S390: Recognize special jumps in prologue parser

Functions compiled with the gcc option `-mhotpatch' may start with a
branch-never BRCL instruction as a 6-byte NOP.  And functions compiled
with `-mstack-size' contain a BRC instruction in their prologue that is
actually a conditional trap.  Both of these special jumps cause the
prologue parser to stop and yield bad unwinding results.

This change makes the prologue analyzer recognize such special jumps and
ignore them.

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_analyze_prologue): Ignore BRC and BRCL
	instructions that do nothing or are conditional traps.

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 0674836..6c33877 100644 (file)
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2016-03-09  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
+       * s390-linux-tdep.c (s390_analyze_prologue): Ignore BRC and BRCL
+       instructions that do nothing or are conditional traps.
+
+2016-03-09  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
        * s390-linux-tdep.c (s390_prologue_frame_unwind_cache): Store
        frame func's PC in info->func before any other failure can occur.
        (s390_frame_this_id): Use frame_id_build_unavailable_stack if

diff --git a/gdb/s390-linux-tdep.c b/gdb/s390-linux-tdep.c
index 155bc66..950696e 100644 (file)
--- a/gdb/s390-linux-tdep.c
+++ b/gdb/s390-linux-tdep.c
@@ -1567,13 +1567,25 @@ s390_analyze_prologue (struct gdbarch *gdbarch,
            break;
        }
 
+      /* BRC/BRCL -- branch relative on condition.  Ignore "branch
+        never", branch to following instruction, and "conditional
+        trap" (BRC +2).  Otherwise terminate search.  */
+      else if (is_ri (insn, op1_brc, op2_brc, &r1, &i2))
+       {
+         if (r1 != 0 && i2 != 1 && i2 != 2)
+           break;
+       }
+      else if (is_ril (insn, op1_brcl, op2_brcl, &r1, &i2))
+       {
+         if (r1 != 0 && i2 != 3)
+           break;
+       }
+
       /* Terminate search when hitting any other branch instruction.  */
       else if (is_rr (insn, op_basr, &r1, &r2)
               || is_rx (insn, op_bas, &r1, &d2, &x2, &b2)
               || is_rr (insn, op_bcr, &r1, &r2)
               || is_rx (insn, op_bc, &r1, &d2, &x2, &b2)
-              || is_ri (insn, op1_brc, op2_brc, &r1, &i2)
-              || is_ril (insn, op1_brcl, op2_brcl, &r1, &i2)
               || is_ril (insn, op1_brasl, op2_brasl, &r2, &i2))
        break;