Date: Tue, 15 Aug 2006 21:34:46 +0800 From: David Xu <davidxu@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: cvs-src@freebsd.org, src-committers@freebsd.org, cvs-all@freebsd.org Subject: Re: cvs commit: src/sys/amd64/amd64 support.S Message-ID: <200608152134.46359.davidxu@freebsd.org> In-Reply-To: <200608150913.52419.jhb@freebsd.org> References: <200608151245.k7FCjpJo077372@repoman.freebsd.org> <200608150913.52419.jhb@freebsd.org>
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On Tuesday 15 August 2006 21:13, John Baldwin wrote:
> On Tuesday 15 August 2006 08:45, David Xu wrote:
> > davidxu 2006-08-15 12:45:51 UTC
> >
> > FreeBSD src repository
> >
> > Modified files:
> > sys/amd64/amd64 support.S
> > Log:
> > Because fuword on AMD64 returns 64bit long integer -1 on fault, clear
> > entire %rax to zero instead of only clearing %eax, otherwise it will
> > leave garbage data in upper 32 bits.
>
> Are you sure that 'xorl %eax,%eax' doesn't actually clear all 64 bits?=20
> This practice of just using xorl rather than xorq is all over the place in
> the amd64 code, and I think I've even seen gcc generate it, so I'm guessi=
ng
> that the xorl actually is a xorq.
=46rom my understanding, they are different.
before my change, generated binary code:
0000000000003ba0 <fusufault>:
3ba0: 65 48 8b 0c 25 20 00 mov %gs:0x20,%rcx
3ba7: 00 00
3ba9: 31 c0 xor %eax,%eax
3bab: 48 89 81 a8 02 00 00 mov %rax,0x2a8(%rcx)
3bb2: 48 ff c8 dec %rax
3bb5: c3 retq
3bb6: 66 data16
3bb7: 66 data16
3bb8: 66 data16
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
after this change:
0000000000003ba0 <fusufault>:
3ba0: 65 48 8b 0c 25 20 00 mov %gs:0x20,%rcx
3ba7: 00 00
3ba9: 48 31 c0 xor %rax,%rax
3bac: 48 89 81 a8 02 00 00 mov %rax,0x2a8(%rcx)
3bb3: 48 ff c8 dec %rax
3bb6: c3 retq
I have only checked fuword while I am working on userland mutex
priority propagating, I have not checked suword and others yet.
David Xu
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