Presenting this for review on behalf of @amharc who is the original author of this patch. This patch handles CFI with basic block sections, which unlike DebugInfo does not support ranges. The DWARF standard explicitly requires emitting separate CFI Frame Descriptor Entries for each contiguous fragment of a function. Thus, the CFI information for all callee-saved registers (possibly including the frame pointer, if necessary) have to be emitted along with redefining the Call Frame Address (CFA), viz. where the current frame starts.
CFI directives are emitted in FDE’s in the object file with a low_pc, high_pc specification. So, a single FDE must point to a contiguous code region unlike debug info which has the support for ranges. This is what complicates CFI for basic block sections.
Now, what happens when we start placing individual basic blocks in unique sections:
- Basic block sections allow the linker to randomly reorder basic blocks in the address space such that a given basic block can become non-contiguous with the original function.
- The different basic block sections can no longer share the cfi_startproc and cfi_endproc directives. So, each basic block section should emit this independently.
- Each (cfi_startproc, cfi_endproc) directive will result in a new FDE that caters to that basic block section.
- Now, this basic block section needs to duplicate the information from the entry block to compute the CFA as it is an independent entity. It cannot refer to the FDE of the original function and hence must duplicate all the stuff that is needed to compute the CFA on its own.
- We are working on a de-duplication patch that can share common information in FDEs in a CIE (Common Information Entry) and we will present this as a follow up patch. This can significantly reduce the duplication overhead and is particularly useful when several basic block sections are created.
- The CFI directives are emitted similarly for registers that are pushed onto the stack, like callee saved registers in the prologue. There are cfi directives that emit how to retrieve the value of the register at that point when the push happened. This has to be duplicated too in a basic block that is floated as a separate section.