gh-92619: Fix bug where the compiler duplicates exit blocks unnecessarily (GH-92620)

Author: iritkatriel

Lib/test/test_dis.py

@@ -1490,7 +1490,7 @@ def _prepare_test_cases():
   Instruction(opname='CALL', opcode=171, arg=1, argval=1, argrepr='', offset=368, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='POP_TOP', opcode=1, arg=None, argval=None, argrepr='', offset=378, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='POP_EXCEPT', opcode=89, arg=None, argval=None, argrepr='', offset=380, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='JUMP_FORWARD', opcode=110, arg=21, argval=426, argrepr='to 426', offset=382, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='JUMP_FORWARD', opcode=110, arg=4, argval=392, argrepr='to 392', offset=382, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='RERAISE', opcode=119, arg=0, argval=0, argrepr='', offset=384, starts_line=22, is_jump_target=True, positions=None),
   Instruction(opname='COPY', opcode=120, arg=3, argval=3, argrepr='', offset=386, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='POP_EXCEPT', opcode=89, arg=None, argval=None, argrepr='', offset=388, starts_line=None, is_jump_target=False, positions=None),
@@ -1502,24 +1502,16 @@ def _prepare_test_cases():
   Instruction(opname='POP_TOP', opcode=1, arg=None, argval=None, argrepr='', offset=420, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CONST', opcode=100, arg=0, argval=None, argrepr='None', offset=422, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='RETURN_VALUE', opcode=83, arg=None, argval=None, argrepr='', offset=424, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='NOP', opcode=9, arg=None, argval=None, argrepr='', offset=426, starts_line=23, is_jump_target=True, positions=None),
-  Instruction(opname='LOAD_GLOBAL', opcode=116, arg=3, argval='print', argrepr='NULL + print', offset=428, starts_line=28, is_jump_target=False, positions=None),
+  Instruction(opname='PUSH_EXC_INFO', opcode=35, arg=None, argval=None, argrepr='', offset=426, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='LOAD_GLOBAL', opcode=116, arg=3, argval='print', argrepr='NULL + print', offset=428, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='LOAD_CONST', opcode=100, arg=10, argval="OK, now we're done", argrepr='"OK, now we\'re done"', offset=440, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='PRECALL', opcode=166, arg=1, argval=1, argrepr='', offset=442, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='CALL', opcode=171, arg=1, argval=1, argrepr='', offset=446, starts_line=None, is_jump_target=False, positions=None),
   Instruction(opname='POP_TOP', opcode=1, arg=None, argval=None, argrepr='', offset=456, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=0, argval=None, argrepr='None', offset=458, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='RETURN_VALUE', opcode=83, arg=None, argval=None, argrepr='', offset=460, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='PUSH_EXC_INFO', opcode=35, arg=None, argval=None, argrepr='', offset=462, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_GLOBAL', opcode=116, arg=3, argval='print', argrepr='NULL + print', offset=464, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='LOAD_CONST', opcode=100, arg=10, argval="OK, now we're done", argrepr='"OK, now we\'re done"', offset=476, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='PRECALL', opcode=166, arg=1, argval=1, argrepr='', offset=478, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='CALL', opcode=171, arg=1, argval=1, argrepr='', offset=482, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='POP_TOP', opcode=1, arg=None, argval=None, argrepr='', offset=492, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='RERAISE', opcode=119, arg=0, argval=0, argrepr='', offset=494, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='COPY', opcode=120, arg=3, argval=3, argrepr='', offset=496, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='POP_EXCEPT', opcode=89, arg=None, argval=None, argrepr='', offset=498, starts_line=None, is_jump_target=False, positions=None),
-  Instruction(opname='RERAISE', opcode=119, arg=1, argval=1, argrepr='', offset=500, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='RERAISE', opcode=119, arg=0, argval=0, argrepr='', offset=458, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='COPY', opcode=120, arg=3, argval=3, argrepr='', offset=460, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='POP_EXCEPT', opcode=89, arg=None, argval=None, argrepr='', offset=462, starts_line=None, is_jump_target=False, positions=None),
+  Instruction(opname='RERAISE', opcode=119, arg=1, argval=1, argrepr='', offset=464, starts_line=None, is_jump_target=False, positions=None),
 ]
 
 # One last piece of inspect fodder to check the default line number handling

Misc/NEWS.d/next/Core and Builtins/2022-05-10-11-34-35.gh-issue-92619.u0V0lY.rst

@@ -0,0 +1 @@
+Make the compiler duplicate an exit block only if none of its instructions have a lineno (previously only the first instruction in the block was checked, leading to unnecessarily duplicated blocks).

Python/compile.c

@@ -9280,7 +9280,15 @@ trim_unused_consts(struct compiler *c, struct assembler *a, PyObject *consts)
 
 static inline int
 is_exit_without_lineno(basicblock *b) {
-    return b->b_exit && b->b_instr[0].i_lineno < 0;
+    if (!b->b_exit) {
+        return 0;
+    }
+    for (int i = 0; i < b->b_iused; i++) {
+        if (b->b_instr[i].i_lineno >= 0) {
+            return 0;
+        }
+    }
+    return 1;
 }
 
 /* PEP 626 mandates that the f_lineno of a frame is correct