[clang] [llvm] [RFC][AMDGPU] Use `bf16` instead of `i16` for bfloat (PR #80908)

[Shilei Tian via llvm-commits]

================
@@ -488,6 +488,49 @@ static bool printImmediateFloat16(uint32_t Imm, const MCSubtargetInfo &STI,
   return true;
 }
 
+static bool printImmediateBFloat16(uint32_t Imm, const MCSubtargetInfo &STI,
+                                   raw_ostream &O) {
+  if (Imm == 0x3F80)
+    O << "1.0";
+  else if (Imm == 0xBF80)
+    O << "-1.0";
+  else if (Imm == 0x3F00)
+    O << "0.5";
+  else if (Imm == 0xBF00)
+    O << "-0.5";
+  else if (Imm == 0x4000)
+    O << "2.0";
+  else if (Imm == 0xC000)
+    O << "-2.0";
+  else if (Imm == 0x4080)
+    O << "4.0";
+  else if (Imm == 0xC080)
+    O << "-4.0";
+  else if (Imm == 0x3E22 && STI.hasFeature(AMDGPU::FeatureInv2PiInlineImm))
+    O << "0.15915494";
+  else
+    return false;
+
+  return true;
+}
+
+void AMDGPUInstPrinter::printImmediateBF16(uint32_t Imm,
+                                           const MCSubtargetInfo &STI,
+                                           raw_ostream &O) {
+  int16_t SImm = static_cast<int16_t>(Imm);
+  if (isInlinableIntLiteral(SImm)) {
+    O << SImm;
+    return;
+  }
+
+  uint16_t HImm = static_cast<uint16_t>(Imm);
+  if (printImmediateBFloat16(HImm, STI, O))
+    return;
+
+  uint64_t Imm16 = static_cast<uint16_t>(Imm);
----------------
shiltian wrote:

Yeah, but it is to make the type promoted to `uint64_t` w/o any ambiguity.

https://github.com/llvm/llvm-project/pull/80908