[1/2] libstdc++: Add dg-require-cpp-feature-test to test feature test macros

Thoughts? There are only a few uses for this presently, but I can see it
being useful often in future. The library exposes which features it
supports in a standardized way, so we can use those in tests to skip
tests for features that aren't available on all targets.

The obvious downside is that it becomes harder to notice if a particular
feature is missing on all targets, because we don't get FAILs we just
skip all tests as UNSUPPORTED. And the checks for whether <version>
correctly defines the macro become redundant, because the test won't
even get run if it doesn't. But we won't use this dg-require for many
tests, only the ones where support is target-dependent because it relies
on something non-standard or not available on all targets (like
nl_langinfo_l or libbacktrace).

-- >8 -

This adds a new dejagnu directive which can be used to make a test
depend on a feature test macro such as __cpp_lib_text_encoding. This is
mroe flexible than writing a new dg-require-xxx for each feature.

libstdc++-v3/ChangeLog:

	* testsuite/lib/dg-options.exp (dg-require-cpp-feature-test):
	New proc.
	* testsuite/lib/libstdc++.exp (check_v3_target_cpp_feature_test):
	New proc.
	* testsuite/std/text_encoding/cons.cc: Use new directive to skip
	the test if the __cpp_lib_text_encoding feature test macro is
	not defined.
	* testsuite/std/text_encoding/requirements.cc: Likewise.
---
 libstdc++-v3/testsuite/lib/dg-options.exp         |  9 +++++++++
 libstdc++-v3/testsuite/lib/libstdc++.exp          | 15 +++++++++++++++
 libstdc++-v3/testsuite/std/text_encoding/cons.cc  |  1 +
 .../testsuite/std/text_encoding/requirements.cc   |  3 ++-
 4 files changed, 27 insertions(+), 1 deletion(-)

Pushed to trunk.

On Sat, 23 Mar 2024 at 00:22, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> Thoughts? There are only a few uses for this presently, but I can see it
> being useful often in future. The library exposes which features it
> supports in a standardized way, so we can use those in tests to skip
> tests for features that aren't available on all targets.
>
> The obvious downside is that it becomes harder to notice if a particular
> feature is missing on all targets, because we don't get FAILs we just
> skip all tests as UNSUPPORTED. And the checks for whether <version>
> correctly defines the macro become redundant, because the test won't
> even get run if it doesn't. But we won't use this dg-require for many
> tests, only the ones where support is target-dependent because it relies
> on something non-standard or not available on all targets (like
> nl_langinfo_l or libbacktrace).
>
> -- >8 -
>
> This adds a new dejagnu directive which can be used to make a test
> depend on a feature test macro such as __cpp_lib_text_encoding. This is
> mroe flexible than writing a new dg-require-xxx for each feature.
>
> libstdc++-v3/ChangeLog:
>
>         * testsuite/lib/dg-options.exp (dg-require-cpp-feature-test):
>         New proc.
>         * testsuite/lib/libstdc++.exp (check_v3_target_cpp_feature_test):
>         New proc.
>         * testsuite/std/text_encoding/cons.cc: Use new directive to skip
>         the test if the __cpp_lib_text_encoding feature test macro is
>         not defined.
>         * testsuite/std/text_encoding/requirements.cc: Likewise.
> ---
>  libstdc++-v3/testsuite/lib/dg-options.exp         |  9 +++++++++
>  libstdc++-v3/testsuite/lib/libstdc++.exp          | 15 +++++++++++++++
>  libstdc++-v3/testsuite/std/text_encoding/cons.cc  |  1 +
>  .../testsuite/std/text_encoding/requirements.cc   |  3 ++-
>  4 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
> index 00ca678a53a..802bfc0b492 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -277,6 +277,15 @@ proc dg-require-target-fs-lwt { args } {
>      return
>  }
>
> +proc dg-require-cpp-feature-test { n args } {
> +    if { ![ check_v3_target_cpp_feature_test $args ] } {
> +       upvar dg-do-what dg-do-what
> +       set dg-do-what [list [lindex ${dg-do-what} 0] "N" "P"]
> +       return
> +    }
> +    return
> +}
> +
>  proc add_options_for_no_pch { flags } {
>      # Remove any inclusion of bits/stdc++.h from the options.
>      regsub -all -- "-include bits/stdc...h" $flags "" flags
> diff --git a/libstdc++-v3/testsuite/lib/libstdc++.exp b/libstdc++-v3/testsuite/lib/libstdc++.exp
> index 7466fb51c58..2b31c950826 100644
> --- a/libstdc++-v3/testsuite/lib/libstdc++.exp
> +++ b/libstdc++-v3/testsuite/lib/libstdc++.exp
> @@ -1134,6 +1134,21 @@ proc v3_check_preprocessor_condition { name cond { inc "" } } {
>      return [v3_try_preprocess name $code $flags]
>  }
>
> +# Return 1 if libstdc++ defines macro for the current target, 0 otherwise.
> +proc check_v3_target_cpp_feature_test { cond } {
> +    global cxxflags
> +    set cxxflags_saved $cxxflags
> +    # Use the latest standard, so that all feature test macros are defined.
> +    # We need to do it here, because this check happens before v3-dg-runtest
> +    # runs its loop to test multiple times with different -std options.
> +    # This should be updated when a new -std is added.
> +    set cxxflags "$cxxflags -std=gnu++26"
> +    set inc "#include <version>"
> +    set result [v3_check_preprocessor_condition cpp_feature_test "$cond" $inc]
> +    set cxxflags $cxxflags_saved
> +    return $result
> +}
> +
>  # Return 1 if Debug Mode is active, 0 otherwise.
>  proc check_v3_target_debug_mode { } {
>      global cxxflags
> diff --git a/libstdc++-v3/testsuite/std/text_encoding/cons.cc b/libstdc++-v3/testsuite/std/text_encoding/cons.cc
> index 8fcc2ec8c3b..4196e32ea8b 100644
> --- a/libstdc++-v3/testsuite/std/text_encoding/cons.cc
> +++ b/libstdc++-v3/testsuite/std/text_encoding/cons.cc
> @@ -1,4 +1,5 @@
>  // { dg-do run { target c++26 } }
> +// { dg-require-cpp-feature-test "__cpp_lib_text_encoding" }
>
>  #include <text_encoding>
>  #include <string_view>
> diff --git a/libstdc++-v3/testsuite/std/text_encoding/requirements.cc b/libstdc++-v3/testsuite/std/text_encoding/requirements.cc
> index a1d5d6baee1..3889b250688 100644
> --- a/libstdc++-v3/testsuite/std/text_encoding/requirements.cc
> +++ b/libstdc++-v3/testsuite/std/text_encoding/requirements.cc
> @@ -1,4 +1,5 @@
>  // { dg-do compile { target c++26 } }
> +// { dg-require-cpp-feature-test __cpp_lib_text_encoding }
>  // { dg-add-options no_pch }
>
>  #include <text_encoding>
>