[llvm] AMDGPU: Add tonearest and towardzero roundings for intrinsic llvm.fptrunc.round (PR #104486)

[Matt Arsenault via llvm-commits]

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@@ -7089,23 +7089,31 @@ bool AMDGPULegalizerInfo::legalizeBVHIntrinsic(MachineInstr &MI,
 }
 
 bool AMDGPULegalizerInfo::legalizeFPTruncRound(MachineInstr &MI,
+                                               MachineRegisterInfo &MRI,
                                                MachineIRBuilder &B) const {
-  unsigned Opc;
-  int RoundMode = MI.getOperand(2).getImm();
+  Register Src = MI.getOperand(1).getReg();
+  if (MRI.getType(Src) != LLT::scalar(32))
+    return false;
 
-  if (RoundMode == (int)RoundingMode::TowardPositive)
-    Opc = AMDGPU::G_FPTRUNC_ROUND_UPWARD;
-  else if (RoundMode == (int)RoundingMode::TowardNegative)
-    Opc = AMDGPU::G_FPTRUNC_ROUND_DOWNWARD;
-  else
+  // Only support towardzero, tonearest, upward and downward.
+  int RoundMode = MI.getOperand(2).getImm();
+  if (RoundMode != (int)RoundingMode::TowardZero &&
+      RoundMode != (int)RoundingMode::TowardNegative &&
+      RoundMode != (int)RoundingMode::TowardPositive &&
+      RoundMode != (int)RoundingMode::TowardNegative)
     return false;
 
-  B.buildInstr(Opc)
+  // "round.towardzero" -> TowardZero 0        -> FP_ROUND_ROUND_TO_ZERO 3
+  // "round.tonearest"  -> NearestTiesToEven 1 -> FP_ROUND_ROUND_TO_NEAREST 0
+  // "round.upward"     -> TowardPositive 2    -> FP_ROUND_ROUND_TO_INF 1
+  // "round.downward    -> TowardNegative 3    -> FP_ROUND_ROUND_TO_NEGINF 2
+  unsigned HW_Mode = (RoundMode + 3) % 4;
+  B.buildInstr(AMDGPU::G_FPTRUNC_ROUND)
       .addDef(MI.getOperand(0).getReg())
-      .addUse(MI.getOperand(1).getReg());
+      .addUse(Src)
+      .addImm(HW_Mode);
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arsenm wrote:

All this is really doing is just remapping the encoded rounding mode values. At that point it should just be a selection pattern 

https://github.com/llvm/llvm-project/pull/104486