gh-109889: fix compiler's redundant NOP detection to look past NOPs with no lineno when looking for the next instruction's lineno (#109987)

Author: iritkatriel

Lib/test/test_compile.py

@@ -1278,6 +1278,11 @@ def f(x):
             while x:
                 0 if 1 else 0
 
+    def test_remove_redundant_nop_edge_case(self):
+        # See gh-109889
+        def f():
+            a if (1 if b else c) else d
+
 @requires_debug_ranges()
 class TestSourcePositions(unittest.TestCase):
     # Ensure that compiled code snippets have correct line and column numbers

Misc/NEWS.d/next/Core and Builtins/2023-09-27-21-35-49.gh-issue-109889.t5hIRT.rst

@@ -0,0 +1,2 @@
+Fix the compiler's redundant NOP detection algorithm to skip over NOPs with
+no line number when looking for the next instruction's lineno.

Python/flowgraph.c

@@ -1017,7 +1017,17 @@ remove_redundant_nops(basicblock *bb) {
                 }
                 /* or if last instruction in BB and next BB has same line number */
                 if (next) {
-                    if (lineno == next->b_instr[0].i_loc.lineno) {
+                    location next_loc = NO_LOCATION;
+                    for (int next_i=0; next_i < next->b_iused; next_i++) {
+                        cfg_instr *instr = &next->b_instr[next_i];
+                        if (instr->i_opcode == NOP && instr->i_loc.lineno == NO_LOCATION.lineno) {
+                            /* Skip over NOPs without location, they will be removed */
+                            continue;
+                        }
+                        next_loc = instr->i_loc;
+                        break;
+                    }
+                    if (lineno == next_loc.lineno) {
                         continue;
                     }
                 }