[ARM] __cxa_end_cleanup: avoid clobbering r4
The fix for 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.
Reviewed By: danielkiss, logan, MaskRay
Differential Revision: https://reviews.llvm.org/D121432
GitOrigin-RevId: 659029302dfb16a29219f931f66e9ad483ad1445
diff --git a/src/cxa_exception.cpp b/src/cxa_exception.cpp
index 36388d5..059d75c 100644
--- a/src/cxa_exception.cpp
+++ b/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)