[PATCH, alpha]: Handle single-stepping of atomic sequences

[Uros Bizjak <ubizjak at gmail dot com>]

Hello!

The gcc simulate-thread.exp testsuite exposed the problem in handling
of alpha atomic sequences. Single-stepping broke link in alpha's LL/SC
atomic sequences.

Attached patch copies handling of atomic sequences from rs6000-tdep.c
and adapts the functionality for alpha.

2011-11-16  Uros Bizjak  <ubizjak@gmail.com>

	* alpha-tdep.c (beq_opcode): New.
	(ldl_l_opcode, ldq_l_opcode, stl_c_opcode, stq_c_opcode): Ditto.
	(alpha_deal_with_atomic_sequence): New function.
	(alpha_gdbarch_init): Handle single stepping of atomic sequences
	with alpha_deal_with_atomic_sequence.

Patched gdb survives simulate-thread.exp testsuite on native
alphaev68-pc-linux-gnu.

I don't have write access, so If OK, please someone commit this patch
to gdb CVS.

Uros.

--- alpha-tdep.c	2011-11-16 23:27:20.080899480 +0100
+++ alpha-tdep.c.ub	2011-11-16 23:27:01.160051444 +0100
@@ -65,6 +65,7 @@ static const int stq_opcode = 0x2d;
 /* Branch instruction format */
 #define BR_RA(insn) MEM_RA(insn)
 
+static const int beq_opcode = 0x39;
 static const int bne_opcode = 0x3d;
 
 /* Operate instruction format */
@@ -762,6 +763,95 @@ alpha_skip_prologue (struct gdbarch *gdb
 }
 
 
+static const int ldl_l_opcode = 0x2a;
+static const int ldq_l_opcode = 0x2b;
+static const int stl_c_opcode = 0x2e;
+static const int stq_c_opcode = 0x2f;
+
+/* Checks for an atomic sequence of instructions beginning with a LDL_L/LDQ_L
+   instruction and ending with a STL_C/STQ_C instruction.  If such a sequence
+   is found, attempt to step through it.  A breakpoint is placed at the end of 
+   the sequence.  */
+
+int 
+alpha_deal_with_atomic_sequence (struct frame_info *frame)
+{
+  struct gdbarch *gdbarch = get_frame_arch (frame);
+  struct address_space *aspace = get_frame_address_space (frame);
+  CORE_ADDR pc = get_frame_pc (frame);
+  CORE_ADDR breaks[2] = {-1, -1};
+  CORE_ADDR loc = pc;
+  CORE_ADDR closing_insn; /* Instruction that closes the atomic sequence.  */
+  unsigned int insn = alpha_read_insn (gdbarch, loc);
+  int insn_count;
+  int index;
+  int last_breakpoint = 0; /* Defaults to 0 (no breakpoints placed).  */  
+  const int atomic_sequence_length = 16; /* Instruction sequence length.  */
+  int bc_insn_count = 0; /* Conditional branch instruction count.  */
+
+  /* Assume all atomic sequences start with a LDL_L/LDQ_L instruction.  */
+  if (INSN_OPCODE (insn) != ldl_l_opcode
+      && INSN_OPCODE (insn) != ldq_l_opcode)
+    return 0;
+
+  /* Assume that no atomic sequence is longer than "atomic_sequence_length" 
+     instructions.  */
+  for (insn_count = 0; insn_count < atomic_sequence_length; ++insn_count)
+    {
+      loc += ALPHA_INSN_SIZE;
+      insn = alpha_read_insn (gdbarch, loc);
+
+      /* Assume that there is at most one conditional branch in the atomic
+         sequence.  If a conditional branch is found, put a breakpoint in 
+         its destination address.  */
+      if (INSN_OPCODE (insn) >= beq_opcode)
+        {
+          int immediate = (insn & 0x001fffff) << 2;
+
+	  immediate = (immediate ^ 0x400000) - 0x400000;
+
+          if (bc_insn_count >= 1)
+            return 0; /* More than one conditional branch found, fallback 
+                         to the standard single-step code.  */
+ 
+	  breaks[1] = loc + ALPHA_INSN_SIZE + immediate;
+
+	  bc_insn_count++;
+	  last_breakpoint++;
+        }
+
+      if (INSN_OPCODE (insn) == stl_c_opcode
+	  || INSN_OPCODE (insn) == stq_c_opcode)
+	break;
+    }
+
+  /* Assume that the atomic sequence ends with a STL_C/STQ_C instruction.  */
+  if (INSN_OPCODE (insn) != stl_c_opcode
+      && INSN_OPCODE (insn) != stq_c_opcode)
+    return 0;
+
+  closing_insn = loc;
+  loc += ALPHA_INSN_SIZE;
+
+  /* Insert a breakpoint right after the end of the atomic sequence.  */
+  breaks[0] = loc;
+
+  /* Check for duplicated breakpoints.  Check also for a breakpoint
+     placed (branch instruction's destination) at the STL_C/STQ_C 
+     instruction, this resets the reservation and take us back to the 
+     LDL_L/LDQ_L instruction at the beginning of the atomic sequence.  */
+  if (last_breakpoint && ((breaks[1] == breaks[0]) 
+      || (breaks[1] == closing_insn)))
+    last_breakpoint = 0;
+
+  /* Effectively inserts the breakpoints.  */
+  for (index = 0; index <= last_breakpoint; index++)
+    insert_single_step_breakpoint (gdbarch, aspace, breaks[index]);
+
+  return 1;
+}
+
+
 /* Figure out where the longjmp will land.
    We expect the first arg to be a pointer to the jmp_buf structure from
    which we extract the PC (JB_PC) that we will land at.  The PC is copied
@@ -1749,6 +1839,9 @@ alpha_gdbarch_init (struct gdbarch_info 
   set_gdbarch_decr_pc_after_break (gdbarch, ALPHA_INSN_SIZE);
   set_gdbarch_cannot_step_breakpoint (gdbarch, 1);
 
+  /* Handles single stepping of atomic sequences.  */
+  set_gdbarch_software_single_step (gdbarch, alpha_deal_with_atomic_sequence);
+
   /* Hook in ABI-specific overrides, if they have been registered.  */
   gdbarch_init_osabi (info, gdbarch);