[builtins] Implement __aeabi_uread4/8 and __aeabi_uwrite4/8.

[efriedma]

Summary

These functions are part of the ARM RTABI (http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf).

Currently LLVM doesn't generate calls to them, but they could be useful for optimizing for size.

[dmgreen]

Nice idea. The rtabi seems to says:

Write functions return the value written, read functions the value read.

With signitures:

int __aeabi_uread4(void *address);
int __aeabi_uwrite4(int value, void *address);
long long __aeabi_uread8(void *address);
long long __aeabi_uwrite8(long long value, void *address);

So writes also return values.

Also, and I kind of trust you know what you are talking about here, but is this using a struct like this always valid? Does packed always means alignment of 1? And is the mayalias needed if the thing we are aliasing with is a char*? (It does make the intent clearer, and the IR looks OK to me.)

[efriedma]

The packed/may_alias struct pattern is the same pattern we use in the x86 immintrin.h for unaligned loads; should do the right thing in general.

Does packed always means alignment of 1?

Yes.

And is the mayalias needed if the thing we are aliasing with is a char*?

Well, if we're not doing LTO with compiler-rt (which isn't really something we support anyway), the may_alias does nothing because there aren't any other memory accesses. The type of the pointer isn't really relevant.

---

Write functions return the value written

Oh, oops, will fix.

[efriedma]

Fixed signature of __aeabi_uwrite4/8.

[dmgreen]

Yeah, I agree. LGTM.

lib/builtins/CMakeLists.txt

set(arm_EABI_SOURCES
   arm/aeabi_frsub.c
   arm/aeabi_idivmod.S
   arm/aeabi_ldivmod.S
   arm/aeabi_memcmp.S
   arm/aeabi_memcpy.S
   arm/aeabi_memmove.S
   arm/aeabi_memset.S
   arm/aeabi_uidivmod.S
-  arm/aeabi_uldivmod.S)
+  arm/aeabi_uldivmod.S
+  arm/aeabi_uread4.c
+  arm/aeabi_uread8.c
+  arm/aeabi_uwrite4.c
+  arm/aeabi_uwrite8.c)
 
 set(arm_Thumb1_JT_SOURCES
   arm/switch16.S
   arm/switch32.S
   arm/switch8.S
   arm/switchu8.S)
 set(arm_Thumb1_SjLj_EH_SOURCES
   arm/restore_vfp_d8_d15_regs.S

lib/builtins/arm/aeabi_uread4.c

+//===-- aeabi_uread4.c - EABI unaligned read implementation ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+#include "../int_lib.h"
+
+AEABI_RTABI si_int __aeabi_uread4(char* p) {
+    struct unaligned_int {
+      si_int v;
+    } __attribute__((__packed__, __may_alias__));
+    return ((struct unaligned_int*)p)->v;
+}

lib/builtins/arm/aeabi_uread8.c

+//===-- aeabi_uread8.c - EABI unaligned read implementation ---------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+#include "../int_lib.h"
+
+AEABI_RTABI di_int __aeabi_uread8(char* p) {
+    struct unaligned_int {
+      di_int v;
+    } __attribute__((__packed__, __may_alias__));
+    return ((struct unaligned_int*)p)->v;
+}

lib/builtins/arm/aeabi_uwrite4.c

+//===-- aeabi_uwrite4.c - EABI unaligned write implementation -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+#include "../int_lib.h"
+
+AEABI_RTABI void __aeabi_uwrite4(si_int value, char* p) {
+    struct unaligned_int {
+      si_int v;
+    } __attribute__((__packed__, __may_alias__));
+    ((struct unaligned_int*)p)->v = value;
+}
+

lib/builtins/arm/aeabi_uwrite8.c

+//===-- aeabi_uwrite8.c - EABI unaligned write implementation -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+#include "../int_lib.h"
+
+AEABI_RTABI void __aeabi_uwrite8(di_int value, char* p) {
+    struct unaligned_int {
+      di_int v;
+    } __attribute__((__packed__, __may_alias__));
+    ((struct unaligned_int*)p)->v = value;
+}
+