gh-109823: Adjust labels in compiler when removing an empty basic block which is a jump target (#109839)

iritkatriel · web-flow · commit d73c12b88c22 · 2023-09-25T18:25:05.000Z
diff --git a/Lib/test/test_compile.py b/Lib/test/test_compile.py
@@ -1272,6 +1272,11 @@ def f():
             else:
                 1 if 1 else 1
 
+    def test_remove_empty_basic_block_with_jump_target_label(self):
+        # See gh-109823
+        def f(x):
+            while x:
+                0 if 1 else 0
 
 @requires_debug_ranges()
 class TestSourcePositions(unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-09-25-14-28-14.gh-issue-109823.kbVTKF.rst b/Misc/NEWS.d/next/Core and Builtins/2023-09-25-14-28-14.gh-issue-109823.kbVTKF.rst
@@ -0,0 +1,2 @@
+Fix bug where compiler does not adjust labels when removing an empty basic
+block which is a jump target.
diff --git a/Python/flowgraph.c b/Python/flowgraph.c
@@ -960,6 +960,7 @@ eliminate_empty_basic_blocks(cfg_builder *g) {
     while(g->g_entryblock && g->g_entryblock->b_iused == 0) {
         g->g_entryblock = g->g_entryblock->b_next;
     }
+    int next_lbl = get_max_label(g->g_entryblock) + 1;
     for (basicblock *b = g->g_entryblock; b != NULL; b = b->b_next) {
         assert(b->b_iused > 0);
         for (int i = 0; i < b->b_iused; i++) {
@@ -969,7 +970,13 @@ eliminate_empty_basic_blocks(cfg_builder *g) {
                 while (target->b_iused == 0) {
                     target = target->b_next;
                 }
-                instr->i_target = target;
+                if (instr->i_target != target) {
+                    if (!IS_LABEL(target->b_label)) {
+                        target->b_label.id = next_lbl++;
+                    }
+                    instr->i_target = target;
+                    instr->i_oparg = target->b_label.id;
+                }
                 assert(instr->i_target && instr->i_target->b_iused > 0);
             }
         }

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+Fix bug where compiler does not adjust labels when removing an empty basic`
	`2`	`+block which is a jump target.`
Original file line number	Diff line number	Diff line change
`@@ -960,6 +960,7 @@ eliminate_empty_basic_blocks(cfg_builder *g) {`
`960`	`960`	`while(g->g_entryblock && g->g_entryblock->b_iused == 0) {`
`961`	`961`	`g->g_entryblock = g->g_entryblock->b_next;`
`962`	`962`	`}`
	`963`	`+ int next_lbl = get_max_label(g->g_entryblock) + 1;`
`963`	`964`	`for (basicblock *b = g->g_entryblock; b != NULL; b = b->b_next) {`
`964`	`965`	`assert(b->b_iused > 0);`
`965`	`966`	`for (int i = 0; i < b->b_iused; i++) {`
`@@ -969,7 +970,13 @@ eliminate_empty_basic_blocks(cfg_builder *g) {`
`969`	`970`	`while (target->b_iused == 0) {`
`970`	`971`	`target = target->b_next;`
`971`	`972`	`}`
`972`		`- instr->i_target = target;`
	`973`	`+ if (instr->i_target != target) {`
	`974`	`+ if (!IS_LABEL(target->b_label)) {`
	`975`	`+ target->b_label.id = next_lbl++;`
	`976`	`+ }`
	`977`	`+ instr->i_target = target;`
	`978`	`+ instr->i_oparg = target->b_label.id;`
	`979`	`+ }`
`973`	`980`	`assert(instr->i_target && instr->i_target->b_iused > 0);`
`974`	`981`	`}`
`975`	`982`	`}`