[binutils-gdb] Optimize byte-aligned copies in copy_bitwise()

[Andreas Arnez <arnez at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=793c128d03113816db85e8d1fa0bcd4982e246ee

commit 793c128d03113816db85e8d1fa0bcd4982e246ee
Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Date:   Thu Nov 24 17:48:04 2016 +0100

    Optimize byte-aligned copies in copy_bitwise()
    
    The function copy_bitwise used for copying DWARF pieces can potentially
    be invoked for large chunks of data.  For instance, consider a large
    struct one of whose members is currently located in a register.  In this
    case copy_bitwise would still copy the data bitwise in a loop, which is
    much slower than necessary.
    
    This change uses memcpy for the large part instead, if possible.
    
    gdb/ChangeLog:
    
    	* dwarf2loc.c (copy_bitwise): Use memcpy for the middle part, if
    	it is byte-aligned.

Diff:
---
 gdb/ChangeLog   |  5 +++++
 gdb/dwarf2loc.c | 27 +++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index d667a37..51787ad 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,4 +1,9 @@
 2016-11-24  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* dwarf2loc.c (copy_bitwise): Use memcpy for the middle part, if
+	it is byte-aligned.
+
+2016-11-24  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 	    Pedro Alves  <palves@redhat.com>
 
 	* dwarf2loc.c (bits_to_str, check_copy_bitwise)
diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 61f197b..128f654 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1548,11 +1548,30 @@ copy_bitwise (gdb_byte *dest, ULONGEST dest_offset,
     {
       size_t len = nbits / 8;
 
-      while (len--)
+      /* Use a faster method for byte-aligned copies.  */
+      if (avail == 0)
 	{
-	  buf |= *(bits_big_endian ? source-- : source++) << avail;
-	  *(bits_big_endian ? dest-- : dest++) = buf;
-	  buf >>= 8;
+	  if (bits_big_endian)
+	    {
+	      dest -= len;
+	      source -= len;
+	      memcpy (dest + 1, source + 1, len);
+	    }
+	  else
+	    {
+	      memcpy (dest, source, len);
+	      dest += len;
+	      source += len;
+	    }
+	}
+      else
+	{
+	  while (len--)
+	    {
+	      buf |= *(bits_big_endian ? source-- : source++) << avail;
+	      *(bits_big_endian ? dest-- : dest++) = buf;
+	      buf >>= 8;
+	    }
 	}
       nbits %= 8;
     }