[compiler-rt] [builtins] Implement __chkstk for arm64 windows

[mstorsjo]

Summary

Do a trivial implementation of the function, similar-ish to the existing one for x86.

[compnerd]

lib/builtins/aarch64/chkstk.S

15

Im not sure what the point of this check is. This is an AArch64 source so would only be compiled for that target.

[mgorny]

lib/builtins/aarch64/chkstk.S

15

We do this in all (most?) of the files for consistency. There were historically cases when tools compiled all files found in the directory — it mostly applies to tests in pre-lit times.

[compnerd]

lib/builtins/aarch64/chkstk.S

19

I'm not sure I understand the reasoning for this entirely. Why the lsl x16, x15, #4 and then subs x16, x16, #1000? Wouldn't it be equal to just doing subs x15, x15, #100 each time and entirely avoid the shift?

25

Can we not use PAGE_SIZE here?

[mstorsjo]

lib/builtins/aarch64/chkstk.S

19

Yes - except we need to preserve x15 on return as well. So since we need to clobber another register, it doesn't matter which range we use for the subtractions.

25

Afaik there's no global PAGE_SIZE define, but we could make one within this file and use that for better readability.

[mstorsjo]

Added some more comments about what registers are clobbered, using a define for the page size. Removed the assumption that we don't need to touch the final page, to be on the safe side (in case a caller would do two of these calls consecutively without touching the stack inbetween).

[compnerd]

Bah, I forgot that __chkstk is special and the normal preserved/clobbered register set is not the same.

lib/builtins/CMakeLists.txt

set(thumb1_SOURCES
     ${thumb1_SOURCES}
     ${arm_EABI_SOURCES})
 endif()
 
 set(aarch64_SOURCES
   ${GENERIC_TF_SOURCES}
   ${GENERIC_SOURCES})
 
+if (MINGW)
+  set(aarch64_SOURCES
+      ${aarch64_SOURCES}
+      aarch64/chkstk.S)
+endif()
+
 set(armhf_SOURCES ${arm_SOURCES})
 set(armv7_SOURCES ${arm_SOURCES})
 set(armv7s_SOURCES ${arm_SOURCES})
 set(armv7k_SOURCES ${arm_SOURCES})
 set(arm64_SOURCES ${aarch64_SOURCES})
 
 # macho_embedded archs
 set(armv6m_SOURCES ${thumb1_SOURCES})

lib/builtins/aarch64/chkstk.S

+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+
+#include "../assembly.h"
+
+// __chkstk routine
+// This routine is windows specific.
+// http://msdn.microsoft.com/en-us/library/ms648426.aspx
+
+// This clobbers registers x16 and x17.
+// Does not modify any memory or the stack pointer.
+
+//      mov     x15, #256 // Number of bytes of stack, in units of 16 byte
+//      bl      __chkstk
+//      sub     sp, sp, x15, lsl #4
+
+#ifdef __aarch64__
+
+#define PAGE_SIZE 4096
+
+        .p2align 2
+DEFINE_COMPILERRT_FUNCTION(__chkstk)
+        lsl    x16, x15, #4
+        mov    x17, sp
+1:
+        sub    x17, x17, #PAGE_SIZE
+        subs   x16, x16, #PAGE_SIZE
+        ldr    xzr, [x17]
+        b.gt   1b
+
+        ret
+END_COMPILERRT_FUNCTION(__chkstk)
+
+#endif // __aarch64__