Home Original page

[discuss] small challenge for instruction selection

H. J. Lu hjl@lucon.org
Thu Jun 16 13:26:00 GMT 2005 
On Thu, Jun 16, 2005 at 01:06:13AM -0600, Jan Beulich wrote:
> >>> "H. J. Lu" <hjl@lucon.org> 16.06.05 03:11:08 >>>
> On Wed, Jun 15, 2005 at 05:29:07PM -0700, Siddha, Suresh B wrote:
> > On Tue, Jun 14, 2005 at 11:15:33PM +0200, Andi Kleen wrote:
> > > > movl $0x80706050,0x40302010(%rdi)
> > > > ret $0xb0a0
> > > > 
> > > > Is 3 bytes overhead with 8+2 bytes contiguous.
> > > 
> > > Nice. Thanks Zachary. Any other calls? :)
> > 
> > with 2 bytes overhead.
> > 
> > 	// mov    %eax,0x8070605040302010
> >         __asm__ __volatile__ ( ".byte 0xa3; .quad  0x8070605040302010");
> >         __asm__ __volatile__ ( "ret $0xa090");
> > 
> > Assembler is not generating the intended code when I use the mnemonic form
> > for the first asm stmt. Disassembly is fine though.
> > 
> >
> >I opened a bug:
> >
> >http://sources.redhat.com/bugzilla/show_bug.cgi?id=1013 
> 
> I don't think that's a bug: include/opcodes/i386.h explicitly disallows this mov form in 64-bit mode; movabs is to be used here instead. This is because for symbolics you'd have an ambiguity resulting otherwise in that you could encode this mov with either 64-bit displacement or sign-extended 32-bit one, with no way for the programmer to indicate which one to choose. Thus you've got to use movabs here to make clear you want a 64-bit disp, and use mov when you rather (and that's very reasonably the default) want a 32-bit one.
> 

If you look at i386.h closely, there are

/* In the 64bit mode the short form mov immediate is redefined to have
   64bit displacement value.  */
{ "mov",   2,   0xa0, X, CpuNo64,bwl_Suf|D|W,                   { Disp16|Disp32, Acc, 0 } },
{ "mov",   2,   0x88, X, 0,      bwlq_Suf|D|W|Modrm,            { Reg, Reg|AnyMem, 0} },
/* In the 64bit mode the short form mov immediate is redefined to have
   64bit displacement value.  */
{ "mov",   2,   0xb0, X, 0,      bwl_Suf|W|ShortForm,           { EncImm, Reg8|Reg16|Reg32, 0 } },
{ "mov",   2,   0xc6, 0, 0,      bwlq_Suf|W|Modrm,              { EncImm, Reg|AnyMem, 0 } },
{ "mov",   2,   0xb0, X, Cpu64,  q_Suf|W|ShortForm,             { Imm64, Reg64, 0 } },
...
{ "movabs",2,   0xa0, X, Cpu64, bwlq_Suf|D|W,                   { Disp64, Acc, 0 } },
{ "movabs",2,   0xb0, X, Cpu64, q_Suf|W|ShortForm,              { Imm64, Reg64, 0 } },

I think there is an oversight. We have

{ "mov",   2,   0xb0, X, Cpu64,  q_Suf|W|ShortForm,             { Imm64, Reg64, 0 } },
...
{ "movabs",2,   0xb0, X, Cpu64, q_Suf|W|ShortForm,              { Imm64, Reg64, 0 } },

But we just missed 

{ "mov",2,   0xa0, X, Cpu64, bwlq_Suf|D|W,                   { Disp64, Acc, 0 } },

I will see what I can do.


H.J.
More information about the Binutils mailing list