Merge pull request #87 from miloyip/issue85simdsegfault

Fix SIMD page fault by using aligned load

include/rapidjson/reader.h

@@ -178,21 +178,56 @@ void SkipWhitespace(InputStream& is) {
 //! Skip whitespace with SSE 4.2 pcmpistrm instruction, testing 16 8-byte characters at once.
 inline const char *SkipWhitespace_SIMD(const char* p) {
 	static const char whitespace[16] = " \n\r\t";
-	__m128i w = _mm_loadu_si128((const __m128i *)&whitespace[0]);
-
-	for (;;) {
-		__m128i s = _mm_loadu_si128((const __m128i *)p);
-		unsigned r = _mm_cvtsi128_si32(_mm_cmpistrm(w, s, _SIDD_UBYTE_OPS | _SIDD_CMP_EQUAL_ANY | _SIDD_BIT_MASK | _SIDD_NEGATIVE_POLARITY));
-		if (r == 0)	// all 16 characters are whitespace
-			p += 16;
-		else {		// some of characters may be non-whitespace
+	static const char whitespaces[4][17] = {
+		"                ",
+		"\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n",
+		"\r\r\r\r\r\r\r\r\r\r\r\r\r\r\r\r",
+		"\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t"};
+
+	// 16-byte align to the lower boundary
+	const char* ap = reinterpret_cast<const char*>(reinterpret_cast<size_t>(p) & ~15);
+
+	// Test first unaligned characters
+	// Cannot make use of _mm_cmpistrm() because it stops when encounters '\0' before p
+	if (ap != p) {
+		const __m128i w0 = _mm_loadu_si128((const __m128i *)&whitespaces[0][0]);
+		const __m128i w1 = _mm_loadu_si128((const __m128i *)&whitespaces[1][0]);
+		const __m128i w2 = _mm_loadu_si128((const __m128i *)&whitespaces[2][0]);
+		const __m128i w3 = _mm_loadu_si128((const __m128i *)&whitespaces[3][0]);
+
+		unsigned char shift = reinterpret_cast<size_t>(p) & 15;
+		const __m128i s = _mm_load_si128(reinterpret_cast<const __m128i*>(ap));
+		__m128i x = _mm_cmpeq_epi8(s, w0);
+		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w1));
+		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w2));
+		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w3));
+		unsigned short r = (unsigned short)~_mm_movemask_epi8(x);
+		r = r >> shift << shift; // Clear results before p
+		if (r != 0) {
 #ifdef _MSC_VER		// Find the index of first non-whitespace
 			unsigned long offset;
-			if (_BitScanForward(&offset, r))
-				return p + offset;
+			_BitScanForward(&offset, r);
+			return ap + offset;
+#else
+			return ap + __builtin_ffs(r) - 1;
+#endif
+		}
+		ap += 16;
+	}
+
+	const __m128i w = _mm_loadu_si128((const __m128i *)&whitespace[0]);
+
+	// The rest of string
+	for (;; ap += 16) {
+		const __m128i s = _mm_load_si128((const __m128i *)ap);
+		const unsigned r = _mm_cvtsi128_si32(_mm_cmpistrm(w, s, _SIDD_UBYTE_OPS | _SIDD_CMP_EQUAL_ANY | _SIDD_BIT_MASK | _SIDD_NEGATIVE_POLARITY));
+		if (r != 0) {	// some of characters is non-whitespace
+#ifdef _MSC_VER			// Find the index of first non-whitespace
+			unsigned long offset;
+			_BitScanForward(&offset, r);
+			return ap + offset;
 #else
-			if (r != 0)
-				return p + __builtin_ffs(r) - 1;
+			return ap + __builtin_ffs(r) - 1;
 #endif
 		}
 	}
@@ -208,28 +243,51 @@ inline const char *SkipWhitespace_SIMD(const char* p) {
 		"\r\r\r\r\r\r\r\r\r\r\r\r\r\r\r\r",
 		"\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t"};
 
-	__m128i w0 = _mm_loadu_si128((const __m128i *)&whitespaces[0][0]);
-	__m128i w1 = _mm_loadu_si128((const __m128i *)&whitespaces[1][0]);
-	__m128i w2 = _mm_loadu_si128((const __m128i *)&whitespaces[2][0]);
-	__m128i w3 = _mm_loadu_si128((const __m128i *)&whitespaces[3][0]);
+	const __m128i w0 = _mm_loadu_si128((const __m128i *)&whitespaces[0][0]);
+	const __m128i w1 = _mm_loadu_si128((const __m128i *)&whitespaces[1][0]);
+	const __m128i w2 = _mm_loadu_si128((const __m128i *)&whitespaces[2][0]);
+	const __m128i w3 = _mm_loadu_si128((const __m128i *)&whitespaces[3][0]);
 
-	for (;;) {
-		__m128i s = _mm_loadu_si128((const __m128i *)p);
+	// 16-byte align to the lower boundary
+	const char* ap = reinterpret_cast<const char*>(reinterpret_cast<size_t>(p) & ~15);
+
+	// Test first unaligned characters
+	if (ap != p) {
+		unsigned char shift = reinterpret_cast<size_t>(p) & 15;
+		const __m128i s = _mm_load_si128(reinterpret_cast<const __m128i*>(ap));
 		__m128i x = _mm_cmpeq_epi8(s, w0);
 		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w1));
 		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w2));
 		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w3));
 		unsigned short r = (unsigned short)~_mm_movemask_epi8(x);
-		if (r == 0)	// all 16 characters are whitespace
-			p += 16;
-		else {		// some of characters may be non-whitespace
+		r = r >> shift << shift; // Clear results before p
+		if (r != 0) {
 #ifdef _MSC_VER		// Find the index of first non-whitespace
 			unsigned long offset;
-			if (_BitScanForward(&offset, r))
-				return p + offset;
+			_BitScanForward(&offset, r);
+			return ap + offset;
+#else
+			return ap + __builtin_ffs(r) - 1;
+#endif
+		}
+		ap += 16;
+	}
+
+	// The rest of string
+	for (;; ap += 16) {
+		const __m128i s = _mm_load_si128((const __m128i *)ap);
+		__m128i x = _mm_cmpeq_epi8(s, w0);
+		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w1));
+		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w2));
+		x = _mm_or_si128(x, _mm_cmpeq_epi8(s, w3));
+		unsigned short r = (unsigned short)~_mm_movemask_epi8(x);
+		if (r != 0) {	// some of characters may be non-whitespace
+#ifdef _MSC_VER			// Find the index of first non-whitespace
+			unsigned long offset;
+			_BitScanForward(&offset, r);
+			return ap + offset;
 #else
-			if (r != 0)
-				return p + __builtin_ffs(r) - 1;
+			return ap + __builtin_ffs(r) - 1;
 #endif
 		}
 	}