Date: Wed, 29 Mar 2000 21:09:45 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: Gary Jennejohn <garyj@muc.de>, freebsd-current@FreeBSD.ORG Subject: Re: Very weird assembly failure (was Re: UP kernel performance and Matt Dillon's patches) Message-ID: <Pine.BSF.4.21.0003292043500.1764-100000@alphplex.bde.org> In-Reply-To: <200003290348.TAA56657@apollo.backplane.com>
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On Tue, 28 Mar 2000, Matthew Dillon wrote:
> I found a couple of minor nits, but only one real bug. In i386/swtch.s
> I forgot to change out a WANT_RESCHED for AST_RESCHED:
>
> sw1a:
> call _chooseproc /* trash ecx, edx, ret eax*/
> testl %eax,%eax
> CROSSJUMP(je, _idle, jne) /* if no proc, idle */
> movl %eax,%ecx
>
> xorl %eax,%eax
> andl $~WANT_RESCHED,_astpending
>
> The problem is that a kernel build is not reporting any errors!
> WANT_RESCHED does not exist at all, anywhere. If I change it to
> a garbage name the kernel still builds. I don't get it.
It seems to be a gas bug. The error is detected if ~WANT_RESCHED is
replaced by WANT_RESCHED. ~WANT_RESCHED is no a simple relocatable
expression, so it isn't clear that gas or elf can handle it. They
don't seem to for the following simpler case:
$ echo "movl $~FOO,%eax" >z.s
$ echo ".globl foo; .set FOO,0x55555555" >z1.s
$ cc -c z.s z1.s
$ ld -o z z.o z1.s
$ objdump --disassemble z
z: file format elf32-i386
Disassembly of section .text:
08048074 <.text>:
8048074: b8 ff ff ff ff movl $0xffffffff,%eax
^^^^^^^^ should be aaaaaaaa
but still has best guess
at time of assembly of z.s
8048079: 90 nop
804807a: 90 nop
804807b: 90 nop
Everthing works right for "FOO" instead of ~FOO.
The aout case gets this wrong in a more obvious way. Gas produces the same
code for "movl $~FOO,%eax" as for "movl $FOO,%eax". Linking to z1.o then
gives the right value for $FOO and the wrong value for $~FOO.
Gas notices the problem for "movl $-FOO,%eax":
z.s: Assembler messages:
z.s:1: Error: Negative of non-absolute symbol FOO
Similarly for the a.out case. Complementation is equivalent to negation
on 2's complement machines, so gas should produce this error for $~FOO too.
Bruce
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