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The BEAM instructions for calling a function don't save the
continuation pointer (return address) on the stack, but to a special
BEAM register called CP. It is the responsibility of the called
function to save CP to the stack frame before calling other functions.
In the earlier implementations of BEAM on Sparc, CP was located in a
CPU register. That meant that the continuation pointer was never
written to memory when calling simple functions that didn't call
other functions at all or ended in a tail-call to another function.
The modern BEAM no longer keeps CP in CPU register. Instead, it is
kept in the `process` struct (in `p->cp`). That means the continuation
pointer must be written to the memory on every call, and if the called
function will call other functions, it will must read the continuation
pointer from `p->cp` and store it on the stack.
This commit eliminates the concept of the CP register and modifies
the call instructions to directly store the continuation pointer on
the stack. That makes allocation and trimming of stack frames slightly
faster. A more important benefit is simplification of code that handles
continuation pointers. Because all continuation pointers are now stored
on the stack, the special case of handling `p->cp` disappears.
Co-authored-by: John Högberg <[email protected]>
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The BeamOp() macro in erl_vm.h is clumsy to use. All users
cast the return value to BeamInstr.
Define new macros that are easier to use. In the future,
we might want to pack an operand into the same word as
the pointer to the instruction, so we will define two macros.
BeamIsOpCode() is used to rewrite code like this:
if (Instr == (BeamInstr) BeamOp(op_i_func_info_IaaI) {
...
}
to:
if (BeamIsOpCode(Instr, op_i_func_info_IaaI)) {
...
}
BeamOpCodeAddr(op_apply_bif) is used when we need the address
for an instruction.
Also elimiminate the global variables em_* in beam_emu.c.
They are not really needed. Use the BeamOpCodeAddr() macro
instead.
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The support is somewhat primitive, since it is determined at
call time if trace on return or exception should be sent.
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