[LLVMdev] X86InstrFormats.td Question

[Nate Begeman]

On Mar 23, 2009, at 12:57 PM, David A. Greene wrote:

> I'm looking at the instruction formats and I can't grok the  
> comments.  For
> example:
>
> // SSSE3 Instruction Templates:
> //
> //   SS38I - SSSE3 instructions with T8 prefix.
> //   SS3AI - SSSE3 instructions with TA prefix.
> //
>
> Where are these prefix names coming from?  I can't find any mention  
> of them in
> the Intel literature.

They come from the fact that the SSSE3 instructions begin with 0F 38  
or 0F 3A.

>
> Also, there's this curious table:
>
> // Prefix byte classes which are used to indicate to the ad-hoc  
> machine code
> // emitter that various prefix bytes are required.
> class OpSize { bit hasOpSizePrefix = 1; }
> class AdSize { bit hasAdSizePrefix = 1; }
> class REX_W  { bit hasREX_WPrefix = 1; }
> class LOCK   { bit hasLockPrefix = 1; }
> class TB     { bits<4> Prefix = 1; }
> class REP    { bits<4> Prefix = 2; }
> class D8     { bits<4> Prefix = 3; }
> class D9     { bits<4> Prefix = 4; }
> class DA     { bits<4> Prefix = 5; }
> class DB     { bits<4> Prefix = 6; }
> class DC     { bits<4> Prefix = 7; }
> class DD     { bits<4> Prefix = 8; }
> class DE     { bits<4> Prefix = 9; }
> class DF     { bits<4> Prefix = 10; }
> class XD     { bits<4> Prefix = 11; }
> class XS     { bits<4> Prefix = 12; }
> class T8     { bits<4> Prefix = 13; }
> class TA     { bits<4> Prefix = 14; }
>
> Are these Prefix values special at all or do they just serve as unique
> identifiers within LLVM?

They are just unique identifiers in LLVM.