Re: [PATCH] [gdb/testsuite] Fix gdb.base/inline-frame-cycle-unwind.exp for s390x

[Simon Marchi <simark@simark.ca>]

On 12/8/25 2:53 PM, Kevin Buettner wrote:
> Ping.
> 
> On Sat,  1 Nov 2025 21:46:51 -0700
> Kevin Buettner <kevinb@redhat.com> wrote:
> 
>> This commit fixes six failures for s390x due to a fundamental
>> difference in unwinding behavior between s390x and other
>> architectures:
>>
>> FAIL: gdb.base/inline-frame-cycle-unwind.exp: bt: cycle at level 5:
>>   backtrace when the unwind is broken at frame 5
>> FAIL: gdb.base/inline-frame-cycle-unwind.exp: bt: cycle at level 3:
>>   backtrace when the unwind is broken at frame 3
>> FAIL: gdb.base/inline-frame-cycle-unwind.exp: bt: cycle at level 1:
>>   backtrace when the unwind is broken at frame 1
>> FAIL: gdb.base/inline-frame-cycle-unwind.exp: bt -no-filters: cycle at level 5:
>>   backtrace when the unwind is broken at frame 5
>> FAIL: gdb.base/inline-frame-cycle-unwind.exp: bt -no-filters: cycle at level 3:
>>   backtrace when the unwind is broken at frame 3
>> FAIL: gdb.base/inline-frame-cycle-unwind.exp: bt -no-filters: cycle at level 1:
>>   backtrace when the unwind is broken at frame 1
>>
>> The core issue is that on s390x, the Canonical Frame Address (CFA) for
>> a function points *into the caller's stack frame*, whereas on x86_64
>> or aarch64 the CFA points *within the current function's frame*.  This
>> architectural difference causes cycle detection to occur later on
>> s390x.
>>
>> The patch resolves this by:
>> - Making expected backtrace output architecture-specific.
>> - For non-s390x targets: expecting the full set of frames up to the
>>   specified level.
>> - For s390x: expecting fewer frames before detecting the cycle
>>   (e.g., level 5 shows 3 frames instead of 5).
>> - Skipping the cycle at level 1 test entirely on s390x since it cannot
>>   be detected at that frame.
>>
>> Tested using recent Fedora releases on s390x, x86_64, and aarch64.

I can't comment on the s390x-specific details, but code-wise the patch
LGTM.  Tom de Vries might have an opinion, he was doing some s390x fixes
recently.

Some nits below.

>> ---
>>  .../gdb.base/inline-frame-cycle-unwind.exp    | 81 +++++++++++++------
>>  1 file changed, 57 insertions(+), 24 deletions(-)
>>
>> diff --git a/gdb/testsuite/gdb.base/inline-frame-cycle-unwind.exp b/gdb/testsuite/gdb.base/inline-frame-cycle-unwind.exp
>> index 7fc47af624f..ccd86eb79ab 100644
>> --- a/gdb/testsuite/gdb.base/inline-frame-cycle-unwind.exp
>> +++ b/gdb/testsuite/gdb.base/inline-frame-cycle-unwind.exp
>> @@ -92,15 +92,31 @@ foreach bt_cmd { "bt" "bt -no-filters" } {
>>  	    gdb_test_no_output "python stop_at_level=5"
>>  	    gdb_test "maint flush register-cache" \
>>  		"Register cache flushed\\."
>> +
>> +	    # Frames expected on all targets
>> +	    set exp [list \
>> +		"#0 \[^\r\n\]* inline_func \\(\\) at \[^\r\n\]*" \
>> +		"#1 \[^\r\n\]* normal_func \\(\\) at \[^\r\n\]*" \
>> +		"#2 \[^\r\n\]* inline_func \\(\\) at \[^\r\n\]*" \
>> +		"#3 \[^\r\n\]* normal_func \\(\\) at \[^\r\n\]*"]
>> +
>> +	    # Additional frames required on non-s390x targets
>> +	    if {![istarget "s390x*-*-*"]} {
>> +		lappend exp \
>> +		    "#4 \[^\r\n\]* inline_func \\(\\) at \[^\r\n\]*" \
>> +		    "#5 \[^\r\n\]* normal_func \\(\\) at \[^\r\n\]*"
>> +	    }
>> +	    #
>> +	    # The final line that should appear for every target

Unexpected `#` by itself above.

>> @@ -108,25 +124,42 @@ foreach bt_cmd { "bt" "bt -no-filters" } {
>>  	    gdb_test_no_output "python stop_at_level=3"
>>  	    gdb_test "maint flush register-cache" \
>>  		"Register cache flushed\\."
>> +	    # Frames expected on all targets
>> +	    set exp [list \
>> +		"#0 \[^\r\n\]* inline_func \\(\\) at \[^\r\n\]*" \
>> +		"#1 \[^\r\n\]* normal_func \\(\\) at \[^\r\n\]*"]
>> +
>> +	    # Additional frames required on non-s390x targets
>> +	    if {![istarget "s390x*-*-*"]} {
>> +		lappend exp \
>> +		    "#2 \[^\r\n\]* inline_func \\(\\) at \[^\r\n\]*" \
>> +		    "#3 \[^\r\n\]* normal_func \\(\\) at \[^\r\n\]*"
>> +	    }
>> +	    #
>> +	    # The final line that should appear for every target

Likewise.

Simon