Select implementation at runtime for memcpy, memmove and mempcpy

This relies upon the insertion of a trampoline function that loads the
address of the appropriate implementation from a static variable. The use of
a static initialiser that points at the function that patches up the function
pointer itself ensure that this is thread-safe.

The method used to determine which is the best implementation takes
advantage of the change in behaviour of interworking branches between ARMv6
and ARMv7. This makes the test OS-independent, and in any case it's
questionable how safe it is to be making system calls when a function as
fundamental as memcpy isn't functional yet, so this is probably a safer
approach than asking the kernel.

The main downside here is that anyone trying the library on an ARMv7 platform
which lacks NEON (the usual example is nVidia Tegra 2) will find it doesn't
work. But then we're not pretending to offer an optimised implementation for
that anyway, and they might as well continue to use glibc.

Author: bavison

Makefile

@@ -1,8 +1,7 @@
-OBJS = memcmp.o memcpymove.o memset.o
-OBJS-A7 = memcpymove-a7.o
-CFLAGS += -std=c99 -O2
+OBJS = architecture.o memcmp.o memcpymove.o memcpymove-a7.o memset.o trampoline.o
+CFLAGS += -std=gnu99 -O2
 
-all: libarmmem.so libarmmem.a libarmmem-a7.so libarmmem-a7.a test
+all: libarmmem.so libarmmem.a test
 
 %.o: %.c
 	$(CROSS_COMPILE)gcc $(CFLAGS) -c -o $@ $^
@@ -16,12 +15,6 @@ libarmmem.so: $(OBJS)
 libarmmem.a: $(OBJS)
 	$(CROSS_COMPILE)ar rcs $@ $^
 
-libarmmem-a7.so: $(OBJS-A7)
-	$(CROSS_COMPILE)gcc -shared -o $@ $^
-
-libarmmem-a7.a: $(OBJS-A7)
-	$(CROSS_COMPILE)ar rcs $@ $^
-
 test: test.o
 	$(CROSS_COMPILE)gcc -o $@ $^
 

architecture.S

@@ -0,0 +1,44 @@
+/*
+Copyright (c) 2015, RISC OS Open Ltd
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright
+      notice, this list of conditions and the following disclaimer in the
+      documentation and/or other materials provided with the distribution.
+    * Neither the name of the copyright holder nor the
+      names of its contributors may be used to endorse or promote products
+      derived from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY
+DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+/* Grubby, but completely OS-independent and USR-mode safe way to
+ * distinguish between ARMv6 and ARMv7 platforms at runtime
+ */
+
+.global architecture
+.hidden architecture
+.func   architecture
+.arm
+architecture:
+        sub     pc, pc, #1  @ is an interworking branch on ARMv7, not ARMv6
+        and     a1, a4, a1  @ second word interpreted as 'B .+0xA' if Thumb
+        mov     a1, #6
+        bx      lr
+.thumb
+        mov     a1, #7
+        bx      lr
+.endfunc

memcpymove-a7.S

@@ -612,7 +612,8 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 .set prefetch_distance, 2
 
-myfunc memcpy
+myfunc memcpy_cortexa7
+.hidden memcpy_cortexa7
 1000:   memcpy  0
 .endfunc
 
@@ -628,7 +629,8 @@ myfunc memcpy
 
 .set prefetch_distance, 2
 
-myfunc memmove
+myfunc memmove_cortexa7
+.hidden memmove_cortexa7
         cmp     a2, a1
         bpl     1000b   /* pl works even over -1 - 0 and 0x7fffffff - 0x80000000 boundaries */
         memcpy  1
@@ -644,9 +646,11 @@ myfunc memmove
  * a1 = pointer to immediately after destination block
  */
 
-myfunc mempcpy
-.global __mempcpy
-__mempcpy:
+myfunc mempcpy_cortexa7
+.hidden mempcpy_cortexa7
+.hidden __mempcpy_cortexa7
+.global __mempcpy_cortexa7
+__mempcpy_cortexa7:
         push    {v1, lr}
         mov     v1, a3
         bl      1000b

memcpymove.S

@@ -531,7 +531,8 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 .set prefetch_distance, 3
 
-myfunc memcpy
+myfunc memcpy_arm1176jzfs
+.hidden memcpy_arm1176jzfs
 1000:   memcpy  0
 .endfunc
 
@@ -547,7 +548,8 @@ myfunc memcpy
 
 .set prefetch_distance, 3
 
-myfunc memmove
+myfunc memmove_arm1176jzfs
+.hidden memmove_arm1176jzfs
         cmp     a2, a1
         bpl     1000b   /* pl works even over -1 - 0 and 0x7fffffff - 0x80000000 boundaries */
         memcpy  1
@@ -563,9 +565,11 @@ myfunc memmove
  * a1 = pointer to immediately after destination block
  */
 
-myfunc mempcpy
-.global __mempcpy
-__mempcpy:
+myfunc mempcpy_arm1176jzfs
+.hidden mempcpy_arm1176jzfs
+.hidden __mempcpy_arm1176jzfs
+.global __mempcpy_arm1176jzfs
+__mempcpy_arm1176jzfs:
         push    {v1, lr}
         mov     v1, a3
         bl      1000b

trampoline.c

@@ -0,0 +1,57 @@
+/*
+Copyright (c) 2015, RISC OS Open Ltd
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright
+      notice, this list of conditions and the following disclaimer in the
+      documentation and/or other materials provided with the distribution.
+    * Neither the name of the copyright holder nor the
+      names of its contributors may be used to endorse or promote products
+      derived from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY
+DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+#define _GNU_SOURCE /* enable mempcpy */
+#include <string.h>
+
+extern int architecture(void);
+
+#define DISPATCH(ret_type, func, args_full, args) \
+                                                  \
+typeof(func) func##_arm1176jzfs;                  \
+typeof(func) func##_cortexa7;                     \
+static typeof(func) func##_initial;               \
+static typeof(func) *p##func = func##_initial;    \
+                                                  \
+ret_type func args_full                           \
+{                                                 \
+    return p##func args;                          \
+}                                                 \
+                                                  \
+static ret_type func##_initial args_full          \
+{                                                 \
+    if (architecture() == 6)                      \
+        p##func = func##_arm1176jzfs;             \
+    else                                          \
+        p##func = func##_cortexa7;                \
+    return p##func args;                          \
+}
+
+DISPATCH(void *, memcpy,   (void *__restrict __dest, __const void *__restrict __src, size_t __n), (__dest, __src, __n))
+DISPATCH(void *, memmove,  (void *           __dest, __const void *           __src, size_t __n), (__dest, __src, __n))
+DISPATCH(void *,__mempcpy, (void *__restrict __dest, __const void *__restrict __src, size_t __n), (__dest, __src, __n))
+DISPATCH(void *, mempcpy,  (void *__restrict __dest, __const void *__restrict __src, size_t __n), (__dest, __src, __n))