[binutils-gdb] S390: Fix displaced stepping of "basr r,0"

[Andreas Arnez]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=5c1eda30f55e9c543de26fec0246f4d7655c81e4

commit 5c1eda30f55e9c543de26fec0246f4d7655c81e4
Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Date:   Fri Jul 13 12:46:14 2018 +0200

    S390: Fix displaced stepping of "basr r,0"
    
    The BASR instruction behaves differently depending on whether the second
    operand is a number from 1 to 15, or zero.  In the former case BASR jumps
    to the address contained in the general register of that number, but in
    the latter case no jump is performed.  GDB's displaced-stepping logic does
    not distinguish these cases, although it should.
    
    This is fixed.  In the case where no jump is performed the PC is adjusted
    to point back after the original instruction.  Otherwise the PC is left
    alone.
    
    gdb/ChangeLog:
    
    	* s390-tdep.c (s390_displaced_step_fixup): Adjust PC for a
    	non-branching basr.

Diff:
---
 gdb/ChangeLog   | 5 +++++
 gdb/s390-tdep.c | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index cc61dab..7b808eb 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2018-07-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* s390-tdep.c (s390_displaced_step_fixup): Adjust PC for a
+	non-branching basr.
+
 2018-07-12  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
 	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
diff --git a/gdb/s390-tdep.c b/gdb/s390-tdep.c
index 77e64af..e962824 100644
--- a/gdb/s390-tdep.c
+++ b/gdb/s390-tdep.c
@@ -492,6 +492,9 @@ s390_displaced_step_fixup (struct gdbarch *gdbarch,
       /* Recompute saved return address in R1.  */
       regcache_cooked_write_unsigned (regs, S390_R0_REGNUM + r1,
 				      amode | (from + insnlen));
+      /* Update PC iff the instruction doesn't actually branch.  */
+      if (insn[0] == op_basr && r2 == 0)
+	regcache_write_pc (regs, from + insnlen);
     }
 
   /* Handle absolute branch instructions.  */