[PATCH] testsuite, mi: avoid a clang bug in 'user-selected-context-sync.exp'

[PATCH] testsuite, mi: avoid a clang bug in 'user-selected-context-sync.exp'

[Tankut Baris Aktemur]

This test causes several timeouts for Clang, taking too long time to
finish.  The reason is, for an infinite loop of the form

   while(1); /* suppose this is line 30.  */

Clang generates code that looks like

   0x00000000004004d4 <+4>:     jmp    0x4004d9 <loop+9>
   0x00000000004004d9 <+9>:     jmp    0x4004d9 <loop+9>

So, the real loop is the instruction at address 0x4004d9.  But a
breakpoint that's defined at the loop line (assume line 30 in this
case) is inserted at address 0x4004d4.

  (gdb) break 30
  Breakpoint 1 at 0x4004d4: file test.c, line 30.

Therefore, continuing a thread that was spinning on the loop does not hit
the breakpoint.  The bug is reported at

  https://bugs.llvm.org/show_bug.cgi?id=49614

Tweak the infinite loop to spin on a variable to avoid this bug.  The
test is unrelated to the bug.

gdb/testsuite/ChangeLog:
2021-03-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.mi/user-selected-context-sync.exp: Spin on a variable in
	the infinite loop to avoid a Clang bug.
---
 gdb/testsuite/gdb.mi/user-selected-context-sync.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/gdb/testsuite/gdb.mi/user-selected-context-sync.c b/gdb/testsuite/gdb.mi/user-selected-context-sync.c
index 500a118a2dc..0f8a966f69d 100644
--- a/gdb/testsuite/gdb.mi/user-selected-context-sync.c
+++ b/gdb/testsuite/gdb.mi/user-selected-context-sync.c
@@ -27,7 +27,8 @@ static pthread_barrier_t barrier;
 static void
 child_sub_function (void)
 {
-  while (1); /* thread loop line */
+  int spin = 1;
+  while (spin); /* thread loop line */
 }
 
 static void *
@@ -57,7 +58,8 @@ main (void)
 
   pthread_barrier_wait (&barrier);
 
-  while (1); /* main break line */
+  int spin = 1;
+  while (spin); /* main break line */
 
   return 0;
 }
-- 
2.17.1

Re: [PATCH] testsuite, mi: avoid a clang bug in 'user-selected-context-sync.exp'

[Simon Marchi]

On 2021-03-29 10:03 a.m., Tankut Baris Aktemur via Gdb-patches wrote:> This test causes several timeouts for Clang, taking too long time to
> finish.  The reason is, for an infinite loop of the form
> 
>    while(1); /* suppose this is line 30.  */
> 
> Clang generates code that looks like
> 
>    0x00000000004004d4 <+4>:     jmp    0x4004d9 <loop+9>
>    0x00000000004004d9 <+9>:     jmp    0x4004d9 <loop+9>
> 
> So, the real loop is the instruction at address 0x4004d9.  But a
> breakpoint that's defined at the loop line (assume line 30 in this
> case) is inserted at address 0x4004d4.
> 
>   (gdb) break 30
>   Breakpoint 1 at 0x4004d4: file test.c, line 30.
> 
> Therefore, continuing a thread that was spinning on the loop does not hit
> the breakpoint.  The bug is reported at
> 
>   https://bugs.llvm.org/show_bug.cgi?id=49614
> 
> Tweak the infinite loop to spin on a variable to avoid this bug.  The
> test is unrelated to the bug.
> 
> gdb/testsuite/ChangeLog:
> 2021-03-29  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
> 
> 	* gdb.mi/user-selected-context-sync.exp: Spin on a variable in
> 	the infinite loop to avoid a Clang bug.
> ---
>  gdb/testsuite/gdb.mi/user-selected-context-sync.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/testsuite/gdb.mi/user-selected-context-sync.c b/gdb/testsuite/gdb.mi/user-selected-context-sync.c
> index 500a118a2dc..0f8a966f69d 100644
> --- a/gdb/testsuite/gdb.mi/user-selected-context-sync.c
> +++ b/gdb/testsuite/gdb.mi/user-selected-context-sync.c
> @@ -27,7 +27,8 @@ static pthread_barrier_t barrier;
>  static void
>  child_sub_function (void)
>  {
> -  while (1); /* thread loop line */
> +  int spin = 1;
> +  while (spin); /* thread loop line */
>  }
>  
>  static void *
> @@ -57,7 +58,8 @@ main (void)
>  
>    pthread_barrier_wait (&barrier);
>  
> -  while (1); /* main break line */
> +  int spin = 1;
> +  while (spin); /* main break line */
>  
>    return 0;
>  }
> 

Thanks, this is OK.  Maybe just add a comment in the code, otherwise
someone might be tempted to "optimize" it.  If they see the comment,
they'll be able to "git blame" and get the full story.

Simon