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[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
This commit is contained in:
Ryan Prichard 2022-03-14 15:43:50 -07:00
parent 56e7d6bd44
commit 659029302d
1 changed files with 13 additions and 8 deletions
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21
libcxxabi/src/cxa_exception.cpp
View File

@ -341,10 +341,11 @@ unwinding with _Unwind_Resume.
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 @@ asm(" .pushsection .text.__cxa_end_cleanup,\"ax\",%progbits\n"
#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)