linux-kernel - [PATCH 2/3] tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax`

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Date:   Sun, 24 Oct 2021 19:43:22 +0200
From:   Willy Tarreau <w@....eu>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/3] tools/nolibc: x86-64: Use `mov $60,%eax` instead of `mov $60,%rax`

From: Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>

Note that mov to 32-bit register will zero extend to 64-bit register.
Thus `mov $60,%eax` has the same effect with `mov $60,%rax`. Use the
shorter opcode to achieve the same thing.
```
  b8 3c 00 00 00       	mov    $60,%eax (5 bytes) [1]
  48 c7 c0 3c 00 00 00 	mov    $60,%rax (7 bytes) [2]
```
Currently, we use [2]. Change it to [1] for shorter code.

Signed-off-by: Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>
Signed-off-by: Willy Tarreau <w@....eu>
---
 tools/include/nolibc/nolibc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index f9afe89ec6f2..4988866af0b5 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -420,7 +420,7 @@ asm(".section .text\n"
     "and $-16, %rsp\n"          // x86 ABI : esp must be 16-byte aligned before call
     "call main\n"               // main() returns the status code, we'll exit with it.
     "mov %eax, %edi\n"          // retrieve exit code (32 bit)
-    "mov $60, %rax\n"           // NR_exit == 60
+    "mov $60, %eax\n"           // NR_exit == 60
     "syscall\n"                 // really exit
     "hlt\n"                     // ensure it does not return
     "");
-- 
2.17.5