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:
2018-01-03  Richard Henderson  <rth@redhat.com>
	    Uros Bizjak  <ubizjak@gmail.com>

	PR gdb/19061
	* alpha-tdep.c (alpha_deal_with_atomic_sequence): Change
	prototype.
	(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, 7 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index bfa2359..b80bc98 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,16 @@
+2018-01-03  Richard Henderson  <rth@redhat.com>
+	    Uros Bizjak  <ubizjak@gmail.com>
+
+	PR gdb/19061
+	* alpha-tdep.c (alpha_deal_with_atomic_sequence): Change
+	prototype.
+	(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.
+
 2017-10-25  Leszek Swirski  <leszeks@google.com>
 
 	* dwarf2read.c (dwarf2_string_attr): Allow DW_FORM_GNU_str_index.
diff --git a/gdb/alpha-tdep.c b/gdb/alpha-tdep.c
index 7c521db..2302e6d 100644
--- a/gdb/alpha-tdep.c
+++ b/gdb/alpha-tdep.c
@@ -766,10 +766,8 @@ static const int stq_c_opcode = 0x2f;
    the sequence.  */
 
 static VEC (CORE_ADDR) *
-alpha_deal_with_atomic_sequence (struct regcache *regcache)
+alpha_deal_with_atomic_sequence (struct gdbarch *gdbarch, CORE_ADDR pc)
 {
-  struct gdbarch *gdbarch = get_regcache_arch (regcache);
-  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.  */
@@ -1721,12 +1719,17 @@ VEC (CORE_ADDR) *
 alpha_software_single_step (struct regcache *regcache)
 {
   struct gdbarch *gdbarch = get_regcache_arch (regcache);
-  CORE_ADDR pc;
-  VEC (CORE_ADDR) *next_pcs = NULL;
+  CORE_ADDR pc, next_pc;
+  VEC (CORE_ADDR) *next_pcs;
 
   pc = regcache_read_pc (regcache);
+  next_pcs = alpha_deal_with_atomic_sequence (gdbarch, pc);
+  if (next_pcs != NULL)
+    return next_pcs;
+
+  next_pc = alpha_next_pc (regcache, pc);
 
-  VEC_safe_push (CORE_ADDR, next_pcs, alpha_next_pc (regcache, pc));
+  VEC_safe_push (CORE_ADDR, next_pcs, next_pc);
   return next_pcs;
 }
 
@@ -1826,7 +1829,7 @@ 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)