[x86 setup] Volatilize asm() statements
asm() statements need to be volatile when:
a. They have side effects (other than value returned).
b. When the value returned can vary over time.
c. When they have ordering constraints that cannot be expressed to gcc.
In particular, the keyboard and timer reads were violating constraint (b),
which resulted in the keyboard/timeout poll getting
loop-invariant-removed when compiling with gcc 4.2.0.
Thanks to an anonymous bug reporter for pointing this out.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/i386/boot/tty.c b/arch/i386/boot/tty.c
index 9c668aa..f3f14bd 100644
--- a/arch/i386/boot/tty.c
+++ b/arch/i386/boot/tty.c
@@ -54,9 +54,9 @@
u16 ax = 0x0200;
u16 cx, dx;
- asm("int $0x1a"
- : "+a" (ax), "=c" (cx), "=d" (dx)
- : : "ebx", "esi", "edi");
+ asm volatile("int $0x1a"
+ : "+a" (ax), "=c" (cx), "=d" (dx)
+ : : "ebx", "esi", "edi");
return dx >> 8;
}
@@ -67,7 +67,7 @@
int getchar(void)
{
u16 ax = 0;
- asm("int $0x16" : "+a" (ax));
+ asm volatile("int $0x16" : "+a" (ax));
return ax & 0xff;
}
@@ -75,9 +75,9 @@
static int kbd_pending(void)
{
u8 pending;
- asm("int $0x16; setnz %0"
- : "=rm" (pending)
- : "a" (0x0100));
+ asm volatile("int $0x16; setnz %0"
+ : "=rm" (pending)
+ : "a" (0x0100));
return pending;
}