I checked gas version 1.38 on my Macbook and version 2.16.91.0.5 on a Linux box at by day job. Both encode xchgw %ax, %ax as 0x66 0x90 in 64-bit mode. Do you have a version that does something different?<br><br><div class="gmail_quote">
On Thu, Oct 6, 2011 at 11:41 PM, Eli Friedman <span dir="ltr"><<a href="mailto:eli.friedman@gmail.com">eli.friedman@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div><div></div><div class="h5">On Thu, Oct 6, 2011 at 11:33 PM, Craig Topper <<a href="mailto:craig.topper@gmail.com">craig.topper@gmail.com</a>> wrote:<br>
> On Thu, Oct 6, 2011 at 11:26 PM, Eli Friedman <<a href="mailto:eli.friedman@gmail.com">eli.friedman@gmail.com</a>><br>
> wrote:<br>
>><br>
>> On Thu, Oct 6, 2011 at 10:35 PM, Craig Topper <<a href="mailto:craig.topper@gmail.com">craig.topper@gmail.com</a>><br>
>> wrote:<br>
>> > Author: ctopper<br>
>> > Date: Fri Oct 7 00:35:38 2011<br>
>> > New Revision: 141353<br>
>> ><br>
>> > URL: <a href="http://llvm.org/viewvc/llvm-project?rev=141353&view=rev" target="_blank">http://llvm.org/viewvc/llvm-project?rev=141353&view=rev</a><br>
>> > Log:<br>
>> > Revert part of r141274. Only need to change encoding for xchg %eax, %eax<br>
>> > in 64-bit mode. This is because in 64-bit mode xchg %eax, %eax implies<br>
>> > zeroing the upper 32-bits of RAX which makes it not a NOP. In 32-bit mode<br>
>> > using NOP encoding is fine.<br>
>> ><br>
>> > Modified:<br>
>> > llvm/trunk/lib/Target/X86/X86InstrInfo.td<br>
>> > llvm/trunk/lib/Target/X86/X86RegisterInfo.td<br>
>> > llvm/trunk/test/MC/X86/x86-32.s<br>
>> > llvm/trunk/test/MC/X86/x86-64.s<br>
>> > llvm/trunk/utils/TableGen/X86DisassemblerTables.cpp<br>
>> ><br>
>> > Modified: llvm/trunk/lib/Target/X86/X86InstrInfo.td<br>
>> > URL:<br>
>> > <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86InstrInfo.td?rev=141353&r1=141352&r2=141353&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86InstrInfo.td?rev=141353&r1=141352&r2=141353&view=diff</a><br>
>> ><br>
>> > ==============================================================================<br>
>> > --- llvm/trunk/lib/Target/X86/X86InstrInfo.td (original)<br>
>> > +++ llvm/trunk/lib/Target/X86/X86InstrInfo.td Fri Oct 7 00:35:38 2011<br>
>> > @@ -1154,11 +1154,15 @@<br>
>> > "xchg{q}\t{$val, $src|$src, $val}", []>;<br>
>> > }<br>
>> ><br>
>> > -def XCHG16ar : I<0x90, AddRegFrm, (outs), (ins GR16_NOAX:$src),<br>
>> > +def XCHG16ar : I<0x90, AddRegFrm, (outs), (ins GR16:$src),<br>
>> > "xchg{w}\t{$src, %ax|AX, $src}", []>, OpSize;<br>
>> > -def XCHG32ar : I<0x90, AddRegFrm, (outs), (ins GR32_NOAX:$src),<br>
>> > - "xchg{l}\t{$src, %eax|EAX, $src}", []>;<br>
>> > -def XCHG64ar : RI<0x90, AddRegFrm, (outs), (ins GR64_NOAX:$src),<br>
>> > +def XCHG32ar : I<0x90, AddRegFrm, (outs), (ins GR32:$src),<br>
>> > + "xchg{l}\t{$src, %eax|EAX, $src}", []>,<br>
>> > Requires<[In32BitMode]>;<br>
>> > +// Uses GR32_NOAX in 64-bit mode to prevent encoding using the 0x90 NOP<br>
>> > encoding.<br>
>> > +// xchg %eax, %eax needs to clear upper 32-bits of RAX so is not a NOP.<br>
>> > +def XCHG32ar64 : I<0x90, AddRegFrm, (outs), (ins GR32_NOAX:$src),<br>
>> > + "xchg{l}\t{$src, %eax|EAX, $src}", []>,<br>
>> > Requires<[In64BitMode]>;<br>
>> > +def XCHG64ar : RI<0x90, AddRegFrm, (outs), (ins GR64:$src),<br>
>> > "xchg{q}\t{$src, %rax|RAX, $src}", []>;<br>
>> ><br>
>> ><br>
>> > @@ -1716,6 +1720,7 @@<br>
>> > def : InstAlias<"xchgq $mem, $val", (XCHG64rm GR64:$val, i64mem:$mem)>;<br>
>> ><br>
>> > // xchg: We accept "xchgX <reg>, %eax" and "xchgX %eax, <reg>" as<br>
>> > synonyms.<br>
>> > -def : InstAlias<"xchgw %ax, $src", (XCHG16ar GR16_NOAX:$src)>;<br>
>> > -def : InstAlias<"xchgl %eax, $src", (XCHG32ar GR32_NOAX:$src)>;<br>
>> > -def : InstAlias<"xchgq %rax, $src", (XCHG64ar GR64_NOAX:$src)>;<br>
>> > +def : InstAlias<"xchgw %ax, $src", (XCHG16ar GR16:$src)>;<br>
>> > +def : InstAlias<"xchgl %eax, $src", (XCHG32ar GR32:$src)>,<br>
>> > Requires<[In32BitMode]>;<br>
>> > +def : InstAlias<"xchgl %eax, $src", (XCHG32ar64 GR32_NOAX:$src)>,<br>
>> > Requires<[In64BitMode]>;<br>
>> > +def : InstAlias<"xchgq %rax, $src", (XCHG64ar GR64:$src)>;<br>
>> ><br>
>> > Modified: llvm/trunk/lib/Target/X86/X86RegisterInfo.td<br>
>> > URL:<br>
>> > <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86RegisterInfo.td?rev=141353&r1=141352&r2=141353&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86RegisterInfo.td?rev=141353&r1=141352&r2=141353&view=diff</a><br>
>> ><br>
>> > ==============================================================================<br>
>> > --- llvm/trunk/lib/Target/X86/X86RegisterInfo.td (original)<br>
>> > +++ llvm/trunk/lib/Target/X86/X86RegisterInfo.td Fri Oct 7 00:35:38<br>
>> > 2011<br>
>> > @@ -390,21 +390,11 @@<br>
>> > (GR32_NOREX sub_32bit)];<br>
>> > }<br>
>> ><br>
>> > -// GR16_NOAX - GR16 registers except AX.<br>
>> > -def GR16_NOAX : RegisterClass<"X86", [i16], 16, (sub GR16, AX)> {<br>
>> > - let SubRegClasses = [(GR8 sub_8bit, sub_8bit_hi)];<br>
>> > -}<br>
>> > -<br>
>> > -// GR32_NOAX - GR32 registers except EAX.<br>
>> > +// GR32_NOAX - GR32 registers except EAX. Used by AddRegFrm of XCHG32<br>
>> > in 64-bit<br>
>> > +// mode to prevent encoding using the 0x90 NOP encoding. xchg %eax,<br>
>> > %eax needs<br>
>> > +// to clear upper 32-bits of RAX so is not a NOP.<br>
>> > def GR32_NOAX : RegisterClass<"X86", [i32], 32, (sub GR32, EAX)> {<br>
>> > - let SubRegClasses = [(GR8 sub_8bit, sub_8bit_hi), (GR16_NOAX<br>
>> > sub_16bit)];<br>
>> > -}<br>
>> > -<br>
>> > -// GR64_NOAX - GR64 registers except RAX.<br>
>> > -def GR64_NOAX : RegisterClass<"X86", [i64], 64, (sub GR64, RAX)> {<br>
>> > - let SubRegClasses = [(GR8 sub_8bit, sub_8bit_hi),<br>
>> > - (GR16_NOAX sub_16bit),<br>
>> > - (GR32_NOAX sub_32bit)];<br>
>> > + let SubRegClasses = [(GR8 sub_8bit, sub_8bit_hi), (GR16 sub_16bit)];<br>
>> > }<br>
>> ><br>
>> > // GR32_NOSP - GR32 registers except ESP.<br>
>> ><br>
>> > Modified: llvm/trunk/test/MC/X86/x86-32.s<br>
>> > URL:<br>
>> > <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/X86/x86-32.s?rev=141353&r1=141352&r2=141353&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/X86/x86-32.s?rev=141353&r1=141352&r2=141353&view=diff</a><br>
>> ><br>
>> > ==============================================================================<br>
>> > --- llvm/trunk/test/MC/X86/x86-32.s (original)<br>
>> > +++ llvm/trunk/test/MC/X86/x86-32.s Fri Oct 7 00:35:38 2011<br>
>> > @@ -948,11 +948,11 @@<br>
>> ><br>
>> > // PR10345<br>
>> > // CHECK: xchgl %eax, %eax<br>
>> > -// CHECK: encoding: [0x87,0xc0]<br>
>> > +// CHECK: encoding: [0x90]<br>
>> > xchgl %eax, %eax<br>
>> ><br>
>> > // CHECK: xchgw %ax, %ax<br>
>> > -// CHECK: encoding: [0x66,0x87,0xc0]<br>
>> > +// CHECK: encoding: [0x66,0x90]<br>
>> > xchgw %ax, %ax<br>
>> ><br>
>> > // CHECK: xchgl %ecx, %eax<br>
>> ><br>
>> > Modified: llvm/trunk/test/MC/X86/x86-64.s<br>
>> > URL:<br>
>> > <a href="http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/X86/x86-64.s?rev=141353&r1=141352&r2=141353&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/X86/x86-64.s?rev=141353&r1=141352&r2=141353&view=diff</a><br>
>> ><br>
>> > ==============================================================================<br>
>> > --- llvm/trunk/test/MC/X86/x86-64.s (original)<br>
>> > +++ llvm/trunk/test/MC/X86/x86-64.s Fri Oct 7 00:35:38 2011<br>
>> > @@ -1173,7 +1173,7 @@<br>
>> ><br>
>> > // PR10345<br>
>> > // CHECK: xchgq %rax, %rax<br>
>> > -// CHECK: encoding: [0x48,0x87,0xc0]<br>
>> > +// CHECK: encoding: [0x48,0x90]<br>
>> > xchgq %rax, %rax<br>
>> ><br>
>> > // CHECK: xchgl %eax, %eax<br>
>> > @@ -1181,7 +1181,7 @@<br>
>> > xchgl %eax, %eax<br>
>> ><br>
>> > // CHECK: xchgw %ax, %ax<br>
>> > -// CHECK: encoding: [0x66,0x87,0xc0]<br>
>> > +// CHECK: encoding: [0x66,0x90]<br>
>> > xchgw %ax, %ax<br>
>><br>
>> Close, but not quite: 0x66 0x90 is a NOP, not xchgw on x86-64.<br>
>><br>
>> -Eli<br>
><br>
> I'm not sure I'm following. Are you saying it should be 0x66 0x87 0xc0 or<br>
> just that the string for the CHECK is wrong?<br>
<br>
</div></div>It should be 0x66 0x87 0xc0; you can double-check with binutils as...<br>
<font color="#888888"><br>
-Eli<br>
</font></blockquote></div><br><br clear="all"><br>-- <br>~Craig<br>