Fix rounding of fixed-point constants where fractional digits should carry into integer part (#2088)

src/asm/lexer.cpp

@@ -1005,14 +1005,20 @@ static uint32_t finishReadingFixedPoint(uint32_t integer) {
 		precision = options.fixPrecision;
 	}
 
-	if (integer >= (1ULL << (32 - precision))) {
-		warning(WARNING_LARGE_CONSTANT, "Magnitude of fixed-point constant is too large");
-		return 0;
-	}
-
 	// Cast to unsigned avoids undefined overflow behavior
 	uint32_t fractional =
 	    static_cast<uint32_t>(round(static_cast<double>(dividend) / divisor * (1ULL << precision)));
+	// Carry from `fractional` to `integer` if `round` rounded up to the next integer
+	assume(fractional <= 1ULL << precision);
+	if (fractional == 1ULL << precision) {
+		++integer;
+		fractional = 0;
+	}
+
+	if (integer >= 1ULL << (32 - precision)) {
+		warning(WARNING_LARGE_CONSTANT, "Magnitude of fixed-point constant is too large");
+		return 0;
+	}
 
 	return (integer << precision) | fractional;
 }

test/asm/fixed-point-carry.asm

@@ -0,0 +1,17 @@
+opt Q8
+println strfmt("%f", 1.999)
+println strfmt("%f", 2.999)
+println strfmt("%f", 3.999)
+println strfmt("%f", 16777215.999)
+
+opt Q16
+println strfmt("%f", 1.999999)
+println strfmt("%f", 2.999999)
+println strfmt("%f", 3.999999)
+println strfmt("%f", 65535.999999)
+
+opt Q24
+println strfmt("%f", 1.999999999)
+println strfmt("%f", 2.999999999)
+println strfmt("%f", 3.999999999)
+println strfmt("%f", 255.999999999)

test/asm/fixed-point-carry.err

@@ -0,0 +1,6 @@
+warning: Magnitude of fixed-point constant is too large [-Wlarge-constant]
+    at fixed-point-carry.asm(5)
+warning: Magnitude of fixed-point constant is too large [-Wlarge-constant]
+    at fixed-point-carry.asm(11)
+warning: Magnitude of fixed-point constant is too large [-Wlarge-constant]
+    at fixed-point-carry.asm(17)

test/asm/fixed-point-carry.out

@@ -0,0 +1,12 @@
+2.00000
+3.00000
+4.00000
+0.00000
+2.00000
+3.00000
+4.00000
+0.00000
+2.00000
+3.00000
+4.00000
+0.00000

test/asm/fixed-point-syntax.out

@@ -5,9 +5,9 @@ $18B
 $62B85
 $4
 $10000
+$20000
 $10000
-$10000
-$10000
+$20000
 $C570A
 $13333
 $13333