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From: Ludovic Courtès
Subject: The mysterious ‘SCM_F_BYTEVECTOR_CONTIGUOUS’
Date: Wed, 11 Jan 2023 18:39:52 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux)

Ludovic Courtès <> skribis:

> This is an updated version of the ‘bytevector-slice’ primitive I used in
> the linker/assembler patch series¹ that I think is ready to go.

While working on this, I noticed I might have to pay attention to
‘SCM_F_BYTEVECTOR_CONTIGUOUS’, as noted in the patch.

But it turns out that flag isn’t really used.  I found two places that
should add it and do not:

diff --git a/libguile/bytevectors.c b/libguile/bytevectors.c
index 6b920c88a..fd7fdad0b 100644
--- a/libguile/bytevectors.c
+++ b/libguile/bytevectors.c
@@ -274,7 +274,8 @@ make_bytevector_from_buffer (size_t len, void *contents,
       c_len = len * (scm_i_array_element_type_sizes[element_type] / 8);
-      SCM_SET_BYTEVECTOR_FLAGS (ret, element_type);
+                                element_type | SCM_F_BYTEVECTOR_CONTIGUOUS);
       SCM_BYTEVECTOR_SET_LENGTH (ret, c_len);
       SCM_BYTEVECTOR_SET_CONTENTS (ret, contents);
diff --git a/module/system/repl/debug.scm b/module/system/repl/debug.scm
diff --git a/module/system/vm/assembler.scm b/module/system/vm/assembler.scm
index 165976363..61e0460ff 100644
--- a/module/system/vm/assembler.scm
+++ b/module/system/vm/assembler.scm
@@ -1857,8 +1857,9 @@ should be .data or .rodata), and return the resulting 
linker object.
   (define tc7-program #x45)
   (define tc7-bytevector #x4d)
-  ;; This flag is intended to be left-shifted by 7 bits.
+  ;; These flags are intended to be left-shifted by 7 bits.
   (define bytevector-immutable-flag #x200)
+  (define bytevector-contiguous-flag #x100)
   (define tc7-array #x5d)
@@ -2026,6 +2027,7 @@ should be .data or .rodata), and return the resulting 
linker object.
                                ;; Bytevector immutable flag also shifted
                                ;; left.
                                (ash (logior bytevector-immutable-flag
+                                            bytevector-contiguous-flag
                                             (array-type-code obj))
           (case word-size
There are probably more.

Fundamentally, I’m not sure what this flag is supposed to mean.  AFAICS,
there’s no way to create a non-contiguous bytevector (or SRFI-4 vector).

This flag was added in 7ed54fd36d2e381aa46ef8a7d2fc13a6776b573a.  My
guess is that it was part of plan that wasn’t carried out in the end.

Andy, thoughts?  :-)


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