[PATCH] [PR gdb/23210] Unset gdbarch significant_addr_bit by default

[Omair Javaid <omair.javaid@linaro.org>]

This patch fixes a bug introduced by fix to AArch64 pointer tagging.

In our fix for tagged pointer support our agreed approach was to sign
extend user-space address after clearing tag bits. This is not same
for all architectures and this patch allows sign extension for
addresses on targets which specifically set significant_addr_bit.

More information about patch that caused the issues and discussion
around tagged pointer support can be found in links below:

https://sourceware.org/ml/gdb-patches/2018-05/msg00000.html
https://sourceware.org/ml/gdb-patches/2017-12/msg00159.html

gdb/ChangeLog:

2018-05-23  Omair Javaid  <omair.javaid@linaro.org>

	* gdbarch.c (verify_gdbarch): Update.
	* utils.c (address_significant): Update.
---
 gdb/gdbarch.c | 3 +--
 gdb/utils.c   | 5 +++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/gdb/gdbarch.c b/gdb/gdbarch.c
index c430ebe..5593911 100644
--- a/gdb/gdbarch.c
+++ b/gdb/gdbarch.c
@@ -615,8 +615,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of stabs_argument_has_addr, invalid_p == 0 */
   /* Skip verify of convert_from_func_ptr_addr, invalid_p == 0 */
   /* Skip verify of addr_bits_remove, invalid_p == 0 */
-  if (gdbarch->significant_addr_bit == 0)
-    gdbarch->significant_addr_bit = gdbarch_addr_bit (gdbarch);
+  /* Skip verify of significant_addr_bit, invalid_p == 0 */
   /* Skip verify of software_single_step, has predicate.  */
   /* Skip verify of single_step_through_delay, has predicate.  */
   /* Skip verify of print_insn, invalid_p == 0 */
diff --git a/gdb/utils.c b/gdb/utils.c
index 9c5bf68..91c0f2b 100644
--- a/gdb/utils.c
+++ b/gdb/utils.c
@@ -2708,10 +2708,11 @@ address_significant (gdbarch *gdbarch, CORE_ADDR addr)
   /* Clear insignificant bits of a target address and sign extend resulting
      address, avoiding shifts larger or equal than the width of a CORE_ADDR.
      The local variable ADDR_BIT stops the compiler reporting a shift overflow
-     when it won't occur.  */
+     when it won't occur. Skip updating of target address if current target has
+     not set gdbarch significant_addr_bit.  */
   int addr_bit = gdbarch_significant_addr_bit (gdbarch);
 
-  if (addr_bit < (sizeof (CORE_ADDR) * HOST_CHAR_BIT))
+  if (addr_bit && (addr_bit < (sizeof (CORE_ADDR) * HOST_CHAR_BIT)))
     {
       CORE_ADDR sign = (CORE_ADDR) 1 << (addr_bit - 1);
       addr &= ((CORE_ADDR) 1 << addr_bit) - 1;
-- 
2.7.4