compiler_rt: Implemented missing handling of denormal numbers when dividing f128 (#30198)

Replaces the "flush denormals to zero" placeholder in divtf3.zig with IEEE 754 denormal support including rounding.

fixes #30179

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/30198
Reviewed-by: Andrew Kelley <andrew@ziglang.org>
Co-authored-by: Chris Boesch <chrboesch@noreply.codeberg.org>
Co-committed-by: Chris Boesch <chrboesch@noreply.codeberg.org>

lib/compiler_rt/divtf3.zig

@@ -222,10 +222,29 @@ inline fn div(a: f128, b: f128) f128 {
                 // The rounded result is normal; return it.
                 return @bitCast(absResult | quotientSign);
             }
+            // Result is denormal with exponent 0
+            return @bitCast(absResult | quotientSign);
+        } else {
+            // For denormals with writtenExponent < 0,
+            // the implicit bit must be shifted into the mantissa (IEEE 754)
+            const shiftAmount = @as(u7, @intCast(1 - writtenExponent));
+
+            // Check for underflow
+            if (shiftAmount > significandBits) {
+                return @bitCast(quotientSign);
+            }
+
+            // Round the quotient before pushing
+            const shouldRound = (residual << 1) > bSignificand;
+            const roundedQuotient = quotient +% @as(u113, @intFromBool(shouldRound));
+
+            // Move to the denormal range and apply the mask
+            const denormQuotient = roundedQuotient >> shiftAmount;
+            const absResult = denormQuotient & significandMask;
+
+            //  Add sign to denormal mantissa and return
+            return @bitCast(absResult | quotientSign);
         }
-        // Flush denormals to zero.  In the future, it would be nice to add
-        // code to round them correctly.
-        return @bitCast(quotientSign);
     } else {
         const round = @intFromBool((residual << 1) >= bSignificand);
         // Clear the implicit bit

lib/compiler_rt/divtf3_test.zig

@@ -46,4 +46,9 @@ test "divtf3" {
     try test__divtf3(0x1.2345f6b77b7a8953365433abcdefp+234, 0x1.edcba987d6bb3aa467754354321fp-4055, 0x50bf2e02f0798d36, 0x5e6fcb6b60044078);
     try test__divtf3(6.72420628622418701252535563464350521E-4932, 2.0, 0x0001000000000000, 0);
     try test__divtf3(1.0, 0x1.ffffffffffffffffffffffffffffp-1, 0x3FFF000000000000, 1);
+
+    try test__divtf3(std.math.floatMin(f128), 2.0, 0x0000800000000000, 0x0000000000000000);
+    try test__divtf3(std.math.floatMin(f128), 4.0, 0x0000400000000000, 0x0000000000000000);
+    try test__divtf3(1.0, 0x1.0000000000000000000000000001p16382, 0x0000ffffffffffff, 0xfffffffffffffffe);
+    try test__divtf3(1.2, std.math.floatMax(f128), 0x00004ccccccccccc, 0xcccccccccccccccd);
 }