gcc -foptimize-sibling-calls cause trouble for perl's signal handler [PATCH]

After change #32012 we have seen test failures for our Linux builds  
as discussed by Jan here:

     http://groups.google.com/group/perl.perl5.porters/msg/ 
7cea860c26fb4464

The failure also seem to be the same as one reported here:

    http://www.nntp.perl.org/group/perl.perl5.porters/2008/03/ 
msg134858.html

What I've managed to figure out is that the problem is that gcc  
optimize out the return from the signal handler when the last  
statement in the function ends up being the call to S_raise_signal 
().  I found that this patch disable this optimization and make all  
tests pass again.  Alternative fix is to compile the file mg.c with  
the -fno-optimize-sibling-calls option.

--- perl-current/mg.c   2008-04-15 01:40:11.000000000 -0700
+++ perl-hack/mg.c      2008-04-23 10:37:44.000000000 -0700
@@ -1406,6 +1406,13 @@
  #endif
     else
         S_raise_signal(aTHX_ sig);
+
+   /* Segfaults observed when the call to S_raise_signal() is
+    * optimized into a jmp instead of a real call.  An explicit
+    * return avoids this optimization.  Alternative to compile
+    * this file with 'gcc -fno-optimize-sibling-calls'
+    */
+   return;
  }

  #if defined(FAKE_PERSISTENT_SIGNAL_HANDLERS) || defined 
(FAKE_DEFAULT_SIGNAL_HANDLERS)

I still don't understand why this ends up being problematic.  Anybody  
able to explain it?  Just seems to  be something about how this  
optimization interacts how the stack is set up when the signal  
handler gets called.

I found this message from somebody having a similar problem on NetBSD.

    http://gcc.gnu.org/ml/gcc/2006-03/msg00409.html

BTW, this shows how the generated assembly code differs after the  
return statement has been added:

--- xx1 2008-04-23 05:36:00.000000000 -0700
+++ xx2 2008-04-23 05:37:27.000000000 -0700
@@ -13,11 +13,11 @@
  .LCFI190:
         .loc 1 1371 0
  .LBB170:
-       movl    PL_thr_key, %ecx
+       movl    PL_thr_key, %eax
         .loc 1 1367 0
         movl    8(%ebp), %ebx
         .loc 1 1371 0
-       pushl   %ecx
+       pushl   %eax
  .LCFI191:
         call    pthread_getspecific
         .loc 1 1389 0
@@ -38,24 +38,28 @@
         je      .L724
  .L725:
         .loc 1 1403 0
-       pushl   %edx
+       pushl   %eax
         pushl   $0
         pushl   $0
         pushl   %ebx
         call    *1180(%ecx)
         addl    $16, %esp
+       .loc 1 1410 0
         movl    -4(%ebp), %ebx
         leave
         ret
         .p2align 4,,7
  .L724:
         .loc 1 1408 0
-       movl    %ebx, 12(%ebp)
-       movl    %ecx, 8(%ebp)
+       pushl   %edx
+       pushl   %edx
+       pushl   %ebx
+       pushl   %ecx
+       call    S_raise_signal
+       addl    $16, %esp
         movl    -4(%ebp), %ebx
         leave
-.LCFI192:
-       jmp     S_raise_signal
+       ret
  .LBE170:
  .LFE113:
         .size   Perl_csighandler, .-Perl_csighandler

• gcc -foptimize-sibling-calls cause trouble for perl's signal handler [PATCH] by Gisle Aas
• Re: gcc -foptimize-sibling-calls cause trouble for perl's signal handler [PATCH] by Gisle Aas