1# CFI directives 2 3## Overview 4 5In 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. 6In case of **compiler-generated** code compilers emit special **debug symbols** in resulting binary code and debuggers/profilers use this information. 7`CFI` (**Call Frame Information**) is a subset of these debug symbols which is responsible for correct **stack unwinding**. 8Unfortunately, hand-written assembly lacks of debug symbols and they should be added manually by a programmer. 9 10## Introduction to CFI 11 12In case you need basics please read related wiki article. 13 14## Bridges annotation 15 16Most `asm` bridges have similar layout: 17- Prolog 18- Data preparation for calls 19- Calls 20- Result handling 21- Epilog 22 23From `CFI` perspective, annotation of `prolog` and `epilog` is needed. 24In prolog we save `lr`, `fp` and `callee` regs on stack. 25So we should explicitly mark these stack slots with help of `CFI` directives. 26`Debugger` or `profiler` will use this information in unwinding: 27 - to find the right previous `frame` (with `fp` and `lr` regs) 28 - to restore `callees` in previous frame 29 30In epilog we read saved `callees` from stack and also `fp`/`lr`. Here we annotate that saved registers are restored. 31 32## Non-standard bridges 33 34There are bridges which `hack` stack memory (setting `stack pointer` to other stack frame), i.e.: 35- Deoptimization, all bridges 36- OsrEntryAfterCFrame 37 38These bridges return control flow to code which has not called them. 39 40 41In 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`). 42
