Add target_version attribute to Common Function Attributes and update
target and target_clones documentation. Move shared detail and examples
to the Function Multiversioning page. Add target-specific details to
target-specific pages.
---
Changes since v1:
- Various typo fixes.
- Reordered content in 'Function multiversioning' section to put implementation
details at the end (as suggested in review).
- Dropped links to outdated wiki page, and a couple of other unhelpful
sentences that the previous version preserved.
I've built and rechecked the info output. Ok for master? And is this ok for
the GCC-14 branch too?
gcc/ChangeLog:
* doc/extend.texi (Common Function Attributes): Update target
and target_clones documentation, and add target_version.
(AArch64 Function Attributes): Add ACLE reference and list
supported features.
(PowerPC Function Attributes): List supported features.
(x86 Function Attributes): Mention function multiversioning.
(Function Multiversioning): Update, and move shared detail here.
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index
e290265d68d33f86a7e7ee9882cc0fd6bed00143..fefac70b5fffc350bf23db74a8fc88fa3bb99bd5
100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -4178,17 +4178,16 @@ and @option{-Wanalyzer-tainted-size}.
Multiple target back ends implement the @code{target} attribute
to specify that a function is to
be compiled with different target options than specified on the
-command line. The original target command-line options are ignored.
-One or more strings can be provided as arguments.
-Each string consists of one or more comma-separated suffixes to
-the @code{-m} prefix jointly forming the name of a machine-dependent
-option. @xref{Submodel Options,,Machine-Dependent Options}.
-
+command line. One or more strings can be provided as arguments.
The @code{target} attribute can be used for instance to have a function
compiled with a different ISA (instruction set architecture) than the
-default. @samp{#pragma GCC target} can be used to specify target-specific
-options for more than one function. @xref{Function Specific Option Pragmas},
-for details about the pragma.
+default.
+
+The options supported by the @code{target} attribute are specific to each
+target; refer to @ref{x86 Function Attributes}, @ref{PowerPC Function
+Attributes}, @ref{ARM Function Attributes}, @ref{AArch64 Function Attributes},
+@ref{Nios II Function Attributes}, and @ref{S/390 Function Attributes}
+for details.
For instance, on an x86, you could declare one function with the
@code{target("sse4.1,arch=core2")} attribute and another with
@@ -4211,39 +4210,26 @@ multiple options is equivalent to separating the option
suffixes with
a comma (@samp{,}) within a single string. Spaces are not permitted
within the strings.
-The options supported are specific to each target; refer to @ref{x86
-Function Attributes}, @ref{PowerPC Function Attributes},
-@ref{ARM Function Attributes}, @ref{AArch64 Function Attributes},
-@ref{Nios II Function Attributes}, and @ref{S/390 Function Attributes}
-for details.
+@samp{#pragma GCC target} can be used to specify target-specific
+options for more than one function. @xref{Function Specific Option Pragmas},
+for details about the pragma.
+
+On x86, the @code{target} attribute can also be used to create multiple
+versions of a function, compiled with different target-specific options.
+@xref{Function Multiversioning} for more details.
@cindex @code{target_clones} function attribute
@item target_clones (@var{options})
The @code{target_clones} attribute is used to specify that a function
-be cloned into multiple versions compiled with different target options
-than specified on the command line. The supported options and restrictions
-are the same as for @code{target} attribute.
-
-For instance, on an x86, you could compile a function with
-@code{target_clones("sse4.1,avx")}. GCC creates two function clones,
-one compiled with @option{-msse4.1} and another with @option{-mavx}.
-
-On a PowerPC, you can compile a function with
-@code{target_clones("cpu=power9,default")}. GCC will create two
-function clones, one compiled with @option{-mcpu=power9} and another
-with the default options. GCC must be configured to use GLIBC 2.23 or
-newer in order to use the @code{target_clones} attribute.
-
-It also creates a resolver function (see
-the @code{ifunc} attribute above) that dynamically selects a clone
-suitable for current architecture. The resolver is created only if there
-is a usage of a function with @code{target_clones} attribute.
-
-Note that any subsequent call of a function without @code{target_clone}
-from a @code{target_clone} caller will not lead to copying
-(target clone) of the called function.
-If you want to enforce such behaviour,
-we recommend declaring the calling function with the @code{flatten} attribute?
+should be cloned into multiple versions compiled with different target options
+than specified on the command line. @xref{Function Multiversioning} for more
+details.
+
+@cindex @code{target_version} function attribute
+@item target_version (@var{options})
+The @code{target_version} attribute is used on AArch64 to create multiple
+versions of a function, compiled with different target-specific options.
+@xref{Function Multiversioning} for more details.
@cindex @code{unavailable} function attribute
@item unavailable
@@ -4734,6 +4720,26 @@ Note that CPU tuning options and attributes such as the
@option{-mcpu=},
@option{-mcpu=} option or the @code{cpu=} attribute conflicts with the
architectural feature rules specified above.
+@subsubsection Function multiversioning
+The @code{target_version} and @code{target_clones} attributes can be used to
+specify multiple versions of a function. Each version enables the specified
+set of architecture extensions, in addition to any extensions that were already
+enabled at the command line or using @code{target} attributes. For general
+details, @pxref{Function Multiversioning}. There are further AArch64-specific
+details available in the
+@uref{https://github.com/ARM-software/acle/blob/main/main/acle.md#function-multi-versioning,
+Arm C Language Extensions (ACLE) specification}.
+
+Some aspects of the ACLE specification are not yet supported. In particular,
+the currently supported feature names are @code{rng}, @code{flagm}, @code{lse},
+@code{fp}, @code{simd}, @code{dotprod}, @code{sm4}, @code{rdma}, @code{rdm}
+(alias of @code{rdma}), @code{crc}, @code{sha2}, @code{sha3}, @code{aes},
+@code{fp16}, @code{fp16fml}, @code{rcpc}, @code{rcpc3}, @code{i8mm},
+@code{bf16}, @code{sve}, @code{f32mm}, @code{f64mm}, @code{sve2},
+@code{sve2-aes}, @code{sve2-bitperm}, @code{sve2-sha3}, @code{sve2-sm4},
+@code{sme}, @code{sb}, @code{predres}, @code{sme-f64f64}, @code{sme-i16i64} and
+@code{sme2}.
+
@node AMD GCN Function Attributes
@subsection AMD GCN Function Attributes
@@ -6278,6 +6284,15 @@ default tuning specified on the command line.
On the PowerPC, the inliner does not inline a
function that has different target options than the caller, unless the
callee has a subset of the target options of the caller.
+
+@cindex @code{target_clones} function attribute
+@item target_clones (@var{options})
+The @code{target_clones} attribute can be used to create multiple versions of a
+function for different supported architectures, with one version for each
+specifier in the options list. One of these version specifiers must be the
+@code{default} version. The other supported target specifiers are
+@code{cpu=power6}, @code{cpu=power7}, @code{cpu=power8}, @code{cpu=power9} and
+@code{cpu=power10}. For more details, @pxref{Function Multiversioning}.
@end table
@node RISC-V Function Attributes
@@ -6872,7 +6887,9 @@ will crash if the wrong kind of handler is used.
@cindex @code{target} function attribute
@item target (@var{options})
As discussed in @ref{Common Function Attributes}, this attribute
-allows specification of target-specific compilation options.
+allows specification of target-specific compilation options. It can also be
+used to create multiple versions of a single function
+(@pxref{Function Multiversioning}).
On the x86, the following options are allowed:
@table @samp
@@ -29431,11 +29448,58 @@ For the effects of the @code{hot} attribute on
functions, see
@section Function Multiversioning
@cindex function versions
-With the GNU C++ front end, for x86 targets, you may specify multiple
-versions of a function, where each function is specialized for a
-specific target feature. At runtime, the appropriate version of the
-function is automatically executed depending on the characteristics of
-the execution platform. Here is an example.
+On some targets it is possible to specify multiple versions of a function,
+where each version of the function is specialized for a different set of target
+features. At runtime, characteristics of the execution platform are checked,
+and the most appropriate version of the function is chosen to be executed
+depending on the available architecture features. One of the versions will be
+a "default" version, which will be chosen if none of the criteria for the other
+versions are met.
+
+Function multiversioning is enabled by annotating the function versions with
one
+of three function attributes.
+
+The @code{target} attribute can be used on x86 targets. Multiversioning with
+the @code{target} attribute is supported only in the C++ frontend. One version
+must be explicitly labelled as the "default" version. The @code{target}
+attributes will normally need to be included in any header file declarations,
+to ensure that the correct function version is called from all translation
+units. In translation units that don't include declarations with
+the @code{target} attributes, any callers will always call the "default"
+version directly.
+
+The @code{target_version} attribute can be used on AArch64 targets.
+Multiversioning with the @code{target_version} attribute is supported only in
+the C++ frontend. This attribute behaves similarly to the @code{target}
+attribute. However, callers in any translation unit will always use the
+versioned function, even if the @code{target_version} attributes aren't
+included in that translation unit. This means that header files don't need to
+include the @code{target_version} attributes, and the function can use a single
+header file declaration (as if it were a normal unversioned function).
+
+The @code{target_clones} attribute can be used on AArch64, PowerPC and x86
+targets. It behaves similarly to the @code{target_version} attribute, except
+that only one copy of the function is included in the source file. The
+attribute takes a list of version specifiers and produces one copy of the
+function for each specifier. This is useful in cases where the compiler is
+capable of generating optimized code (with autovectorization, for example)
+using architecture features enabled only in the more specialized function
+versions. For example, on PowerPC, compiling a function with
+@code{target_clones("default,cpu=power9")} will create two function clones -
+one compiled with @option{-mcpu=power9}, and another with the default options.
+The @code{target_clones} attribute is available in the C, C++, D and Ada
+frontends.
+
+Function multiversioning attributes do not propagate from a versioned
+function to its callees, although a callee can still be optimised using the
+caller's extra target features if it is inlined directly into the caller.
+
+For details of the target options supported on each target, refer to
+@ref{AArch64 Function Attributes}, @ref{PowerPC Function Attributes},
+and @ref{x86 Function Attributes}.
+
+Here is an example of function multiversioning on x86 using the @code{target}
+attribute.
@smallexample
__attribute__ ((target ("default")))
@@ -29474,16 +29538,28 @@ int main ()
@}
@end smallexample
-In the above example, four versions of function foo are created. The
-first version of foo with the target attribute "default" is the default
-version. This version gets executed when no other target specific
-version qualifies for execution on a particular platform. A new version
-of foo is created by using the same function signature but with a
-different target string. Function foo is called or a pointer to it is
-taken just like a regular function. GCC takes care of doing the
-dispatching to call the right version at runtime. Refer to the
-@uref{https://gcc.gnu.org/wiki/FunctionMultiVersioning, GCC wiki on
-Function Multiversioning} for more details.
+In the above example, four versions of function foo are created. The first
+version of @samp{foo}, with the target attribute @samp{"default"}, is the
+default version. This version gets executed when no other target specific
+version qualifies for execution on a particular platform. Other versions of
+@samp{foo} are created by using the same function signature but with a
+different target string. The function @samp{foo} can be called or a pointer to
+it can be taken just like for a regular function. GCC takes care of doing the
+dispatching to call the right version at runtime.
+
+Function multiversioning is implemented using the STT_GNU_IFUNC symbol type
+extension to the ELF standard. This is the same mechanism used by the
+@code{ifunc} attribute (@pxref{Common Function Attributes}). However, in this
+case the compiler automatically generates a resolver function that checks which
+features are available at runtime. This resolver uses GLIBC's hardware
+capability bits, and therefore requires GCC to be configured to use GLIBC 2.23
+or newer. The resolver is run once at startup, and the resulting function
+pointer is then stored in the dynamic symbol table. When using the
+@code{target_version} or @code{target_clones} attributes, the resolved ifunc
+symbol uses the normal symbol name. When using the @code{target} attribute,
+the normal symbol name is instead used by the default function version (which
+is why @code{target} attributes for multiversioning need to be included in any
+header file declarations as well).
@node Type Traits
@section Type Traits
