arkcompiler_runtime_core/docs/cfi_directives.md

CFI directives

Overview

In some situations hand-written assembly code is preferred to compiler-generated, i.e. for performance reasons (usage of special cryptographic/vector instructions) or if there is a need for manual stack/registers manipulation. In case of compiler-generated code compilers emit special debug symbols in resulting binary code and debuggers/profilers use this information.
CFI (Call Frame Information) is a subset of these debug symbols which is responsible for correct stack unwinding. Unfortunately, hand-written assembly lacks of debug symbols and they should be added manually by a programmer.

Introduction to CFI

In case you need basics please read related wiki article.

Bridges annotation

Most asm bridges have similar layout:

• Prolog
• Data preparation for calls
• Calls
• Result handling
• Epilog

From CFI perspective, annotation of prolog and epilog is needed. In prolog we save lr, fp and callee regs on stack. So we should explicitly mark these stack slots with help of CFI directives. Debugger or profiler will use this information in unwinding:

• to find the right previous frame (with fp and lr regs)
• to restore callees in previous frame

In epilog we read saved callees from stack and also fp/lr. Here we annotate that saved registers are restored.

Non-standard bridges

There are bridges which hack stack memory (setting stack pointer to other stack frame), i.e.:

• Deoptimization, all bridges
• OsrEntryAfterCFrame

These bridges return control flow to code which has not called them.

In that case we "say" to debugger that we are not going to return to previous frame. So we directly specify other frame in which we will return + spot slots for callees (it can be callee slots in cframe or boundary frame).