[llvm] [mlir] [APFloat] Report the sign and the zero a conversion cannot represent (PR #216056)

[Matthias Springer via llvm-commits]

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@@ -2334,6 +2334,59 @@ TEST(APFloatTest, Float8E8M0FNUNaNConvert) {
   }
 }
 
+// Test that converting into a format that cannot represent the sign, or that
+// has no encoding for zero, reports the loss. Callers decide whether to keep
+// the result from losesInfo, and the value that comes out is not the value
+// that went in: the sign bit has nowhere to go, and zero is replaced by the
+// smallest normalized value.
+TEST(APFloatTest, ConvertLosesUnrepresentableSignAndZero) {
+  // Neither format has a sign; only Float8E8M0FNU also lacks a zero.
+  const fltSemantics *NoSignSemantics[] = {&APFloat::Float8E8M0FNU(),
+                                           &APFloat::Float8E5M3FNU()};
+
+  for (const fltSemantics *Sem : NoSignSemantics) {
+    for (double Value : {-2.0, -0.0}) {
+      APFloat test(Value);
+      bool losesInfo = false;
+      APFloat::opStatus status =
+          test.convert(*Sem, APFloat::rmNearestTiesToEven, &losesInfo);
+      EXPECT_TRUE(losesInfo);
+      EXPECT_EQ(status, APFloat::opInexact);
+    }
+
+    // The magnitude alone still converts exactly.
+    APFloat test(2.0);
+    bool losesInfo = true;
+    APFloat::opStatus status =
+        test.convert(*Sem, APFloat::rmNearestTiesToEven, &losesInfo);
+    EXPECT_FALSE(losesInfo);
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matthias-springer wrote:

Same here.

https://github.com/llvm/llvm-project/pull/216056