Fix APFloat::toString on Float8E5M2 values.

[reedwm]

Summary

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

[bkramer]

looks good

[reedwm]

Thanks for reviewing! Since I don't have commit access, can you commit this?

llvm/lib/Support/APFloat.cpp

void IEEEFloat::toString(SmallVectorImpl<char> &Str, unsigned FormatPrecision,
   }
 
   AdjustToPrecision(significand, exp, FormatPrecision);
 
   SmallVector<char, 256> buffer;
 
   // 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);
 
   bool inTrail = true;
   while (significand != 0) {
     // digit <- significand % 10
     // significand <- significand / 10
     APInt::udivrem(significand, ten, significand, digit);

llvm/unittests/ADT/APFloatTest.cpp

for (int j = 0; j < 256; j++) {
       z16.remainder(y16);
       z16.convert(APFloat::Float8E4M3FN(), APFloat::rmNearestTiesToEven,
                   &losesInfo);
       EXPECT_TRUE(z.bitwiseIsEqual(z16)) << "i=" << i << ", j=" << j;
     }
   }
 }
 
+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());
   EXPECT_TRUE(DPosZeroToDouble.isPosZero());
   APFloat DNegZero(-0.0);
   APFloat DNegZeroToDouble(DNegZero.convertToDouble());
   EXPECT_TRUE(DNegZeroToDouble.isNegZero());