[LLVMdev] Issue describing instructions with tablegen

[Hal Finkel]

----- Original Message -----
> From: "Johnny Val" <johnnydval at gmail.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Tuesday, July 8, 2014 4:45:59 AM
> Subject: [LLVMdev] Issue describing instructions with tablegen
> 
> 
> 
> Hi,
> 
> I'm writing a backend for a fairly odd architecture. It has many
> different register banks which are are identified by a few bits in
> the instruction. These identifier bits are variable length whilst
> the instruction length stays constant. An example instruction would
> be:
> 
> ASM: mov R0, A1
> BIN: 0b0100011100 | 111 | 00 | 0101 | 1
> 
> The first "section" is the mov opcode. The next section (111) says
> that the src register bank is R and 00 identifies R0 in that bank
> (the R bank has 4 registers). Likewise, 0101 identifies register
> bank A and 1 identifies A1 (the A bank only has 2 registers, so 1
> bit is enough). I hope that explains the variable length properties
> of the instructions/identifiers.

I think that you're looking at this the wrong way: The register encoding should be the bank+id, and so you only need one mov instruction. If you have instructions that are specific to a bank, and thus don't encode the full register value, then only take the necessary bits in the encoding specification for those particular instructions.

 -Hal

> 
> There are 9 banks, using my naive approach of one instruction
> description per permutation this results in 81 different
> instructions (and 810 lines). I wrote a simple perl script to auto
> generate all these instructions, the result looks like:
> 
> def MOV_r_r : MOVrr <(outs RRegs:$dst), (ins RRegs:$src),
> [/* No pattern */]> {
> bits<2> dst;
> bits<2> src;
> ..........
> }
> 
> Is there a better way of doing this? This instruction "style" is used
> throughout almost every instruction. Ending up with a ~800 line .td
> file for every single instruction seems wrong. Did I miss some
> tablegen functionality that would be perfect for this?
> 
> If this is the best way of approaching the problem, would I need to
> have 81 if statements in my XXXXInstrInfo::copyPhysReg to identify
> which instruction to use when copying between registers?
> 
> Thanks,
> 
> Johnny
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory