[RFC PATCH] tree-ssa-sink: do not sink to in front of setjmp

[Alexander Monakov <amonakov@ispras.ru>]

Greetings!

While testing our patch that reimplements -Wclobbered on GIMPLE we found
a case where tree-ssa-sink moves a statement to a basic block in front
of a setjmp call.

I am confident that this is unintended and should be considered invalid
GIMPLE. One of the edges incoming to a setjmp BB will be an edge from
the ABNORMAL_DISPATCHER block, corresponding to transfer of control flow
from a longjmp-like function and resulting in a "second return" from
setjmp. When that happens, it is not possible for GIMPLE statements in
front of setjmp to be somehow re-executed after longjmp.

I am unsure how this could be prevented "once and for all" so the
following patch just attacks one place (out of three) in
tree-ssa-sink by checking 'bb_has_abnormal_pred (sinkbb)'. Alexey
(Cc'ed) bootstrapped and regtested the patch on trunk.

The testcase is just

struct __jmp_buf_tag { };
typedef struct __jmp_buf_tag jmp_buf[1];
struct globals { jmp_buf listingbuf; };
extern struct globals *const ptr_to_globals;
void foo()
{
  if ( _setjmp ( ((*ptr_to_globals).listingbuf )))
    ;
}

Before tree-ssa-sink we have

void foo ()
{
   struct globals * ptr_to_globals.0_1;
   struct __jmp_buf_tag[1] * _2;

   <bb 2> :
   ptr_to_globals.0_1 = ptr_to_globals;
   _2 = &ptr_to_globals.0_1->listingbuf;

   <bb 3> :
   _setjmp (_2);
   goto <bb 5>; [INV]

   <bb 4> :
   .ABNORMAL_DISPATCHER (0);

   <bb 5> :
   return;

}

And tree-ssa-sink yields (see BB 3)

Sinking _2 = &ptr_to_globals.0_1->listingbuf;
  from bb 2 to bb 3
void foo ()
{
   struct globals * ptr_to_globals.0_1;
   struct __jmp_buf_tag[1] * _2;

   <bb 2> :
   ptr_to_globals.0_1 = ptr_to_globals;

   <bb 3> :
   _2 = &ptr_to_globals.0_1->listingbuf;
   _setjmp (_2);
   goto <bb 5>; [INV]

   <bb 4> :
   .ABNORMAL_DISPATCHER (0);

   <bb 5> :
   return;

}

The patch:

diff --git a/gcc/tree-ssa-sink.c b/gcc/tree-ssa-sink.c
index c5d67840be3..317e2563114 100644
--- a/gcc/tree-ssa-sink.c
+++ b/gcc/tree-ssa-sink.c
@@ -461,6 +461,9 @@ statement_sink_location (gimple *stmt, basic_block frombb,
 	  else
 	    *togsi = gsi_after_labels (sinkbb);
 
+	  if (bb_has_abnormal_pred (sinkbb))
+	    return false;
+
 	  return true;
 	}
     }