From mboxrd@z Thu Jan  1 00:00:00 1970
From: erik quanstrom <quanstro@quanstro.net>
Date: Mon, 21 Nov 2011 10:08:18 -0500
To: 9fans@9fans.net
Message-ID: <93653898c4ceb27355b5fa1e548176aa@brasstown.quanstro.net>
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
Subject: [9fans] sysrfork fp bug?
Topicbox-Message-UUID: 421cb034-ead7-11e9-9d60-3106f5b1d025

sysrfork() does *not* do a procsave before forking.  thus the
floating point registers in the new process are just going to
be a copy of whatever was last saved, and perhaps nothing.

on my atom system (but not some others), this attached
program does fault.  i wrote a bit of it in assembler to control
what was in F0.  the c compiler wasn't keeping things in a
register.

have i missed something or do we need a procsave() in fork?

- erik

---
sed 's/.//' >fp.c <<'//GO.SYSIN DD fp.c'
-#include <u.h>
-#include <libc.h>
-
-char *theformat = "%g\n";
-
-void
-main(void)
-{
-	extern void dofp(void);
-
-	dofp();
-}
//GO.SYSIN DD fp.c
echo myfp.s
sed 's/.//' >myfp.s <<'//GO.SYSIN DD myfp.s'
-TEXT	dofp(SB), $0
-	SUBL		$12, SP
-	FLD1
-	CALL		fork(SB)
-
-	MOVL		$2000, AX
-	MOVL		AX, 0x0(SP)
-	CALL		sleep(SB)
-
-	MOVL		theformat(SB), AX
-	MOVL		AX, 0x0(SP)
-	FMOVDP	F0,0x4(SP)
-	CALL		print(SB)
-	ADDL		$12, SP
-	RET
//GO.SYSIN DD myfp.s