[mlir] Use double format when parsing bfloat16 hexadecimal values

[rriddle]

Summary

bfloat16 doesn't have a valid APFloat format, so we have to use double semantics when storing it. This change makes sure that hexadecimal values can be round-tripped properly given this fact.

[merge_guards_bot]

Unit tests: fail. 61743 tests passed, 1 failed and 780 were skipped.

failed: MLIR.IR/invalid.mlir

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[rriddle]

Fix invalid test.

[merge_guards_bot]

Unit tests: pass. 61744 tests passed, 0 failed and 780 were skipped.

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[rriddle]

Rebase

[merge_guards_bot]

Unit tests: unknown.

clang-tidy: unknown.

clang-format: unknown.

Build artifacts: console-log.txt

mlir/lib/Parser/Parser.cpp

if (!type.isa<FloatType>())
     return (emitError("floating point value not valid for specified type"),
             nullptr);
   return FloatAttr::get(type, isNegative ? -val.getValue() : val.getValue());
 }
 
 /// Construct a float attribute bitwise equivalent to the integer literal.
 static FloatAttr buildHexadecimalFloatLiteral(Parser *p, FloatType type,
                                               uint64_t value) {
-  int width = type.getIntOrFloatBitWidth();
-  APInt apInt(width, value);
+  // FIXME: bfloat is currently stored as a double internally because it doesn't
+  // have valid APFloat semantics.
+  if (type.isF64() || type.isBF16()) {
+    APFloat apFloat(type.getFloatSemantics(), APInt(/*numBits=*/64, value));
+    return p->builder.getFloatAttr(type, apFloat);
+  }
+
+  APInt apInt(type.getWidth(), value);
   if (apInt != value) {
     p->emitError("hexadecimal float constant out of range for type");
     return nullptr;
   }
   APFloat apFloat(type.getFloatSemantics(), apInt);
   return p->builder.getFloatAttr(type, apFloat);
 }
 
if (!type) {
     // Default to i64 if not type is specified.
     if (!consumeIf(Token::colon))
       type = builder.getIntegerType(64);
     else if (!(type = parseType()))
       return nullptr;
   }
 
   if (auto floatType = type.dyn_cast<FloatType>()) {
-    // TODO(zinenko): Update once hex format for bfloat16 is supported.
-    if (type.isBF16())
-      return emitError(loc,
-                       "hexadecimal float literal not supported for bfloat16"),
-             nullptr;
     if (isNegative)
       return emitError(
                  loc,
                  "hexadecimal float literal should not have a leading minus"),
              nullptr;
     if (!isHex) {
       emitError(loc, "unexpected decimal integer literal for a float attribute")
               .attachNote()

mlir/test/IR/invalid.mlir

%def = "foo.region_with_def"() ({
     "foo.use" (%def) : (i32) -> ()
     "foo.yield" () : () -> ()
   }) : () -> (i32)
   return
 }
 
 // -----
 
-func @hexadecimal_bf16() {
-  // expected-error @+1 {{hexadecimal float literal not supported for bfloat16}}
-  "foo"() {value = 0xffff : bf16} : () -> ()
-}
-
-// -----
-
 func @hexadecimal_float_leading_minus() {
   // expected-error @+1 {{hexadecimal float literal should not have a leading minus}}
   "foo"() {value = -0x7fff : f16} : () -> ()
 }
 
 // -----
 
 func @hexadecimal_float_literal_overflow() {

mlir/test/IR/parser.mlir

func @f64_special_values() {
   %4 = constant 0x7FF0000000000000 : f64
   // F64 negative infinity.
   // CHECK: constant 0xFFF0000000000000 : f64
   %5 = constant 0xFFF0000000000000 : f64
 
   return
 }
 
+// FIXME: bfloat16 currently uses f64 as a storage format. This test should be
+// changed when that gets fixed.
+// CHECK-LABEL: @bfloat16_special_values
+func @bfloat16_special_values() {
+  // bfloat16 signaling NaNs.
+  // CHECK: constant 0x7FF0000000000001 : bf16
+  %0 = constant 0x7FF0000000000001 : bf16
+  // CHECK: constant 0x7FF8000000000000 : bf16
+  %1 = constant 0x7FF8000000000000 : bf16
+
+  // bfloat16 quiet NaNs.
+  // CHECK: constant 0x7FF0000001000000 : bf16
+  %2 = constant 0x7FF0000001000000 : bf16
+  // CHECK: constant 0xFFF0000001000000 : bf16
+  %3 = constant 0xFFF0000001000000 : bf16
+
+  // bfloat16 positive infinity.
+  // CHECK: constant 0x7FF0000000000000 : bf16
+  %4 = constant 0x7FF0000000000000 : bf16
+  // bfloat16 negative infinity.
+  // CHECK: constant 0xFFF0000000000000 : bf16
+  %5 = constant 0xFFF0000000000000 : bf16
+
+  return
+}
+
 // We want to print floats in exponential notation with 6 significant digits,
 // but it may lead to precision loss when parsing back, in which case we print
 // the decimal form instead.
 // CHECK-LABEL: @f32_potential_precision_loss()
 func @f32_potential_precision_loss() {
   // CHECK: constant -1.23697901 : f32
   %0 = constant -1.23697901 : f32
   return