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py/vm.c: Document the SELECTIVE_EXC_IP optimisation. #8870

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py/vm.c: Document the SELECTIVE_EXC_IP optimisation. #8870

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20 changes: 18 additions & 2 deletions py/vm.c
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Original file line number Diff line number Diff line change
Expand Up @@ -202,14 +202,30 @@
// MP_VM_RETURN_YIELD, ip, sp valid, yielded value in *sp
// MP_VM_RETURN_EXCEPTION, exception in state[0]
mp_vm_return_kind_t MICROPY_WRAP_MP_EXECUTE_BYTECODE(mp_execute_bytecode)(mp_code_state_t *code_state, volatile mp_obj_t inject_exc) {

#define SELECTIVE_EXC_IP (0)
// When disabled, code_state->ip is updated unconditionally during op
// dispatch, and this is subsequently used in the exception handler
// (either NLR jump or direct RAISE). This is good for code size because it
// happens in a single place but is more work than necessary, as many opcodes
// cannot raise. Enabling SELECTIVE_EXC_IP means that code_state->ip
// is "selectively" updated only during handling of opcodes that might raise.
// This costs about 360 bytes on PYBV11 for a 1-3% performance gain (e.g. 3%
// in bm_fft.py). On rp2040, there is zero code size diff for a 0-1% gain.
// (Both with computed goto enabled).
#if SELECTIVE_EXC_IP
#define MARK_EXC_IP_SELECTIVE() { code_state->ip = ip; } /* stores ip 1 byte past last opcode */
// Note: Because ip has already been advanced by one byte in the dispatch, the
// value of ip here is one byte past the last opcode.
#define MARK_EXC_IP_SELECTIVE() { code_state->ip = ip; }
// No need to update in dispatch.
#define MARK_EXC_IP_GLOBAL()
#else
#define MARK_EXC_IP_SELECTIVE()
#define MARK_EXC_IP_GLOBAL() { code_state->ip = ip; } /* stores ip pointing to last opcode */
// Immediately before dispatch, save the current ip, which will be the opcode
// about to be dispatched.
#define MARK_EXC_IP_GLOBAL() { code_state->ip = ip; }
#endif

#if MICROPY_OPT_COMPUTED_GOTO
#include "py/vmentrytable.h"
#define DISPATCH() do { \
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