lib: sfloat: Fix conversion to special values

The conversion function returns special values even when the input is valid number. This commit makes sure that no special value is returned if input is valid. Signed-off-by: Dominik Kilian <[email protected]>
adsz-nordic · Jan 24, 2023 · 855cc65 · 855cc65
1 parent 8ce1a91
commit 855cc65
Show file tree

Hide file tree

Showing 2 changed files with 33 additions and 9 deletions.
diff --git a/lib/sfloat/sfloat.c b/lib/sfloat/sfloat.c
@@ -29,6 +29,10 @@
 #define SFLOAT_EXP_BIT_POS (12)
 /* Exponent mask in the SFLOAT encoding. */
 #define SFLOAT_EXP_MASK (0xF000)
+/* Value of the exponent that is in SFLOAT special values. */
+#define SFLOAT_SPECIAL_EXP 0
+/* Minimum value of mantissa in SFLOAT special values. */
+#define SFLOAT_SPECIAL_MANTISSA_MIN 2046
 
 /* Size in bits of the float sign encoding. */
 #define FLOAT_SIGN_BIT_SIZE 1
@@ -46,6 +50,8 @@
 
 /* Helper macro for getting absolute value. */
 #define ABS(a) ((a < 0) ? (-a) : (a))
+/* Helper macro for dividing two integers with rounding upwards (ceiling). */
+#define DIV_AND_CEIL(a, b) (((a) + ((b) - 1)) / (b))
 
 /* Float type should use binary32 notation from the IEEE 754-2008 specification. */
 BUILD_ASSERT(sizeof(float) == sizeof(uint32_t));
@@ -140,6 +146,12 @@ static struct sfloat_desc sfloat_desc_from_float(float float_num)
 		sfloat.exponent = (~sfloat.exponent & 0x0F) + 1;
 	}
 
+	/* Make sure that special sfloat values are not returned */
+	if ((sfloat.exponent == SFLOAT_SPECIAL_EXP) && (mantissa >= SFLOAT_SPECIAL_MANTISSA_MIN)) {
+		sfloat.exponent++;
+		mantissa = DIV_AND_CEIL(SFLOAT_SPECIAL_MANTISSA_MIN, 10);
+	}
+
 	sfloat.mantissa = mantissa & SFLOAT_MANTISSA_MASK;
 	if (float_enc.sign) {
 		sfloat.mantissa = (~sfloat.mantissa & SFLOAT_MANTISSA_MASK) + 1;

diff --git a/tests/lib/sfloat/src/main.c b/tests/lib/sfloat/src/main.c
@@ -69,14 +69,26 @@ ZTEST(sfloat_suite, test_sfloat_from_float)
 		{-20479000000.0, {0x7800}}, /* - 10^(7) * 2048 = -20480000000.00000 */
 
 		/* Mantisssa - edge cases */
-		{2047.0, {0x07FF}}, /* 10^(0) * 2047 = 2047.00000 */
-		{2048.0, {0x10CD}}, /* 10^(1) * 205 = 2050.00000 */
-		{-2047.0, {0x0801}}, /* - 10^(0) * 2047 = -2047.00000 */
-		{-2048.0, {0x0800}}, /* - 10^(0) * 2048 = -2048.00000 */
-		{2047.5, {0x10CD}}, /* 10^(1) * 205 = 2050.00000 */
-		{2048.5, {0x10CD}}, /* 10^(1) * 205 = 2050.00000 */
-		{-2047.5, {0x0800}}, /* - 10^(0) * 2048 = -2048.00000 */
-		{-2048.5, {0x1F33}}, /* - 10^(1) * 205 = -2050.00000 */
+		{20470.0, {0x17FF}}, /* 10^(1) * 2047 = 20470.0000 */
+		{20480.0, {0x20CD}}, /* 10^(2) * 205 = 20500.0000 */
+		{-20470.0, {0x1801}}, /* - 10^(1) * 2047 = -20470.0000 */
+		{-20480.0, {0x1800}}, /* - 10^(1) * 2048 = -20480.0000 */
+		{20475.0, {0x20CD}}, /* 10^(2) * 205 = 20500.0000 */
+		{20485.0, {0x20CD}}, /* 10^(2) * 205 = 20500.0000 */
+		{-20475.0, {0x1800}}, /* - 10^(1) * 2048 = -20480.0000 */
+		{-20485.0, {0x2F33}}, /* - 10^(2) * 205 = -20500.0000 */
+
+		/* Avoiding special values */
+		{2045.0, {0x07FD}}, /* 10^(0) * 2045 = 2045.0000 */
+		{2046.0, {0x10CD}}, /* 10^(1) * 205 = 2050.0000 (0x07FE is special) */
+		{2047.0, {0x10CD}}, /* 10^(1) * 205 = 2050.0000 (0x07FF is special) */
+		{2048.0, {0x10CD}}, /* 10^(1) * 205 = 2050.0000 (0x07FF is special) */
+		{2049.0, {0x10CD}}, /* 10^(1) * 205 = 2050.0000 */
+		{-2045.0, {0x0803}}, /* - 10^(0) * 2045 = 2045.0000 */
+		{-2046.0, {0x1F33}}, /* - 10^(1) * 205 = 2050.0000 (0x0802 is special) */
+		{-2047.0, {0x1F33}}, /* - 10^(1) * 205 = 2050.0000 (0x0801 is special) */
+		{-2048.0, {0x1F33}}, /* - 10^(1) * 205 = 2050.0000 (0x0800 is special) */
+		{-2049.0, {0x1F33}}, /* - 10^(1) * 205 = 2050.0000 */
 
 		/* Special values */
 		{(0.0 / 0.0), {SFLOAT_NAN}}, /* SFLOAT NaN */
@@ -95,7 +107,7 @@ ZTEST(sfloat_suite, test_sfloat_from_float)
 		float in_float = test_vector[i].in_float;
 
 		res = sfloat_from_float(in_float);
-		zassume_true(res.val == out_sfloat.val,
+		zassert_true(res.val == out_sfloat.val,
 			"comparison failed for the sample[%d] equal to %f: 0x%04X != 0x%04X",
 			i, in_float, res.val, out_sfloat.val);
 	}