[llvm] [AMDGPU] Add CodeGen support for GFX12 s_mul_u64 (PR #75825)

[Matt Arsenault via llvm-commits]

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@@ -2094,6 +2094,75 @@ bool AMDGPURegisterBankInfo::foldInsertEltToCmpSelect(
   return true;
 }
 
+// Break s_mul_u64 into 32-bit vector operations.
+void AMDGPURegisterBankInfo::applyMappingSMULU64(
+    MachineIRBuilder &B, const OperandsMapper &OpdMapper) const {
+  SmallVector<Register, 2> DefRegs(OpdMapper.getVRegs(0));
+  SmallVector<Register, 2> Src0Regs(OpdMapper.getVRegs(1));
+  SmallVector<Register, 2> Src1Regs(OpdMapper.getVRegs(2));
+
+  // All inputs are SGPRs, nothing special to do.
+  if (DefRegs.empty()) {
+    assert(Src0Regs.empty() && Src1Regs.empty());
+    applyDefaultMapping(OpdMapper);
+    return;
+  }
+
+  assert(DefRegs.size() == 2);
+  assert(Src0Regs.size() == Src1Regs.size() &&
+         (Src0Regs.empty() || Src0Regs.size() == 2));
+
+  MachineRegisterInfo &MRI = OpdMapper.getMRI();
+  MachineInstr &MI = OpdMapper.getMI();
+  Register DstReg = MI.getOperand(0).getReg();
+  LLT HalfTy = LLT::scalar(32);
+
+  // Depending on where the source registers came from, the generic code may
+  // have decided to split the inputs already or not. If not, we still need to
+  // extract the values.
+
+  if (Src0Regs.empty())
+    split64BitValueForMapping(B, Src0Regs, HalfTy, MI.getOperand(1).getReg());
+  else
+    setRegsToType(MRI, Src0Regs, HalfTy);
+
+  if (Src1Regs.empty())
+    split64BitValueForMapping(B, Src1Regs, HalfTy, MI.getOperand(2).getReg());
+  else
+    setRegsToType(MRI, Src1Regs, HalfTy);
+
+  setRegsToType(MRI, DefRegs, HalfTy);
+
+  // The multiplication is done as follows:
+  //
+  //                            Op1H  Op1L
+  //                          * Op0H  Op0L
+  //                       --------------------
+  //                       Op1H*Op0L  Op1L*Op0L
+  //          + Op1H*Op0H  Op1L*Op0H
+  // -----------------------------------------
+  // (Op1H*Op0L + Op1L*Op0H + carry)  Op1L*Op0L
+  //
+  //  We drop Op1H*Op0H because the result of the multiplication is a 64-bit
+  //  value and that would overflow.
+  //  The low 32-bit value is Op1L*Op0L.
+  //  The high 32-bit value is Op1H*Op0L + Op1L*Op0H + carry (from
+  //  Op1L*Op0L).
+
+  Register Hi = B.buildUMulH(HalfTy, Src0Regs[0], Src1Regs[0]).getReg(0);
+  MRI.setRegBank(Hi, AMDGPU::VGPRRegBank);
----------------
arsenm wrote:

It's a bit ugly but ApplyRegBankMapping is used in some other places to avoid explicitly inserting setRegBank at every point.

https://github.com/llvm/llvm-project/pull/75825