[AMDGPU][MC][GFX10] Corrected constant bus checks to exclude null

[dp]

Summary

Currently null operand is incorrectly counted against 2 scalar value limit. null should be free to use.

AMD SWDEV-157286

[artem.tamazov]

Interesting question. Does null SGPR reads occupy constant bus?

[artem.tamazov]

In D65229#1652664, @artem.tamazov wrote:

Interesting question. Does null SGPR reads occupy constant bus?

I see. “Null” is an inline.

[artem.tamazov]

LGTM

llvm/trunk/lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp

unsigned AMDGPUAsmParser::findImplicitSGPRReadInVOP(const MCInst &Inst) const {
   for (unsigned i = 0; i < Num; ++i) {
     unsigned Reg = Desc.ImplicitUses[i];
     switch (Reg) {
     case AMDGPU::FLAT_SCR:
     case AMDGPU::VCC:
     case AMDGPU::VCC_LO:
     case AMDGPU::VCC_HI:
     case AMDGPU::M0:
-    case AMDGPU::SGPR_NULL:
       return Reg;
     default:
       break;
     }
   }
   return AMDGPU::NoRegister;
 }
 
default:
     return 2;
   }
 }
 
 bool AMDGPUAsmParser::usesConstantBus(const MCInst &Inst, unsigned OpIdx) {
   const MCOperand &MO = Inst.getOperand(OpIdx);
   if (MO.isImm()) {
     return !isInlineConstant(Inst, OpIdx);
+  } else if (MO.isReg()) {
+    auto Reg = MO.getReg();
+    const MCRegisterInfo *TRI = getContext().getRegisterInfo();
+    return isSGPR(mc2PseudoReg(Reg), TRI) && Reg != SGPR_NULL;
+  } else {
+    return true;
   }
-  return !MO.isReg() ||
-         isSGPR(mc2PseudoReg(MO.getReg()), getContext().getRegisterInfo());
 }
 
 bool AMDGPUAsmParser::validateConstantBusLimitations(const MCInst &Inst) {
   const unsigned Opcode = Inst.getOpcode();
   const MCInstrDesc &Desc = MII.get(Opcode);
   unsigned ConstantBusUseCount = 0;
   unsigned NumLiterals = 0;
   unsigned LiteralSize;

llvm/trunk/test/MC/AMDGPU/gfx10-constant-bus.s

 v_lshlrev_b64 v[5:6], 0x3f717273, 0x3f717273
 // GFX10-ERR: error: invalid operand (violates constant bus restrictions)
 
 v_mad_u64_u32 v[5:6], s12, v1, 0x12345678, 0x12345678
 // GFX10: v_mad_u64_u32 v[5:6], s12, v1, 0x12345678, 0x12345678 ; encoding: [0x05,0x0c,0x76,0xd5,0x01,0xff,0xfd,0x03,0x78,0x56,0x34,0x12]
 
 v_mad_u64_u32 v[5:6], s12, s1, 0x12345678, 0x12345678
 // GFX10-ERR: error: invalid operand (violates constant bus restrictions)
+
+//-----------------------------------------------------------------------------------------
+// null is free
+
+v_bfe_u32 v5, s1, s2, null
+// GFX10: v_bfe_u32 v5, s1, s2, null      ; encoding: [0x05,0x00,0x48,0xd5,0x01,0x04,0xf4,0x01]