Re: inline build_string performance

[Dmitry Antipov <dmantipov@yandex.ru>]

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On 06/26/2012 08:42 PM, Paul Eggert wrote:

> That sounds worthwhile for critical paths.
>
> How about reverting the build_string change, and defining a new
> inline function build_literal intended for when the argument is a
> string literal and for when speed is more important than conserving
> code space?  That would give us speed where speed matters and where
> we know it'll be faster, while avoiding code bloat otherwise.
> build_string and  build_literal would have identical semantics,
> but different performance properties.

This may be implemented without reverting previous stuff. It will also gives
other (non-GCC) compilers a chance to demonstrate their optimization skills.

Dmitry




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=== modified file 'src/alloc.c'
--- src/alloc.c	2012-06-26 14:41:01 +0000
+++ src/alloc.c	2012-06-26 17:26:02 +0000
@@ -2533,6 +2533,25 @@
   return string;
 }
 
+/* Fast version used when both STR and NBYTES are compile-time
+   constants, and all characters from STR are ASCII.  */
+
+Lisp_Object
+build_literal (const char *str, ptrdiff_t nbytes)
+{
+  Lisp_Object string;
+  struct Lisp_String *s;
+
+  eassert (nbytes > 0);
+
+  s = allocate_string ();
+  allocate_string_data (s, nbytes, nbytes);
+  memcpy (s->data, str, nbytes);
+  string_chars_consed += nbytes;
+
+  XSETSTRING (string, s);
+  return string;
+}
 
 \f
 /***********************************************************************

=== modified file 'src/lisp.h'
--- src/lisp.h	2012-06-26 05:00:30 +0000
+++ src/lisp.h	2012-06-26 17:24:48 +0000
@@ -2716,12 +2716,28 @@
 /* Make a string from the data at STR, treating it as multibyte if the
    data warrants.  */
 
+#ifdef __GNUC__
+
+extern Lisp_Object build_literal (const char *str, ptrdiff_t nbytes);
+
+#define build_string(str)				\
+  ({ Lisp_Object __val;					\
+    if (__builtin_constant_p (str) && strlen (str) > 0)	\
+      __val = build_literal (str, strlen (str));	\
+    else						\
+      __val = make_string (str, strlen (str));		\
+    __val; })
+
+#else
+
 static inline Lisp_Object
 build_string (const char *str)
 {
   return make_string (str, strlen (str));
 }
 
+#endif
+
 extern Lisp_Object pure_cons (Lisp_Object, Lisp_Object);
 EXFUN (Fgarbage_collect, 0);
 extern void make_byte_code (struct Lisp_Vector *);