Support 'UNSUPPORTED: [...]: exception handling disabled' for libstdc++ testing (was: Support in the GCC(/C++) test suites for '-fno-exceptions')

[Thomas Schwinge <thomas@codesourcery.com>]

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Hi!

On 2023-06-06T20:31:21+0100, Jonathan Wakely <jwakely@redhat.com> wrote:
> On Tue, 6 Jun 2023 at 20:14, Thomas Schwinge <thomas@codesourcery.com>
> wrote:
>> This issue comes up in context of me working on C++ support for GCN and
>> nvptx target.  Those targets shall default to '-fno-exceptions' -- or,
>> "in other words", '-fexceptions' is not supported.  (Details omitted
>> here.)
>>
>> It did seem clear to me that with such a configuration it'll be hard to
>> get clean test results.  Then I found code in
>> 'gcc/testsuite/lib/gcc-dg.exp:gcc-dg-prune':
>>
>>     # If exceptions are disabled, mark tests expecting exceptions to be
>> enabled
>>     # as unsupported.
>>     if { ![check_effective_target_exceptions_enabled] } {
>>         if [regexp "(^|\n)\[^\n\]*: error: exception handling disabled"
>> $text] {
>>             return "::unsupported::exception handling disabled"
>>         }
>>
>> ..., which, in a way, sounds as if the test suite generally is meant to
>> produce useful results for '-fno-exceptions', nice surprise!
>>
>> Running x86_64-pc-linux-gnu (not yet GCN, nvptx) 'make check' with:
>>
>>     RUNTESTFLAGS='--target_board=unix/-fno-exceptions\{,-m32\}'
>>
>> ..., I find that indeed this does work for a lot of test cases, where we
>> then get (random example):
>>
>>      PASS: g++.dg/coroutines/pr99710.C  (test for errors, line 23)
>>     -PASS: g++.dg/coroutines/pr99710.C (test for excess errors)
>>     +UNSUPPORTED: g++.dg/coroutines/pr99710.C: exception handling disabled
>>
>> ..., due to:
>>
>>      [...]/g++.dg/coroutines/pr99710.C: In function 'task my_coro()':
>>     +[...]/g++.dg/coroutines/pr99710.C:18:10: error: exception handling
>> disabled, use '-fexceptions' to enable
>>      [...]/g++.dg/coroutines/pr99710.C:23:7: error: await expressions are
>> not permitted in handlers
>>      compiler exited with status 1
>>
>> But, we're nowhere near clean test results: PASS -> FAIL as well as
>> XFAIL -> XPASS regressions, due to 'error: exception handling disabled'
>> precluding other diagnostics seems to be one major issue.
>>
>> Is there interest in me producing the obvious (?) changes to those test
>> cases, such that compiler g++ as well as target library libstdc++ test
>> results are reasonably clean?  (If you think that's all "wasted effort",
>> then I suppose I'll just locally ignore any FAILs/XPASSes/UNRESOLVEDs
>> that appear in combination with
>> 'UNSUPPORTED: [...]: exception handling disabled'.)
>
> I would welcome that for libstdc++.

Assuming no issues found in testing, OK to push the attached
"Support 'UNSUPPORTED: [...]: exception handling disabled' for libstdc++ testing"?
(Thanks, Jozef!)

> I do sometimes run the libstdc++ tests
> with "unusual" options, like -fno-exceptions and -fno-rtti (e.g. today I've
> been fixing FAILs that only happen with -fexcess-precision=standard). I
> just manually ignore the tests that fail for -fno-exceptions, but it would
> be great if they were automatically skipped as UNSUPPORTED.
>
> We already have a handful of tests that use #if __cpp_exceptions to make
> those parts conditional on exception support. We also have exactly one test
> that is currently UNSUPPORTED when -fno-exceptions is used:
> testsuite/18_support/nested_exception/rethrow_if_nested-term.cc:// {
> dg-skip-if "" { *-*-* } { "-fno-exceptions" } }

ACK -- that'll only work for explicit '-fno-exceptions', but not for
implicit (say, via 'CC1PLUS_SPEC'), right?  So, indeed:

> That could be changed to use an effective target keyword instead.

I'll look into that later.

> To add an effective-target to the libstdc++ testsuite would be as simple as:
>
> --- a/libstdc++-v3/testsuite/lib/libstdc++.exp
> +++ b/libstdc++-v3/testsuite/lib/libstdc++.exp
> @@ -1421,6 +1421,14 @@ proc check_effective_target_tzdb { } {
>     }]
> }
>
> +# Return 1 if exception handling is enabled.
> +proc check_effective_target_exceptions_enabled { } {
> +    return [check_v3_target_prop_cached et_eh {
> +       set cond "defined __cpp_exceptions"
> +       return [v3_check_preprocessor_condition eh $cond]
> +    }]
> +}
> +

Well, we don't even need to do that, because:

> However, you probably want to add it to the main testsuite instead, which
> would be a little more involved (the v3_check_preprocessor_condition proc
> is specific to libstdc++).

..., this has already been done in Subversion r279246
(Git commit a9046e9853024206bec092dd63e21e152cb5cbca)
"[MSP430] -Add fno-exceptions multilib" (thanks, Jozef!):

    --- gcc/testsuite/lib/target-supports.exp
    +++ gcc/testsuite/lib/target-supports.exp
    @@ -8990,6 +8990,24 @@ proc check_effective_target_exceptions {} {
         return 1
     }

    +# Used to check if the testing configuration supports exceptions.
    +# Returns 0 if exceptions are unsupported or disabled (e.g. by passing
    +# -fno-exceptions). Returns 1 if exceptions are enabled.
    +proc check_effective_target_exceptions_enabled {} {
    +    return [check_cached_effective_target exceptions_enabled {
    +   if { [check_effective_target_exceptions] } {
    +       return [check_no_compiler_messages exceptions_enabled assembly {
    +           void foo (void)
    +           {
    +               throw 1;
    +           }
    +       }]
    +   } else {
    +       # If exceptions aren't supported, then they're not enabled.
    +       return 0
    +   }
    +    }]
    +}

     proc check_effective_target_tiny {} {
         return [check_cached_effective_target tiny {

..., and it even already has one usage in libstdc++, per your
commit 4c27c6584d0c15926f57ac40f931e238cf0b3110
"libstdc++: Make testsuite usable with -fno-exceptions":

    --- libstdc++-v3/testsuite/23_containers/vector/bool/72847.cc
    +++ libstdc++-v3/testsuite/23_containers/vector/bool/72847.cc
    @@ -15,7 +15,7 @@
     // with this library; see the file COPYING3.  If not see
     // <http://www.gnu.org/licenses/>.

    -// { dg-skip-if "" { *-*-* } { "-fno-exceptions" } }
    +// { dg-require-effective-target exceptions_enabled }

     #include <vector>
     #include <ext/throw_allocator.h>

;-)


Grüße
 Thomas


>> For a start, the libstdc++ test suite needs
>> 'UNSUPPORTED: [...]: exception handling disabled' enabled/ported.  (I'll
>> do that.)  Otherwise, a number of test cases need DejaGnu directives
>> conditionalized on 'target exceptions_enabled'.  (Or,
>> 'error: exception handling disabled' made a "really late" diagnostic, so
>> that it doesn't preclude other diagnostics?  I'll have a look.  Well,
>> maybe something like: in fact do not default to '-fno-exceptions', but
>> instead emit 'error: exception handling disabled' only if in a "really
>> late" pass we run into exceptions-related constructs that we cannot
>> support.  That'd also avoid PASS -> UNSUPPORTED "regressions" when
>> exception handling in fact gets optimized away, for example.  I like that
>> idea, conceptually -- but is it feasible to implement..?)
>>
>
> IMHO just defining an effective target keyword and then using that in test
> selectors seems simpler, and doesn't require changes to the compiler, just
> the tests.


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From d12157a17683ff400f911751e2f2d74394f9ff5d Mon Sep 17 00:00:00 2001
From: Thomas Schwinge <thomas@codesourcery.com>
Date: Wed, 7 Jun 2023 08:46:38 +0200
Subject: [PATCH] Support 'UNSUPPORTED: [...]: exception handling disabled' for
 libstdc++ testing

Verbatim copy of what was added to 'gcc/testsuite/lib/gcc-dg.exp:gcc-dg-prune'
in Subversion r279246 (Git commit a9046e9853024206bec092dd63e21e152cb5cbca)
"[MSP430] -Add fno-exceptions multilib".

This greatly improves 'make check-target-libstdc++-v3' results for, for
example, x86_64-pc-linux-gnu with:

    RUNTESTFLAGS='--target_board=unix/-fno-exceptions\{,-m32\}'

	libstdc++-v3/
	* testsuite/lib/prune.exp (libstdc++-dg-prune): Support
	'UNSUPPORTED: [...]: exception handling disabled'.
---
 libstdc++-v3/testsuite/lib/prune.exp | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/libstdc++-v3/testsuite/lib/prune.exp b/libstdc++-v3/testsuite/lib/prune.exp
index 2d2349e7cba..be6d16c26e5 100644
--- a/libstdc++-v3/testsuite/lib/prune.exp
+++ b/libstdc++-v3/testsuite/lib/prune.exp
@@ -79,6 +79,18 @@ proc libstdc++-dg-prune { system text } {
     # Ignore dsymutil warning (tool bug is actually in the linker)
     regsub -all "(^|\n)\[^\n\]*could not find object file symbol for symbol\[^\n\]*" $text "" text
 
+    # If exceptions are disabled, mark tests expecting exceptions to be enabled
+    # as unsupported.
+    if { ![check_effective_target_exceptions_enabled] } {
+	if [regexp "(^|\n)\[^\n\]*: error: exception handling disabled" $text] {
+	    return "::unsupported::exception handling disabled"
+	}
+
+	if [regexp "(^|\n)\[^\n\]*: error: #error .__cpp_exceptions." $text] {
+	    return "::unsupported::exception handling disabled"
+	}
+    }
+
     foreach p $additional_prunes {
 	if { [string length $p] > 0 } {
 	    # Following regexp matches a complete line containing $p.
-- 
2.34.1