[newlib-cygwin] Avoid ARM SWI Seek when querying file position

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=3017f23f1cfdf31dbebdeaa32e45aca15c0b77b6

commit 3017f23f1cfdf31dbebdeaa32e45aca15c0b77b6
Author: Andy Koppe <andy.koppe@gmail.com>
Date:   Fri Aug 31 12:26:02 2018 +0100

    Avoid ARM SWI Seek when querying file position
    
    Issuing an ARM semi-hosting Seek command when just querying file
    position with SEEK_CUR and offset zero is unnecessary, because unlike
    the lseek() Unix system call the Seek command does not actually return
    the file position. For that reason, syscalls.c for ARM keeps track of
    file position in the 'poslog', so we can just return that.
    
    Moreover, since the Seek command only accepts an absolute file position,
    SEEK_CUR operations are implemented by adding the relative offset to the
    position in the poslog. If the host implements non-binary files with
    implicit carriage return characters but doesn't discount those implicit
    CRs when implementing Seek (by just mapping straight to Windows file
    operations), this actually ended up wrongly changing file position when
    using SEEK_CUR with offset zero or functions like ftell() or fgetpos()
    that are based on that.
    
    Also, use off_t rather than int for the poslog.

Diff:
---
 newlib/libc/sys/arm/syscalls.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/newlib/libc/sys/arm/syscalls.c b/newlib/libc/sys/arm/syscalls.c
index e0cf0ac..6e70467 100644
--- a/newlib/libc/sys/arm/syscalls.c
+++ b/newlib/libc/sys/arm/syscalls.c
@@ -76,7 +76,7 @@ static int monitor_stderr;
 typedef struct
 {
   int handle;
-  int pos;
+  off_t pos;
 }
 poslog;
 
@@ -234,9 +234,16 @@ _swilseek (int file, off_t ptr, int dir)
 
   if (dir == SEEK_CUR)
     {
+      off_t pos;
       if (slot == MAX_OPEN_FILES)
 	return -1;
-      ptr = openfiles[slot].pos + ptr;
+      pos = openfiles[slot].pos;
+
+      /* Avoid SWI SEEK command when just querying file position. */
+      if (ptr == 0)
+	return pos;
+
+      ptr += pos;
       dir = SEEK_SET;
     }