On Wed, 15 Jan 2025, Jakub Jelinek wrote:
> Hi!
>
> When writing the gcc-15/changes.html patch posted earlier, I've been
> wondering where significant part of the Basic asm chapter went and the
> problem was the insertion of a new @node in the middle of the Basic Asm
> @node, plus not mentioning the new @node in the @menu. So the asm constexpr
> node was not normally visible and the Remarks for the section neither.
>
> The following patch moves it before Asm Labels, removes the spots where it
> described what hasn't been actually committed (constant expression can only
> be a container with data/size member functions) and fixes up the toplevel
> extended asm documentation (it was in the Basic Asm remarks and Extended Asm
> section's remark still said it is not valid).
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
> 2025-01-15 Jakub Jelinek <ja...@redhat.com>
>
> * doc/extend.texi (Using Assembly Language with C): Add Asm constexprs
> to @menu.
> (Basic Asm): Move @node asm constexprs before Asm Labels, rename to
> Asm constexprs, change wording so that it is clearer that the constant
> expression actually must not return a string literal, just some specific
> container and other wording tweaks. Only talk about top-level for basic
> asms in this @node, move restrictions on top-level extended asms to ...
> (Extended Asm): ... here.
>
> --- gcc/doc/extend.texi.jj 2025-01-02 11:47:30.559217524 +0100
> +++ gcc/doc/extend.texi 2025-01-15 22:35:27.836185257 +0100
> @@ -10865,6 +10865,8 @@ for a C symbol, or to place a C variable
> * Basic Asm:: Inline assembler without operands.
> * Extended Asm:: Inline assembler with operands.
> * Constraints:: Constraints for @code{asm} operands
> +* Asm constexprs:: C++11 constant expressions instead of string
> + literals.
> * Asm Labels:: Specifying the assembler name to use for a C symbol.
> * Explicit Register Variables:: Defining variables residing in specified
> registers.
> @@ -10920,23 +10922,6 @@ Some assemblers allow semicolons as a li
> note that some assembler dialects use semicolons to start a comment.
> @end table
>
> -@node asm constexprs
> -With gnu++11 or later the string can also be a compile time constant
> expression
> -inside parens. The constant expression can return a string or a container
> -with data and size members, following similar rules as C++26
> @code{static_assert}
> -message. Any string is converted to the character set of the source code.
> -When this feature is available the @code{__GXX_CONSTEXPR_ASM__} cpp symbol
> is defined.
> -
> -@example
> -#include <string>
> -constexpr std::string_view genfoo() @{ return "foo"; @}
> -
> -void function()
> -@{
> - asm((genfoo()));
> -@}
> -@end example
> -
> @subsubheading Remarks
> Using extended @code{asm} (@pxref{Extended Asm}) typically produces
> smaller, safer, and more efficient code, and in most cases it is a
> @@ -10944,15 +10929,10 @@ better solution than basic @code{asm}.
> with the @code{naked} attribute require only basic @code{asm}
> (@pxref{Function Attributes}).
>
> -Extended @code{asm} statements may be used both inside a C
> -function or at file scope (``top-level''), where
> -you can use this technique to emit assembler directives,
> -define assembly language macros that can be invoked elsewhere in the file,
> -or write entire functions in assembly language.
> -Extended @code{asm} statements outside of functions may not use any
> -qualifiers, may not specify clobbers, may not use @code{%}, @code{+} or
> -@code{&} modifiers in constraints and can only use constraints which don't
> -allow using any register.
> +Basic @code{asm} statements may be used both inside a C function or at
> +file scope (``top-level''), where you can use this technique to emit
> +assembler directives, define assembly language macros that can be invoked
> +elsewhere in the file, or write entire functions in assembly language.
>
> Safely accessing C data and calling functions from basic @code{asm} is more
> complex than it may appear. To access C data, it is better to use extended
> @@ -11074,26 +11054,26 @@ perform a jump to one of the labels list
> This is a literal string that is the template for the assembler code. It is
> a
> combination of fixed text and tokens that refer to the input, output,
> and goto parameters. @xref{AssemblerTemplate}. With gnu++11 or later it can
> -also be a constant expression inside parens (see @ref{asm constexprs}).
> +also be a constant expression inside parens (see @ref{Asm constexprs}).
>
> @item OutputOperands
> A comma-separated list of the C variables modified by the instructions in
> the
> @var{AssemblerTemplate}. An empty list is permitted. @xref{OutputOperands}.
> With gnu++11 or later the strings can also be constant expressions inside
> parens
> -(see @ref{asm constexprs})
> +(see @ref{Asm constexprs})
>
> @item InputOperands
> A comma-separated list of C expressions read by the instructions in the
> @var{AssemblerTemplate}. An empty list is permitted. @xref{InputOperands}.
> With gnu++11 or later the strings can also be constant expressions inside
> parens
> -(see @ref{asm constexprs})
> +(see @ref{Asm constexprs})
>
> @item Clobbers
> A comma-separated list of registers or other values changed by the
> @var{AssemblerTemplate}, beyond those listed as outputs.
> An empty list is permitted. @xref{Clobbers and Scratch Registers}.
> With gnu++11 or later the strings can also be constant expressions inside
> parens
> -(see @ref{asm constexprs})
> +(see @ref{Asm constexprs})
>
> @item GotoLabels
> When you are using the @code{goto} form of @code{asm}, this section contains
> @@ -11115,9 +11095,17 @@ within C code. This may help you to maxi
> code or to access assembly instructions that are not readily available to C
> programs.
>
> -Note that extended @code{asm} statements must be inside a function. Only
> -basic @code{asm} may be outside functions (@pxref{Basic Asm}).
> -Functions declared with the @code{naked} attribute also require basic
> +Similarly to basic @code{asm}, extended @code{asm} statements may be used
> +both inside a C function or at file scope (``top-level''), where you can
> +use this technique to emit assembler directives, define assembly language
> +macros that can be invoked elsewhere in the file, or write entire functions
> +in assembly language.
> +Extended @code{asm} statements outside of functions may not use any
> +qualifiers, may not specify clobbers, may not use @code{%}, @code{+} or
> +@code{&} modifiers in constraints and can only use constraints which don't
> +allow using any register.
> +
> +Functions declared with the @code{naked} attribute require basic
> @code{asm} (@pxref{Function Attributes}).
>
> While the uses of @code{asm} are many and varied, it may help to think of an
> @@ -12641,6 +12629,26 @@ The list below describes the supported m
> @include md.texi
> @raisesections
>
> +@node Asm constexprs
> +@subsection C++11 constant expressions instead of string literals
> +
> +With gnu++11 or later the string can also be a compile time constant
> expression
> +inside parens. The constant expression can return a container with data and
> size
> +member functions, following similar rules as C++26 @code{static_assert}
> +message. Any string is converted to the character set of the source code.
> +When this feature is available the @code{__GXX_CONSTEXPR_ASM__} preprocessor
> +macro is predefined.
> +
> +@example
> +#include <string>
> +constexpr std::string_view genfoo() @{ return "foo"; @}
> +
> +void function()
> +@{
> + asm((genfoo()));
> +@}
> +@end example
> +
> @node Asm Labels
> @subsection Controlling Names Used in Assembler Code
> @cindex assembler names for identifiers
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
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