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?
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
