struct CallFrame

Each frame records its caller's FP and PC (return address). A call pushes

the PC and the prologue then pushes the caller's FP (x86), or the prologue

pushes the return-address register and PC together (other CPUs). Since the

stack grows down, the PC is always just after the FP in memory. It then

sets the FP to point at (or above) the FP, PC pair just pushed. On x86 it's

unavoidable that the FP is two words below the CFA (SP at call site), since

the call itself puts the PC there; the FP points directly to the FP, PC pair

describing the caller. On ARM, the compiler will often place the FP, PC

pair at the bottom of the new frame instead of the top; the FP points

directly to the FP, PC pair describing the caller, but there's no guarantee

where the FP is in relation to the CFA. On RISC-V, the FP is set to the CFA

(SP at call site / entry); so it points *just past* the FP, PC pair

describing the caller, but it's also guaranteed to be the CFA.