Implement BULK_MEMORY_THRESHOLD

Author: TerrorJack

Makefile

@@ -22,6 +22,13 @@ BUILD_LIBC_TOP_HALF ?= yes
 # The directory where we're store intermediate artifacts.
 OBJDIR ?= $(CURDIR)/build
 
+# When the length is no larger than this threshold, we consider the
+# overhead of bulk memory opcodes to outweigh the performance benefit,
+# and fall back to the original musl implementation. See
+# https://github.com/WebAssembly/wasi-libc/pull/263 for relevant
+# discussion
+BULK_MEMORY_THRESHOLD ?= 32
+
 # Variables from this point on are not meant to be overridable via the
 # make command-line.
 
@@ -394,6 +401,9 @@ $(MUSL_PRINTSCAN_NO_FLOATING_POINT_OBJS): CFLAGS += \
 $(BULK_MEMORY_OBJS): CFLAGS += \
         -mbulk-memory
 
+$(BULK_MEMORY_OBJS): CFLAGS += \
+        -DBULK_MEMORY_THRESHOLD=$(BULK_MEMORY_THRESHOLD)
+
 $(LIBWASI_EMULATED_SIGNAL_MUSL_OBJS): CFLAGS += \
 	    -D_WASI_EMULATED_SIGNAL
 

libc-top-half/musl/src/string/memcpy.c

@@ -5,8 +5,9 @@
 void *memcpy(void *restrict dest, const void *restrict src, size_t n)
 {
 #if defined(__wasm_bulk_memory__)
-	return __builtin_memcpy(dest, src, n);
-#else
+	if (n > BULK_MEMORY_THRESHOLD)
+	  return __builtin_memcpy(dest, src, n);
+#endif
 	unsigned char *d = dest;
 	const unsigned char *s = src;
 
@@ -124,5 +125,4 @@ void *memcpy(void *restrict dest, const void *restrict src, size_t n)
 
 	for (; n; n--) *d++ = *s++;
 	return dest;
-#endif // __wasm_bulk_memory__
 }

libc-top-half/musl/src/string/memmove.c

@@ -9,8 +9,9 @@ typedef __attribute__((__may_alias__)) size_t WT;
 void *memmove(void *dest, const void *src, size_t n)
 {
 #if defined(__wasm_bulk_memory__)
-	return __builtin_memmove(dest, src, n);
-#else
+	if (n > BULK_MEMORY_THRESHOLD)
+		return __builtin_memmove(dest, src, n);
+#endif
 	char *d = dest;
 	const char *s = src;
 
@@ -42,5 +43,4 @@ void *memmove(void *dest, const void *src, size_t n)
 	}
 
 	return dest;
-#endif // __wasm_bulk_memory__
 }

libc-top-half/musl/src/string/memset.c

@@ -4,8 +4,9 @@
 void *memset(void *dest, int c, size_t n)
 {
 #if defined(__wasm_bulk_memory__)
-	return __builtin_memset(dest, c, n);
-#else
+	if (n > BULK_MEMORY_THRESHOLD)
+		return __builtin_memset(dest, c, n);
+#endif
 	unsigned char *s = dest;
 	size_t k;
 
@@ -90,5 +91,4 @@ void *memset(void *dest, int c, size_t n)
 #endif
 
 	return dest;
-#endif // __wasm_bulk_memory__
 }