Test case showing reassignment block scoping problem

This demonstrates a situation we don't handle well today. The basic structure is that you have some variable defined at the top level, then some control flow like if/switch where _all_ branches reassign the variable, then some code after that references the resulting phi node: ```javascript let x1; // ... mutate/read x1 if (cond) { x2 = {}; } else { x3 = {}; } x4 = phi(x2, x3); ``` We currently group x3, x3, and x4 into a scope together, but note that...there's no `let` declaration for any of those! This means that it looks like the scope for x2 and x3 start in the consequent/alternate, but the true scope spans from before-after the if. I'm inclined to say that LeaveSSA should run _before_ scope analysis, and produce something like the following in this case: ```javascript let x1; // ...mutate/read x1 let x2; // new variable declaration for the new version of x if (cond) { x2 = {}; } else { x2 = {}; } x2; ``` This then allows us to construct a correct range for x2, which starts in the other block.
2025-11-01 09:12:30 +00:00 · 2022-11-22 10:42:36 -08:00
parent 4a3640f25c
commit d2e5c31fc6
2 changed files with 191 additions and 0 deletions
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+
+## Input
+
+```javascript
+function foo(a, b, c) {
+  let x = [];
+  if (a) {
+    x.push(a);
+  }
+  let y = <div>{x}</div>;
+
+  switch (b) {
+    case 0: {
+      x = [];
+      x.push(b);
+      break;
+    }
+    default: {
+      x = [];
+      x.push(c);
+    }
+  }
+  return (
+    <div>
+      {y}
+      {x}
+    </div>
+  );
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$4_@0[1:5] = Array []
+  [2] If (read a$1) then:bb2 else:bb1
+bb2:
+  predecessor blocks: bb0
+  [3] Call mutate x$4_@0.push(read a$1)
+  [4] Goto bb1
+bb1:
+  predecessor blocks: bb2 bb0
+  [5] Const mutate $6_@1 = "div"
+  [6] Let mutate y$5_@2 = JSX <read $6_@1>{freeze x$4_@0}</read $6_@1>
+  [7] Const mutate $7_@4[7:14] = 0
+  [8] Switch (read b$2)
+    Case read $7_@4: bb5
+    Default: bb4
+bb5:
+  predecessor blocks: bb1
+  [9] Reassign mutate x$4_@4[7:14] = Array []
+  [10] Call mutate x$4_@4.push(read b$2)
+  [11] Goto bb3
+bb4:
+  predecessor blocks: bb1
+  [12] Reassign mutate x$4_@4[7:14] = Array []
+  [13] Call mutate x$4_@4.push(read c$3)
+  [14] Goto bb3
+bb3:
+  predecessor blocks: bb5 bb4
+  [15] Const mutate $8_@5 = "div"
+  [16] Const mutate $9_@6 = "\n      "
+  [17] Const mutate $10_@7 = "\n      "
+  [18] Const mutate $11_@8 = "\n    "
+  [19] Const mutate $12_@9 = JSX <read $8_@5>{read $9_@6}{read y$5_@2}{read $10_@7}{freeze x$4_@4}{read $11_@8}</read $8_@5>
+  [20] Return read $12_@9
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$4_@0[1:5] = Array []
+    "]
+    bb0_instrs --> bb0_terminal(["If (read a$1)"])
+  end
+  subgraph bb2
+    bb2_instrs["
+      [3] Call mutate x$4_@0.push(read a$1)
+    "]
+    bb2_instrs --> bb2_terminal(["Goto"])
+  end
+  subgraph bb1
+    bb1_instrs["
+      [5] Const mutate $6_@1 = 'div'
+      [6] Let mutate y$5_@2 = JSX <read $6_@1>{freeze x$4_@0}</read $6_@1>
+      [7] Const mutate $7_@4[7:14] = 0
+    "]
+    bb1_instrs --> bb1_terminal(["Switch (read b$2)"])
+  end
+  subgraph bb5
+    bb5_instrs["
+      [9] Reassign mutate x$4_@4[7:14] = Array []
+      [10] Call mutate x$4_@4.push(read b$2)
+    "]
+    bb5_instrs --> bb5_terminal(["Goto"])
+  end
+  subgraph bb4
+    bb4_instrs["
+      [12] Reassign mutate x$4_@4[7:14] = Array []
+      [13] Call mutate x$4_@4.push(read c$3)
+    "]
+    bb4_instrs --> bb4_terminal(["Goto"])
+  end
+  subgraph bb3
+    bb3_instrs["
+      [15] Const mutate $8_@5 = 'div'
+      [16] Const mutate $9_@6 = '\n      '
+      [17] Const mutate $10_@7 = '\n      '
+      [18] Const mutate $11_@8 = '\n    '
+      [19] Const mutate $12_@9 = JSX <read $8_@5>{read $9_@6}{read y$5_@2}{read $10_@7}{freeze x$4_@4}{read $11_@8}</read $8_@5>
+    "]
+    bb3_instrs --> bb3_terminal(["Return read $12_@9"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "then" --> bb2
+  bb0_terminal -- "else" --> bb1
+  bb2_terminal --> bb1
+  bb1_terminal -- "read $7_@4" --> bb5
+  bb1_terminal -- "default" --> bb4
+  bb1_terminal -- "fallthrough" --> bb3
+  bb5_terminal --> bb3
+  bb4_terminal --> bb3
+
+```
+
+## Code
+
+```javascript
+function foo$0(a$1, b$2, c$3) {
+  let x$4 = [];
+  bb1: if (a$1) {
+    x$4.push(a$1);
+  }
+
+  let y$5 = <div>{x$4}</div>;
+
+  bb3: switch (b$2) {
+    case 0: {
+      x$4 = [];
+      x$4.push(b$2);
+      break bb3;
+    }
+
+    default: {
+      x$4 = [];
+      x$4.push(c$3);
+    }
+  }
+
+  return (
+    <div>
+      {y$5}
+      {x$4}
+    </div>
+  );
+}
+
+```
+      
@@ -0,0 +1,25 @@
+function foo(a, b, c) {
+  let x = [];
+  if (a) {
+    x.push(a);
+  }
+  let y = <div>{x}</div>;
+
+  switch (b) {
+    case 0: {
+      x = [];
+      x.push(b);
+      break;
+    }
+    default: {
+      x = [];
+      x.push(c);
+    }
+  }
+  return (
+    <div>
+      {y}
+      {x}
+    </div>
+  );
+}