master: read-byte transform: refill the buffer instead of just calling n-bin

The branch "master" has been updated in SBCL:
       via  4727a49892c11ebe996753f4a6e420666ac9c065 (commit)
      from  ae55fe64435d4015b8d640532196885b3323d4a7 (commit)

- Log -----------------------------------------------------------------
commit 4727a49892c11ebe996753f4a6e420666ac9c065
Author: Stas Boukarev <[email protected]>
Date:   Fri Aug 7 22:40:04 2026 +0300

    read-byte transform: refill the buffer instead of just calling n-bin
    
    Which does not touch the buffer.
---
 src/compiler/fndb.lisp    |  3 +++
 src/compiler/srctran.lisp | 25 ++++++++++++++-----------
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/src/compiler/fndb.lisp b/src/compiler/fndb.lisp
index 389e6a820..95f88562b 100644
--- a/src/compiler/fndb.lisp
+++ b/src/compiler/fndb.lisp
@@ -1789,6 +1789,9 @@
 (defknown read-byte (stream &optional t t) t ()
   :derive-type (read-elt-type-deriver nil 'integer nil))
 
+(defknown fast-read-byte-refill (stream t t) t ()
+  :derive-type (read-elt-type-deriver nil '(unsigned-byte 8) nil))
+
 (defknown (prin1 print princ) (t &optional stream-designator)
   t
   (any)
diff --git a/src/compiler/srctran.lisp b/src/compiler/srctran.lisp
index 9eeef6208..9c9416ae6 100644
--- a/src/compiler/srctran.lisp
+++ b/src/compiler/srctran.lisp
@@ -9480,20 +9480,23 @@
 (deftransform read-byte ((stream &optional eof-error-p eof-value))
   (delete-lvar-cast-if (specifier-type 'stream) stream)
   `(if (sb-impl::ansi-stream-p stream)
-       (let ((index (ansi-stream-in-index stream)))
-         ;; ANSI stream with no in-buffer must have IN-INDEX = +ANSI-STREAM-IN-BUFFER-LENGTH+
-         (if (/= index sb-impl::+ansi-stream-in-buffer-length+)
-             (prog1 (aref (truly-the vector (ansi-stream-in-buffer stream)) index)
-               (setf (ansi-stream-in-index stream) (1+ index)))
+       (let ((index (ansi-stream-in-index stream))
+             (buffer (ansi-stream-in-buffer stream)))
+         (if buffer
+             (if (/= index sb-impl::+ansi-stream-in-buffer-length+)
+                 (prog1 (aref (truly-the vector (ansi-stream-in-buffer stream)) index)
+                   (setf (ansi-stream-in-index stream) (1+ index)))
+                 (fast-read-byte-refill stream ,(if eof-error-p 'eof-error-p t)
+                                        eof-value))
              (funcall (ansi-stream-bin stream) stream ,(if eof-error-p 'eof-error-p t)
                       eof-value)))
        (locally (declare (notinline read-byte))
-           (read-byte stream
-                      ,@(loop for (lvar var) on (list eof-error-p 'eof-error-p
-                                                      eof-value 'eof-value)
-                              by #'cddr
-                              when lvar
-                              collect var)))))
+         (read-byte stream
+                    ,@(loop for (lvar var) on (list eof-error-p 'eof-error-p
+                                                    eof-value 'eof-value)
+                            by #'cddr
+                            when lvar
+                            collect var)))))
 
 (deftransform peek-char ((&optional peek-type stream eof-error-p eof-value recursive-p)
                          (null &rest t))

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