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[AArch64] Async unwind - Fix MTE codegen emitting frame adjustments i…
…n a loop When untagging the stack, the compiler may emit a sequence like: ``` .LBB0_1: st2g sp, [sp], #32 sub x8, x8, #32 cbnz x8, .LBB0_1 stg sp, [sp], #16 ``` These stack adjustments cannot be described by CFI instructions. This patch disables merging of SP update with untagging, i.e. makes the compiler use an additional scratch register (there should be plenty available at this point as we are in the epilogue) and generate: ``` mov x9, sp mov x8, #256 stg x9, [x9], #16 .LBB0_1: sub x8, x8, #32 st2g x9, [x9], #32 cbnz x8, .LBB0_1 add sp, sp, #272 ``` Merging is disabled only when we need to generate asynchronous unwind tables. Reviewed By: eugenis Differential Revision: https://reviews.llvm.org/D114548
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