Fix PR19061, gdb hangs/spins-on-cpu when debugging any program on Alpha

This fixes PR19061, where gdb hangs/spins-on-cpu when debugging any
program on Alpha.

(This patch is Uros' forward port of the patch from comment #5
of the PR [1].)

Patch was tested on alphaev68-linux-gnu, also tested with gcc's
testsuite, where it fixed all hangs in guality.exp and
simulate-thread.exp testcases.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=19061#c5

gdb/ChangeLog:
2017-12-15  Richard Henderson  <rth@redhat.com>
	    Uros Bizjak  <ubizjak@gmail.com>

	PR gdb/19061
	* alpha-tdep.c (alpha_software_single_step): Call
	alpha_deal_with_atomic_sequence here.
	(set_gdbarch_software_single_step): Set to
	alpha_software_single_step.
	* nat/linux-ptrace.h [__alpha__]: Define GDB_ARCH_IS_TRAP_BRKPT
	and GDB_ARCH_IS_TRAP_HWBKPT.

3 files changed, 28 insertions, 6 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 292929d..6a5c74b 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@
+2017-12-15  Richard Henderson  <rth@redhat.com>
+	    Uros Bizjak  <ubizjak@gmail.com>
+
+	PR gdb/19061
+	* alpha-tdep.c (alpha_software_single_step): Call
+	alpha_deal_with_atomic_sequence here.
+	(set_gdbarch_software_single_step): Set to
+	alpha_software_single_step.
+	* nat/linux-ptrace.h [__alpha__]: Define GDB_ARCH_IS_TRAP_BRKPT
+	and GDB_ARCH_IS_TRAP_HWBKPT.
+
 2017-12-15  Yao Qi  <yao.qi@linaro.org>
 
 	* unittests/memory-map-selftests.c: Wrap test with HAVE_LIBEXPAT.
diff --git a/gdb/alpha-tdep.c b/gdb/alpha-tdep.c
index 21199bd..07144ce 100644
--- a/gdb/alpha-tdep.c
+++ b/gdb/alpha-tdep.c
@@ -767,10 +767,8 @@ static const int stq_c_opcode = 0x2f;
    the sequence.  */
 
 static std::vector<CORE_ADDR>
-alpha_deal_with_atomic_sequence (struct regcache *regcache)
+alpha_deal_with_atomic_sequence (struct gdbarch *gdbarch, CORE_ADDR pc)
 {
-  struct gdbarch *gdbarch = regcache->arch ();
-  CORE_ADDR pc = regcache_read_pc (regcache);
   CORE_ADDR breaks[2] = {-1, -1};
   CORE_ADDR loc = pc;
   CORE_ADDR closing_insn; /* Instruction that closes the atomic sequence.  */
@@ -1723,9 +1721,17 @@ alpha_next_pc (struct regcache *regcache, CORE_ADDR pc)
 std::vector<CORE_ADDR>
 alpha_software_single_step (struct regcache *regcache)
 {
-  CORE_ADDR pc = alpha_next_pc (regcache, regcache_read_pc (regcache));
+  struct gdbarch *gdbarch = regcache->arch ();
+
+  CORE_ADDR pc = regcache_read_pc (regcache);
+
+  std::vector<CORE_ADDR> next_pcs
+    = alpha_deal_with_atomic_sequence (gdbarch, pc);
+  if (!next_pcs.empty ())
+    return next_pcs;
 
-  return {pc};
+  CORE_ADDR next_pc = alpha_next_pc (regcache, pc);
+  return {next_pc};
 }
 
 
@@ -1821,7 +1827,7 @@ alpha_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   set_gdbarch_cannot_step_breakpoint (gdbarch, 1);
 
   /* Handles single stepping of atomic sequences.  */
-  set_gdbarch_software_single_step (gdbarch, alpha_deal_with_atomic_sequence);
+  set_gdbarch_software_single_step (gdbarch, alpha_software_single_step);
 
   /* Hook in ABI-specific overrides, if they have been registered.  */
   gdbarch_init_osabi (info, gdbarch);
diff --git a/gdb/nat/linux-ptrace.h b/gdb/nat/linux-ptrace.h
index 5954945..8a8c4c6 100644
--- a/gdb/nat/linux-ptrace.h
+++ b/gdb/nat/linux-ptrace.h
@@ -155,6 +155,8 @@ struct buffer;
    Beginning with Linux 4.6, the MIPS port reports proper TRAP_BRKPT and
    TRAP_HWBKPT codes, so we also match them.
 
+   The Alpha kernel uses TRAP_BRKPT for all traps.
+
    The generic Linux target code should use GDB_ARCH_IS_TRAP_* instead
    of TRAP_* to abstract out these peculiarities.  */
 #if defined __i386__ || defined __x86_64__
@@ -166,6 +168,9 @@ struct buffer;
 #elif defined __mips__
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == SI_KERNEL || (X) == TRAP_BRKPT)
 # define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == SI_KERNEL || (X) == TRAP_HWBKPT)
+#elif defined __alpha__
+# define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
+# define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == TRAP_BRKPT)
 #else
 # define GDB_ARCH_IS_TRAP_BRKPT(X) ((X) == TRAP_BRKPT)
 # define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == TRAP_HWBKPT)