read/write_pieced_value: Improve logic for buffer allocation

So far the main loop in read_pieced_value and write_pieced_value is
structured like this:

(1) Prepare a buffer and some variables we may need;

(2) depending on the DWARF piece type to be handled, use the buffer and
    the prepared variables, ignore them, or even recalculate them.

This approach reduces readability and may also lead to unnecessary copying
of data.  This patch moves the preparations to the places where sufficient
information is available and removes some of the variables involved.

gdb/ChangeLog:

	* dwarf2loc.c (read_pieced_value): Move the buffer allocation and
	some other preparations to the places where sufficient information
	is available.
	(write_pieced_value): Likewise.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* dwarf2loc.c (read_pieced_value): Move the buffer allocation and
+	some other preparations to the places where sufficient information
+	is available.
+	(write_pieced_value): Likewise.
+
 2017-06-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
 	* dwarf2loc.c (bits_to_bytes): New function.

gdb/dwarf2loc.c

@@ -1794,8 +1794,7 @@ read_pieced_value (struct value *v)
     {
       struct dwarf_expr_piece *p = &c->pieces[i];
       size_t this_size, this_size_bits;
-      long dest_offset_bits, source_offset_bits, source_offset;
+      long dest_offset_bits, source_offset_bits;
-      const gdb_byte *intermediate_buffer;
 
       /* Compute size, source, and destination offsets for copying, in
 	 bits.  */
@@ -1813,11 +1812,6 @@ read_pieced_value (struct value *v)
       if (this_size_bits > max_offset - offset)
 	this_size_bits = max_offset - offset;
 
-      this_size = bits_to_bytes (source_offset_bits, this_size_bits);
-      buffer.reserve (this_size);
-      source_offset = source_offset_bits / 8;
-      intermediate_buffer = buffer.data ();
-
       /* Copy from the source to DEST_BUFFER.  */
       switch (p->location)
 	{
@@ -1843,13 +1837,11 @@ read_pieced_value (struct value *v)
 					   this_size, buffer.data (),
 					   &optim, &unavail))
 	      {
-		/* Just so garbage doesn't ever shine through.  */
-		memset (buffer.data (), 0, this_size);
-
 		if (optim)
 		  mark_value_bits_optimized_out (v, offset, this_size_bits);
 		if (unavail)
 		  mark_value_bits_unavailable (v, offset, this_size_bits);
+		break;
 	      }
 
 	    copy_bitwise (contents, dest_offset_bits,
@@ -1859,12 +1851,15 @@ read_pieced_value (struct value *v)
 	  break;
 
 	case DWARF_VALUE_MEMORY:
+	  this_size = bits_to_bytes (source_offset_bits, this_size_bits);
+	  buffer.reserve (this_size);
+
 	  read_value_memory (v, offset,
 			     p->v.mem.in_stack_memory,
-			     p->v.mem.addr + source_offset,
+			     p->v.mem.addr + source_offset_bits / 8,
 			     buffer.data (), this_size);
 	  copy_bitwise (contents, dest_offset_bits,
-			intermediate_buffer, source_offset_bits % 8,
+			buffer.data (), source_offset_bits % 8,
 			this_size_bits, bits_big_endian);
 	  break;
 
@@ -1892,18 +1887,18 @@ read_pieced_value (struct value *v)
 
 	case DWARF_VALUE_LITERAL:
 	  {
-	    size_t n = this_size;
+	    ULONGEST literal_size_bits = 8 * p->v.literal.length;
+	    size_t n = this_size_bits;
 
-	    if (n > p->v.literal.length - source_offset)
+	    /* Cut off at the end of the implicit value.  */
-	      n = (p->v.literal.length >= source_offset
+	    if (source_offset_bits >= literal_size_bits)
-		   ? p->v.literal.length - source_offset
+	      break;
-		   : 0);
+	    if (n > literal_size_bits - source_offset_bits)
-	    if (n != 0)
+	      n = literal_size_bits - source_offset_bits;
-	      intermediate_buffer = p->v.literal.data + source_offset;
 
 	    copy_bitwise (contents, dest_offset_bits,
-			  intermediate_buffer, source_offset_bits % 8,
+			  p->v.literal.data, source_offset_bits,
-			  this_size_bits, bits_big_endian);
+			  n, bits_big_endian);
 	  }
 	  break;
 
@@ -1960,9 +1955,7 @@ write_pieced_value (struct value *to, struct value *from)
     {
       struct dwarf_expr_piece *p = &c->pieces[i];
       size_t this_size_bits, this_size;
-      long dest_offset_bits, source_offset_bits, dest_offset, source_offset;
+      long dest_offset_bits, source_offset_bits;
-      int need_bitwise;
-      const gdb_byte *source_buffer;
 
       this_size_bits = p->size;
       if (bits_to_skip > 0 && bits_to_skip >= this_size_bits)
@@ -1978,24 +1971,6 @@ write_pieced_value (struct value *to, struct value *from)
       if (this_size_bits > max_offset - offset)
 	this_size_bits = max_offset - offset;
 
-      this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
-      source_offset = source_offset_bits / 8;
-      dest_offset = dest_offset_bits / 8;
-
-      /* Check whether the data can be transferred byte-wise.  */
-      if (dest_offset_bits % 8 == 0 && source_offset_bits % 8 == 0
-	  && this_size_bits % 8 == 0)
-	{
-	  source_buffer = contents + source_offset;
-	  need_bitwise = 0;
-	}
-      else
-	{
-	  buffer.reserve (this_size);
-	  source_buffer = buffer.data ();
-	  need_bitwise = 1;
-	}
-
       switch (p->location)
 	{
 	case DWARF_VALUE_REGISTER:
@@ -2047,21 +2022,44 @@ write_pieced_value (struct value *to, struct value *from)
 	  }
 	  break;
 	case DWARF_VALUE_MEMORY:
-	  if (need_bitwise)
+	  {
+	    CORE_ADDR start_addr = p->v.mem.addr + dest_offset_bits / 8;
+
+	    if (dest_offset_bits % 8 == 0 && this_size_bits % 8 == 0
+		&& source_offset_bits % 8 == 0)
+	      {
+		/* Everything is byte-aligned; no buffer needed.  */
+		write_memory (start_addr,
+			      contents + source_offset_bits / 8,
+			      this_size_bits / 8);
+		break;
+	      }
+
+	    this_size = bits_to_bytes (dest_offset_bits, this_size_bits);
+	    buffer.reserve (this_size);
+
+	    if (dest_offset_bits % 8 != 0 || this_size_bits % 8 != 0)
+	      {
+		if (this_size <= 8)
+		  {
+		    /* Perform a single read for small sizes.  */
+		    read_memory (start_addr, buffer.data (), this_size);
+		  }
+		else
 		  {
 		    /* Only the first and last bytes can possibly have any
 		       bits reused.  */
-	      read_memory (p->v.mem.addr + dest_offset, buffer.data (), 1);
+		    read_memory (start_addr, buffer.data (), 1);
-	      read_memory (p->v.mem.addr + dest_offset + this_size - 1,
+		    read_memory (start_addr + this_size - 1,
 				 &buffer[this_size - 1], 1);
-	      copy_bitwise (buffer.data (), dest_offset_bits % 8,
+		  }
-			    contents, source_offset_bits,
-			    this_size_bits,
-			    bits_big_endian);
 	      }
 
-	  write_memory (p->v.mem.addr + dest_offset,
+	    copy_bitwise (buffer.data (), dest_offset_bits % 8,
-			source_buffer, this_size);
+			  contents, source_offset_bits,
+			  this_size_bits, bits_big_endian);
+	    write_memory (start_addr, buffer.data (), this_size);
+	  }
 	  break;
 	default:
 	  mark_value_bytes_optimized_out (to, 0, TYPE_LENGTH (value_type (to)));