1 files changed, 29 insertions, 22 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 2a53dbbdd07..075b9c590c8 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -3906,7 +3906,8 @@ union read_non_regular
   struct
   {
     emacs_fd fd;
-    ptrdiff_t inserted, trytry;
+    char *buf;
+    ptrdiff_t bufsize;
   } s;
   GCALIGNED_UNION_MEMBER
 };
@@ -3916,11 +3917,7 @@ static Lisp_Object
 read_non_regular (Lisp_Object state)
 {
   union read_non_regular *data = XFIXNUMPTR (state);
-  intmax_t nbytes
-    = emacs_fd_read (data->s.fd,
-                     ((char *) BEG_ADDR + PT_BYTE - BEG_BYTE
-                      + data->s.inserted),
-                     data->s.trytry);
+  intmax_t nbytes = emacs_fd_read (data->s.fd, data->s.buf, data->s.bufsize);
   return make_int (nbytes < 0 ? -errno : nbytes);
 }
 
@@ -4854,18 +4851,18 @@ by calling `format-decode', which see.  */)
 
     while (inserted < total)
       {
-        ptrdiff_t this;
-
-        if (gap_size == 0)
+        char *buf;
+        ptrdiff_t bufsize, this;
+        if (gap_size < total - inserted && gap_size < sizeof read_buf)
           {
-            /* The size estimate was wrong.  Make the gap 50% larger.  */
-            make_gap (GAP_SIZE >> 1);
-            gap_size = GAP_SIZE - inserted;
+            buf = read_buf;
+            bufsize = sizeof read_buf;
+          }
+        else
+          {
+            buf = (char *) BEG_ADDR + PT_BYTE - BEG_BYTE + inserted;
+            bufsize = min (min (gap_size, total - inserted), READ_BUF_SIZE);
           }
-
-        /* 'try' is reserved in some compilers (Microsoft C).  */
-        ptrdiff_t trytry = min (gap_size,
-                                min (total - inserted, READ_BUF_SIZE));
 
         if (!seekable && end_offset == TYPE_MAXIMUM (off_t))
           {
@@ -4875,7 +4872,7 @@ by calling `format-decode', which see.  */)
             /* Read from the file, capturing `quit'.  When an
                error occurs, end the loop, and arrange for a quit
                to be signaled after decoding the text we read.  */
-            union read_non_regular data = {{fd, inserted, trytry}};
+            union read_non_regular data = {{fd, buf, bufsize}};
             nbytes = internal_condition_case_1
               (read_non_regular, make_pointer_integer (&data),
                Qerror, read_non_regular_quit);
@@ -4894,11 +4891,7 @@ by calling `format-decode', which see.  */)
              part of the buffer until all the reading is done, so a
              C-g here doesn't do any harm.  */
           {
-            this = emacs_fd_read (fd,
-                                  ((char *) BEG_ADDR + PT_BYTE - BEG_BYTE
-                                   + inserted),
-                                  trytry);
-            if (this < 0)
+            this = emacs_fd_read (fd, buf, bufsize);                                        if (this < 0)
               this = -errno;
           }
 
@@ -4908,6 +4901,20 @@ by calling `format-decode', which see.  */)
             break;
           }
 
+        if (buf == read_buf)
+          {
+            if (gap_size < this)
+              {
+                /* Size estimate was low.  Make the gap at least 50% larger,
+                   and big enough so that the next loop iteration will
+                   use the gap directly instead of copying via read_buf.  */
+                make_gap (max (GAP_SIZE >> 1,
+                               this - gap_size + sizeof read_buf));
+                gap_size = GAP_SIZE - inserted;
+              }
+            memcpy (BEG_ADDR + PT_BYTE - BEG_BYTE + inserted,
+                    read_buf, this);
+          }
         gap_size -= this;
         inserted += this;
       }