Fix APFloat::toString on Float8E5M2 values.

Before, an APInt with value 10 was created, whose width was the significand width. But 10 cannot fit in Float8E5M2's significand.

Differential Revision: https://reviews.llvm.org/D138540

Author: reedwm

llvm/lib/Support/APFloat.cpp

@@ -4099,6 +4099,11 @@ void IEEEFloat::toString(SmallVectorImpl<char> &Str, unsigned FormatPrecision,
 
   // Fill the buffer.
   unsigned precision = significand.getBitWidth();
+  if (precision < 4) {
+    // We need enough precision to store the value 10.
+    precision = 4;
+    significand = significand.zext(precision);
+  }
   APInt ten(precision, 10);
   APInt digit(precision, 0);
 

llvm/unittests/ADT/APFloatTest.cpp

@@ -5152,6 +5152,26 @@ TEST(APFloatTest, Float8E4M3FNExhaustivePair) {
   }
 }
 
+TEST(APFloatTest, F8ToString) {
+  for (APFloat::Semantics S :
+       {APFloat::S_Float8E5M2, APFloat::S_Float8E4M3FN}) {
+    SCOPED_TRACE("Semantics=" + std::to_string(S));
+    for (int i = 0; i < 256; i++) {
+      SCOPED_TRACE("i=" + std::to_string(i));
+      APFloat test(APFloat::Float8E5M2(), APInt(8, i));
+      llvm::SmallString<128> str;
+      test.toString(str);
+
+      if (test.isNaN()) {
+        EXPECT_EQ(str, "NaN");
+      } else {
+        APFloat test2(APFloat::Float8E5M2(), str);
+        EXPECT_TRUE(test.bitwiseIsEqual(test2));
+      }
+    }
+  }
+}
+
 TEST(APFloatTest, IEEEdoubleToDouble) {
   APFloat DPosZero(0.0);
   APFloat DPosZeroToDouble(DPosZero.convertToDouble());