random: Use branchless implementation for mask generation in Randomizer::getBytesFromString() (php#10522)

* random: Add `max_offset` local to Randomizer::getBytesFromString()

* random: Use branchless implementation for mask generation in Randomizer::getBytesFromString()

This was benchmarked against clzl with a standalone script with random inputs
and is slightly faster. clzl requires an additional branch to handle the
source_length = 1 / max_offset = 0 case.

* Improve comment for masking in Randomizer::getBytesFromString()

Author: TimWolla

Diff for: ext/random/randomizer.c

@@ -388,6 +388,7 @@ PHP_METHOD(Random_Randomizer, getBytesFromString)
 	ZEND_PARSE_PARAMETERS_END();
 
 	const size_t source_length = ZSTR_LEN(source);
+	const size_t max_offset = source_length - 1;
 
 	if (source_length < 1) {
 		zend_argument_value_error(1, "cannot be empty");
@@ -401,9 +402,9 @@ PHP_METHOD(Random_Randomizer, getBytesFromString)
 
 	retval = zend_string_alloc(length, 0);
 
-	if (source_length > 0x100) {
+	if (max_offset > 0xff) {
 		while (total_size < length) {
-			uint64_t offset = randomizer->algo->range(randomizer->status, 0, source_length - 1);
+			uint64_t offset = randomizer->algo->range(randomizer->status, 0, max_offset);
 
 			if (EG(exception)) {
 				zend_string_free(retval);
@@ -413,26 +414,14 @@ PHP_METHOD(Random_Randomizer, getBytesFromString)
 			ZSTR_VAL(retval)[total_size++] = ZSTR_VAL(source)[offset];
 		}
 	} else {
-		uint64_t mask;
-		if (source_length <= 0x1) {
-			mask = 0x0;
-		} else if (source_length <= 0x2) {
-			mask = 0x1;
-		} else if (source_length <= 0x4) {
-			mask = 0x3;
-		} else if (source_length <= 0x8) {
-			mask = 0x7;
-		} else if (source_length <= 0x10) {
-			mask = 0xF;
-		} else if (source_length <= 0x20) {
-			mask = 0x1F;
-		} else if (source_length <= 0x40) {
-			mask = 0x3F;
-		} else if (source_length <= 0x80) {
-			mask = 0x7F;
-		} else {
-			mask = 0xFF;
-		}
+		uint64_t mask = max_offset;
+		// Copy the top-most bit into all lower bits.
+		// Shifting by 4 is sufficient, because max_offset
+		// is guaranteed to fit in an 8-bit integer at this
+		// point.
+		mask |= mask >> 1;
+		mask |= mask >> 2;
+		mask |= mask >> 4;
 
 		int failures = 0;
 		while (total_size < length) {
@@ -445,7 +434,7 @@ PHP_METHOD(Random_Randomizer, getBytesFromString)
 			for (size_t i = 0; i < randomizer->status->last_generated_size; i++) {
 				uint64_t offset = (result >> (i * 8)) & mask;
 
-				if (offset >= source_length) {
+				if (offset > max_offset) {
 					if (++failures > PHP_RANDOM_RANGE_ATTEMPTS) {
 						zend_string_free(retval);
 						zend_throw_error(random_ce_Random_BrokenRandomEngineError, "Failed to generate an acceptable random number in %d attempts", PHP_RANDOM_RANGE_ATTEMPTS);

Diff for: ext/random/tests/03_randomizer/methods/getBytesFromString_fast_path.phpt

@@ -47,6 +47,32 @@ for ($i = 1; $i <= strlen($allBytes); $i *= 2) {
     echo PHP_EOL;
 }
 
+// Test lengths that are one more than the powers of two. For these
+// the maximum offset will be a power of two and thus a minimal number
+// of bits will be set in the offset.
+for ($i = 1; ($i + 1) <= strlen($allBytes); $i *= 2) {
+    $oneMore = $i + 1;
+
+    echo "{$oneMore}:", PHP_EOL;
+
+    $wrapper = new TestWrapperEngine($xoshiro);
+    $r = new Randomizer($wrapper);
+    $result = $r->getBytesFromString(substr($allBytes, 0, $oneMore), 20000);
+
+    $count = [];
+    for ($j = 0; $j < strlen($result); $j++) {
+        $b = $result[$j];
+        $count[ord($b)] ??= 0;
+        $count[ord($b)]++;
+    }
+
+    // We expect that each possible value appears at least once, if
+    // not is is very likely that some bits were erroneously masked away.
+    var_dump(count($count));
+
+    echo PHP_EOL;
+}
+
 echo "Slow Path:", PHP_EOL;
 
 $wrapper = new TestWrapperEngine($xoshiro);
@@ -107,6 +133,30 @@ int(128)
 int(2500)
 int(256)
 
+2:
+int(2)
+
+3:
+int(3)
+
+5:
+int(5)
+
+9:
+int(9)
+
+17:
+int(17)
+
+33:
+int(33)
+
+65:
+int(65)
+
+129:
+int(129)
+
 Slow Path:
 int(20000)
 int(256)