Re: For a cheaper ‘bytevector->pointer’

[Ludovic Courtès]

Hello!

Andy Wingo <address@hidden> skribis:

> Honestly I would prefer not to do this.  If I understand correctly, the
> problem is in FFI calls -- you have a bytevector and you want to pass it
> as a pointer.  In that case the "right" optimization is to avoid the
> scm_tc7_pointer altogether and instead having an unboxed raw pointer.
> The idioms used in FFI are local enough that a compiler can do this.

I agree!  I have a patch from the 2.0 era (attached), but it doesn’t
work because all the tc3s are already taken.  I don’t think this has
changed but I could well be missing something about the tag space.
WDYT?

> More broadly -- the current FFI is an interpreter but it should be a
> compiler.  When a call happens, the code interprets the description of
> the ABI.  Instead, pointer->function should ideally *compile* a
> trampoline.  In an ideal world this compilation can happen
> ahead-of-time, when the .go file is compiled.

Yes, agreed.

> In the short term, what about allowing bytevectors as arguments
> whereever a pointer is allowed?  Perhaps it's bad to expand the domain
> of these functions but it may be the right trade-off.

So in practice, every time there’s '* in the FFI, it’d accept a
bytevector, right?

I would prefer immediate pointers if that’s possible, and then one of
the two other solutions.

Thanks!

Ludo’.

commit c705f743031b305051549928cd91e5cfdfef7ec7
Author: Ludovic Courtès <address@hidden>
Date:   Sun Jan 30 23:28:13 2011 +0100

    Attempt to support "immediate pointers".
    
    Problem is, 3 is not a valid "immediate tag", because that would prevent
    using an immediate number as the car of a pair.

diff --git a/libguile/evalext.c b/libguile/evalext.c
index ff2ff0ec0..c9dcf8b96 100644
--- a/libguile/evalext.c
+++ b/libguile/evalext.c
@@ -1,5 +1,6 @@
-/* Copyright (C) 1998,1999,2000,2001,2002,2003, 2006, 2008, 2009, 2010 Free 
Software Foundation, Inc.
- * 
+/* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2006, 2008,
+ *   2009, 2010, 2011 Free Software Foundation, Inc.
+ *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
  * as published by the Free Software Foundation; either version 3 of
@@ -72,6 +73,8 @@ SCM_DEFINE (scm_self_evaluating_p, "self-evaluating?", 1, 0, 
0,
     case scm_tc3_imm24:
        /* characters, booleans, other immediates */
       return scm_from_bool (!scm_is_null_and_not_nil (obj));
+    case scm_tc3_aligned_pointer:
+      return SCM_BOOL_T;
     case scm_tc3_cons:
       switch (SCM_TYP7 (obj))
        {
diff --git a/libguile/foreign.c b/libguile/foreign.c
index 52da23f6e..d00d4a975 100644
--- a/libguile/foreign.c
+++ b/libguile/foreign.c
@@ -60,7 +60,7 @@ SCM_SYMBOL (sym_null, "%null-pointer");
 SCM_SYMBOL (sym_null_pointer_error, "null-pointer-error");
 
 /* The cell representing the null pointer.  */
-static SCM null_pointer;
+static SCM null_pointer =  SCM_PACK (scm_tc3_aligned_pointer);
 
 #if SIZEOF_VOID_P == 4
 # define scm_to_uintptr   scm_to_uint32
@@ -139,8 +139,9 @@ scm_from_pointer (void *ptr, scm_t_pointer_finalizer 
finalizer)
 {
   SCM ret;
 
-  if (ptr == NULL && finalizer == NULL)
-    ret = null_pointer;
+  if (SCM_LIKELY (((scm_t_uintptr) ptr & 3) == 0 && finalizer == NULL))
+    /* Return an immediate pointer.  */
+    ret = SCM_PACK ((scm_t_bits) ptr | scm_tc3_aligned_pointer);
   else
     {
       ret = scm_cell (scm_tc7_pointer, (scm_t_bits) ptr);
@@ -1125,7 +1126,6 @@ scm_init_foreign (void)
 #endif
              );
 
-  null_pointer = scm_cell (scm_tc7_pointer, 0);
   scm_define (sym_null, null_pointer);
 }
 
diff --git a/libguile/foreign.h b/libguile/foreign.h
index b29001962..bf16126e5 100644
--- a/libguile/foreign.h
+++ b/libguile/foreign.h
@@ -49,12 +49,18 @@ typedef enum scm_t_foreign_type scm_t_foreign_type;
 
 typedef void (*scm_t_pointer_finalizer) (void *);
 
-#define SCM_POINTER_P(x)                                                \
-  (!SCM_IMP (x) && SCM_TYP7(x) == scm_tc7_pointer)
+#define SCM_POINTER_P(x)                       \
+  (SCM_IMP (x)                                 \
+   ? SCM_ITAG3 (x) == scm_tc3_aligned_pointer  \
+   : SCM_TYP7 (x) == scm_tc7_pointer)
+
 #define SCM_VALIDATE_POINTER(pos, x)           \
   SCM_MAKE_VALIDATE (pos, x, POINTER_P)
+
 #define SCM_POINTER_VALUE(x)                   \
-  ((void *) SCM_CELL_WORD_1 (x))
+  (SCM_IMP (x)                                 \
+   ? (void *) ((scm_t_uintptr) (x) & ~3UL)     \
+   : (void *) SCM_CELL_WORD_1 (x))
 
 SCM_API SCM scm_from_pointer (void *, scm_t_pointer_finalizer);
 
diff --git a/libguile/gc.c b/libguile/gc.c
index 91250ba57..1754f6b1d 100644
--- a/libguile/gc.c
+++ b/libguile/gc.c
@@ -1,4 +1,5 @@
-/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2002, 2003, 2006, 2008, 
2009, 2010 Free Software Foundation, Inc.
+/* Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003,
+ *   2006, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -746,8 +747,9 @@ scm_i_tag_name (scm_t_bits tag)
       return "cons (immediate car)";
     case scm_tcs_cons_nimcar:
       return "cons (non-immediate car)";
+    case scm_tc3_aligned_pointer:
     case scm_tc7_pointer:
-      return "foreign";
+      return "pointer";
     case scm_tc7_hashtable:
       return "hashtable";
     case scm_tc7_fluid:
diff --git a/libguile/goops.c b/libguile/goops.c
index c597044f5..feb61ff22 100644
--- a/libguile/goops.c
+++ b/libguile/goops.c
@@ -211,6 +211,9 @@ SCM_DEFINE (scm_class_of, "class-of", 1, 0, 0,
       else
         return scm_class_unknown;
 
+    case scm_tc3_aligned_pointer:
+      return class_foreign;
+
     case scm_tc3_cons:
       switch (SCM_TYP7 (x))
        {
diff --git a/libguile/hash.c b/libguile/hash.c
index 0dcd1c29e..7ceea43e0 100644
--- a/libguile/hash.c
+++ b/libguile/hash.c
@@ -143,6 +143,18 @@ scm_i_utf8_string_hash (const char *str, size_t len)
   return h;
 }
 
+static unsigned long
+pointer_hash (SCM obj)
+{
+  /* Pointer objects are typically used to store addresses of heap
+     objects.  On most platforms, these are at least 3-byte
+     aligned (on x86_64-*-gnu, `malloc' returns 4-byte aligned
+     addresses), so get rid of the least significant bits.  */
+  scm_t_uintptr significant_bits;
+
+  significant_bits = (scm_t_uintptr) SCM_POINTER_VALUE (obj) >> 4UL;
+  return (size_t) significant_bits;
+}
 
 /* Dirk:FIXME:: why downcase for characters? (2x: scm_hasher, scm_ihashv) */
 /* Dirk:FIXME:: scm_hasher could be made static. */
@@ -155,6 +167,8 @@ scm_hasher(SCM obj, unsigned long n, size_t d)
   case scm_tc3_int_1: 
   case scm_tc3_int_2:
     return SCM_I_INUM(obj) % n;   /* SCM_INUMP(obj) */
+  case scm_tc3_aligned_pointer:
+    return pointer_hash (obj) % n;
   case scm_tc3_imm24:
     if (SCM_CHARP(obj))
       return (unsigned)(scm_c_downcase(SCM_CHAR(obj))) % n;
@@ -214,16 +228,7 @@ scm_hasher(SCM obj, unsigned long n, size_t d)
     case scm_tc7_symbol:
       return scm_i_symbol_hash (obj) % n;
     case scm_tc7_pointer:
-      {
-       /* Pointer objects are typically used to store addresses of heap
-          objects.  On most platforms, these are at least 3-byte
-          aligned (on x86_64-*-gnu, `malloc' returns 4-byte aligned
-          addresses), so get rid of the least significant bits.  */
-       scm_t_uintptr significant_bits;
-
-       significant_bits = (scm_t_uintptr) SCM_POINTER_VALUE (obj) >> 4UL;
-       return (size_t) significant_bits  % n;
-      }
+      return pointer_hash (obj) % n;
     case scm_tc7_wvect:
     case scm_tc7_vector:
       {
diff --git a/libguile/print.c b/libguile/print.c
index 679327a8a..f5af19131 100644
--- a/libguile/print.c
+++ b/libguile/print.c
@@ -499,6 +499,9 @@ iprin1 (SCM exp, SCM port, scm_print_state *pstate)
          scm_ipruk ("immediate", exp, port);
        }
       break;
+    case scm_tc3_aligned_pointer:
+      scm_i_pointer_print (exp, port, pstate);
+      break;
     case scm_tc3_cons:
       switch (SCM_TYP7 (exp))
        {
diff --git a/libguile/tags.h b/libguile/tags.h
index 9e0e3059d..913064d9f 100644
--- a/libguile/tags.h
+++ b/libguile/tags.h
@@ -3,8 +3,8 @@
 #ifndef SCM_TAGS_H
 #define SCM_TAGS_H
 
-/* Copyright (C) 
1995,1996,1997,1998,1999,2000,2001,2002,2003,2004,2008,2009,2010
- * Free Software Foundation, Inc.
+/* Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002,
+ *   2003, 2004, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -386,7 +386,7 @@ typedef scm_t_uintptr scm_t_bits;
 #define scm_tc3_cons            0
 #define scm_tc3_struct          1
 #define scm_tc3_int_1           (scm_tc2_int + 0)
-#define scm_tc3_unused          3
+#define scm_tc3_aligned_pointer  3
 #define scm_tc3_imm24           4
 #define scm_tc3_tc7_1           5
 #define scm_tc3_int_2           (scm_tc2_int + 4)