before it.
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`textflag.h` and `funcdata.h` are themselves ordinary `#define` files: the
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flag names and the runtime macros are preprocessor definitions, not language
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keywords. That is why a missing include produces a parser error at the first
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use of `NOSPLIT` rather than a complaint about the name.
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## #ifdef, #ifndef, #else, #endif
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```text
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#ifdef GOOS_windows
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#define SYSCALL_INT 0x2b
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#endif
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```
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Selection is by defined-name only: `#ifdef`, `#ifndef`, `#else`, `#endif`,
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nesting freely. There is no `#if defined(x) && y`, because the preprocessor
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evaluates no expressions; reach that with a build-tag Go file generating a
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header, which is exactly how the runtime's own `go_asm.h` and defs headers
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are produced.
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## What preprocessing does not cover
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The preprocessor is textual and runs first, so it knows nothing of assembly
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semantics: it does not check that a macro expansion is a legal instruction,
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and it does not participate in the constant expression evaluator, which runs
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later, in the parser. A constant folded with `#define` and a constant folded
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in an operand expression end at the same value through different doors;
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GOOBJ.md records both in the object identically.
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