[llvm] [AMDGPU][MC] Implement fft and rotate modes for ds_swizzle_b32 (PR #108064)

[Matt Arsenault via llvm-commits]

================
@@ -8141,6 +8143,56 @@ AMDGPUAsmParser::parseSwizzleBitmaskPerm(int64_t &Imm) {
   return true;
 }
 
+bool AMDGPUAsmParser::parseSwizzleFFT(int64_t &Imm) {
+  using namespace llvm::AMDGPU::Swizzle;
+
+  if (!skipToken(AsmToken::Comma, "expected a comma"))
+    return false;
+
+  SMLoc Loc = getLoc();
+
+  int64_t Type;
+
+  if (!parseExpr(Type))
+    return false;
+
+  if (Type != FFT_NO_SWIZZLE && Type != FFT_SWIZZLE_00 &&
+      Type != FFT_SWIZZLE_10) {
+    const std::string ErrMsg = "invalid FFT swizzle type: must be " +
+                               std::to_string(FFT_SWIZZLE_00) + ", " +
+                               std::to_string(FFT_SWIZZLE_10) + ", or " +
+                               std::to_string(FFT_NO_SWIZZLE);
+    Error(Loc, ErrMsg);
+    return false;
+  }
+
+  Imm = FFT_MODE_ENC | Type;
+  return true;
+}
+
+bool AMDGPUAsmParser::parseSwizzleRotate(int64_t &Imm) {
+  using namespace llvm::AMDGPU::Swizzle;
+
+  SMLoc Loc;
+  int64_t Direction;
+
+  if (!parseSwizzleOperand(Direction, 0, 1,
+                           "direction must be 0 (left) or 1 (right)", Loc))
+    return false;
+
+  int64_t RotateSize;
+  const std::string ErrorMsg =
+      "number of threads to rotate must be in the interval [0," +
+      std::to_string(ROTATE_MAX_SIZE) + "]";
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arsenm wrote:

shouldn't need to pre-create the error string, also these all use Twine? 

https://github.com/llvm/llvm-project/pull/108064