gptmbr: use cltq to extend a zero %eax into %edx

Using cltq (cdq) to clear %edx when %eax is zero is cheaper by one
byte.

diff --git a/mbr/gptmbr.S b/mbr/gptmbr.S
index 9005183..85b9fcd 100644 (file)
--- a/mbr/gptmbr.S
+++ b/mbr/gptmbr.S
@@ -112,7 +112,7 @@ next:
 
        /* Load partition table header */
        xorl    %eax,%eax
-       xorl    %edx,%edx
+       cltd
        incw    %ax             /* %edx:%eax = 1 */
        movw    $phdr, %bx
        pushw   %bx             /* -8(%bp) phdr == bootsect */