[AMDGPU] Ignore unused bits in VINTERP encoding

[foad]

Summary

In the GFX11 VINTERP encoding bits 23, 59 and 60 are unused. Change the
disassembler to ignore these bits.

[foad]

We are not consistent about handling "unused" bits. I'm not even sure what to call them - in the SP3 encoding tables they are just greyed out. But I think the correct behaviour is for the assembler to set them to 0, and the disassembler to ignore them. This is what SP3 seems to do.

[dp]

LGTM.

llvm/test/MC/Disassembler/AMDGPU/gfx11_dasm_vinterp.txt

6

Maybe add a comment describing the purpose of the test?

llvm/lib/Target/AMDGPU/VINTERPInstructions.td

class VINTERPe_gfx11 <bits<7> op, VOPProfile P> : Enc64 {
   bits<9> src1;
   bits<3> src2_modifiers;
   bits<9> src2;
   bits<1> clamp;
   bits<3> waitexp;
 
   let Inst{31-26} = 0x33; // VOP3P encoding
   let Inst{25-24} = 0x1; // VINTERP sub-encoding
-  let Inst{23}    = 0; // reserved
 
   let Inst{7-0}   = vdst;
   let Inst{10-8}  = waitexp;
   let Inst{11}    = !if(P.HasOpSel, src0_modifiers{2}, 0); // op_sel(0)
   let Inst{12}    = !if(P.HasOpSel, src1_modifiers{2}, 0); // op_sel(1)
   let Inst{13}    = !if(P.HasOpSel, src2_modifiers{2}, 0); // op_sel(2)
   let Inst{14}    = !if(P.HasOpSel, src0_modifiers{3}, 0); // op_sel(3)
   let Inst{15}    = clamp;

llvm/test/MC/Disassembler/AMDGPU/gfx11_dasm_vinterp.txt

 # RUN: llvm-mc -arch=amdgcn -mcpu=gfx1100 -disassemble %s | FileCheck -strict-whitespace -check-prefix=GFX11 %s
 
 # GFX11: v_interp_p10_f32 v0, v1, v2, v3{{$}}
 0x00,0x00,0x00,0xcd,0x01,0x05,0x0e,0x04
 
+# Check that unused bits in the encoding are ignored.
+# GFX11: v_interp_p10_f32 v0, v1, v2, v3{{$}}
+0x00,0x00,0x80,0xcd,0x01,0x05,0x0e,0x1c
+
 # GFX11: v_interp_p10_f32 v1, v10, v20, v30{{$}}
 0x01,0x00,0x00,0xcd,0x0a,0x29,0x7a,0x04
 
 # GFX11: v_interp_p10_f32 v2, v11, v21, v31{{$}}
 0x02,0x00,0x00,0xcd,0x0b,0x2b,0x7e,0x04
 
 # GFX11: v_interp_p10_f32 v3, v12, v22, v32{{$}}
 0x03,0x00,0x00,0xcd,0x0c,0x2d,0x82,0x04