[libcxx-commits] [PATCH] D121432: [ARM] __cxa_end_cleanup: avoid clobbering r4

[Ryan Prichard via Phabricator via libcxx-commits]

rprichard created this revision.
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The fix for D111703 <https://reviews.llvm.org/D111703> clobbered r4 both to:

- Save/restore the original lr.
- Load the address of _Unwind_Resume for LIBCXXABI_BAREMETAL.

This patch saves and restores lr without clobbering any extra
registers.

For LIBCXXABI_BAREMETAL, it is still necessary to clobber one extra
register to hold the address of _Unwind_Resume, but it seems better to
use ip/r12 (intended for linker veneers/trampolines) than r4 for this
purpose.

The function also clobbers r0 for the _Unwind_Resume function's
parameter, but that is unavoidable.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121432

Files:
  libcxxabi/src/cxa_exception.cpp


Index: libcxxabi/src/cxa_exception.cpp
===================================================================
--- libcxxabi/src/cxa_exception.cpp
+++ libcxxabi/src/cxa_exception.cpp
@@ -341,10 +341,11 @@
 According to ARM EHABI 8.4.1, __cxa_end_cleanup() should not clobber any
 register, thus we have to write this function in assembly so that we can save
 {r1, r2, r3}.  We don't have to save r0 because it is the return value and the
-first argument to _Unwind_Resume().  In addition, we are saving lr in order to
-align the stack to 16 bytes and lr will be used to identify the caller and its
-frame information. _Unwind_Resume never return and we need to keep the original
-lr so just branch to it.
+first argument to _Unwind_Resume().  The function also saves/restores r4 to
+keep the stack aligned and to provide a temp register.  _Unwind_Resume never
+returns and we need to keep the original lr so just branch to it.  When
+targeting bare metal, the function also clobbers ip/r12 to hold the address of
+_Unwind_Resume, which may be too far away for an ordinary branch.
 */
 __attribute__((used)) static _Unwind_Exception *
 __cxa_end_cleanup_impl()
@@ -381,15 +382,19 @@
 #if defined(__ARM_FEATURE_BTI_DEFAULT)
     "	bti\n"
 #endif
-    "	push	{r1, r2, r3, lr}\n"
+    "	push	{r1, r2, r3, r4}\n"
+    "	mov	r4, lr\n"
     "	bl	__cxa_end_cleanup_impl\n"
-    "	pop	{r1, r2, r3, r4}\n"
     "	mov	lr, r4\n"
 #if defined(LIBCXXABI_BAREMETAL)
     "	ldr	r4,	=_Unwind_Resume\n"
-    "	bx	r4\n"
+    "	mov	ip,	r4\n"
+#endif
+    "	pop	{r1, r2, r3, r4}\n"
+#if defined(LIBCXXABI_BAREMETAL)
+    "	bx	ip\n"
 #else
-    "   b _Unwind_Resume\n"
+    "	b	_Unwind_Resume\n"
 #endif
     "	.popsection");
 #endif // defined(_LIBCXXABI_ARM_EHABI)


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