From: Timo Kreuzer <timo.kreuzer@reactos.org>
Date: Sat, 26 Dec 2015 20:34:15 +0000 (+0000)
Subject: [CRT]
X-Git-Tag: ReactOS-0.4.0~19^2~5
X-Git-Url: https://git.reactos.org/?p=reactos.git;a=commitdiff_plain;h=654e3f77437a9027838cfc35ff0ff3764ac4a9a7

[CRT]
Fix x86 asm implementation of strlen / wcslen.

svn path=/trunk/; revision=70430
---

diff --git a/reactos/lib/sdk/crt/string/i386/tcslen.inc b/reactos/lib/sdk/crt/string/i386/tcslen.inc
index b5256d586c9..1f3842f5c33 100644
--- a/reactos/lib/sdk/crt/string/i386/tcslen.inc
+++ b/reactos/lib/sdk/crt/string/i386/tcslen.inc
@@ -7,23 +7,34 @@ PUBLIC _tcslen
 
 FUNC _tcslen
     FPO 0, 1, 1, 1, 0, FRAME_FPO
+
+    /* Save edi and eflags (according to the x86 ABI, we don't need to do that
+       but since the native function doesn't change the direction flag, we don't
+       either */
     push edi
-    mov edi, [esp + 8]
+    pushfd
+
+    /* Load the string pointer into edi */
+    mov edi, [esp + 12]
+
+    /* Set eax to 0, since we want to compare with 0 */
     xor eax, eax
-    test edi, edi
-    jz _tcslen_end
 
+    /* Set ecx to -1 (i.e. 0xFFFFFFFF) */
     mov ecx, -1
+
+    /* Clear direction flag */
     cld
 
+    /* Now compare the characters until a 0 is found */
     repne _tscas
 
+    /* Calculate the count (eax = -ecx - 1) */
     not ecx
-    dec ecx
-
-    mov eax, ecx
+    lea eax, [ecx-1]
 
-_tcslen_end:
+    /* Restore eflags/edi and return the result */
+    popfd
     pop edi
     ret
 ENDFUNC