[3.12] GH-127953: Make line number lookup O(1) regardless of the size of the code object (GH-129127)

Include/cpython/code.h

@@ -76,11 +76,12 @@ typedef struct {
 } _PyCoCached;
 
 /* Ancillary data structure used for instrumentation.
-   Line instrumentation creates an array of
-   these. One entry per code unit.*/
+   Line instrumentation creates this with sufficient
+   space for one entry per code unit. The total size
+   of the data will be `bytes_per_entry * Py_SIZE(code)` */
 typedef struct {
-    uint8_t original_opcode;
-    int8_t line_delta;
+    uint8_t bytes_per_entry;
+    uint8_t data[1];
 } _PyCoLineInstrumentationData;
 
 /* Main data structure used for instrumentation.

Misc/NEWS.d/next/Core_and_Builtins/2024-12-30-15-49-31.gh-issue-127953.B4_6L9.rst

@@ -0,0 +1,2 @@
+The time to handle a ``LINE`` event in sys.monitoring (and sys.settrace) is
+now independent of the number of lines in the code object.

Objects/codeobject.c

@@ -853,6 +853,9 @@ PyCode_Addr2Line(PyCodeObject *co, int addrq)
     if (addrq < 0) {
         return co->co_firstlineno;
     }
+    if (co->_co_monitoring && co->_co_monitoring->lines) {
+        return _Py_Instrumentation_GetLine(co, addrq/sizeof(_Py_CODEUNIT));
+    }
     assert(addrq >= 0 && addrq < _PyCode_NBYTES(co));
     PyCodeAddressRange bounds;
     _PyCode_InitAddressRange(co, &bounds);

Python/ceval.c

@@ -860,17 +860,25 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
     {
         _Py_CODEUNIT *prev = frame->prev_instr;
         _Py_CODEUNIT *here = frame->prev_instr = next_instr;
-        _PyFrame_SetStackPointer(frame, stack_pointer);
-        int original_opcode = _Py_call_instrumentation_line(
-                tstate, frame, here, prev);
-        stack_pointer = _PyFrame_GetStackPointer(frame);
-        if (original_opcode < 0) {
-            next_instr = here+1;
-            goto error;
+        int original_opcode = 0;
+        if (tstate->tracing) {
+            PyCodeObject *code = frame->f_code;
+            int index = (int)(here - _PyCode_CODE(code));
+            original_opcode = code->_co_monitoring->lines->data[index*code->_co_monitoring->lines->bytes_per_entry];
         }
-        next_instr = frame->prev_instr;
-        if (next_instr != here) {
-            DISPATCH();
+        else {
+            _PyFrame_SetStackPointer(frame, stack_pointer);
+            original_opcode = _Py_call_instrumentation_line(
+                    tstate, frame, here, prev);
+            stack_pointer = _PyFrame_GetStackPointer(frame);
+            if (original_opcode < 0) {
+                next_instr = here+1;
+                goto error;
+            }
+            next_instr = frame->prev_instr;
+            if (next_instr != here) {
+                DISPATCH();
+            }
         }
         if (_PyOpcode_Caches[original_opcode]) {
             _PyBinaryOpCache *cache = (_PyBinaryOpCache *)(next_instr+1);