Go CFG: merge result-var 'result-init' node into 'result-zero-init'

A named result variable previously created two additional nodes:
'result-zero-init:i' computing the zero value, and 'result-init:i'
writing that value into the result variable. The result-init node
existed only to write the value already computed by result-zero-init.

Make the result-zero-init instruction perform the write itself
(InitResultInstruction is now keyed on result-zero-init with its own
value as RHS) and drop the separate result-init node. All zero-value
constant semantics stay on the same node, so GVN and constant analysis
are unchanged; the only data-flow change is the SSA def relabeling
(result-init -> result-zero-init) with no flow loss.

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphShared.qll

@@ -569,15 +569,9 @@ module GoCfg {
           tag = "param-init:" + i.toString()
         )
         or
-        // Result variable init (on the function body)
-        exists(int i, Go::FuncDef fd |
-          n = fd.getBody() and
-          exists(fd.getBody()) and
-          exists(fd.getResultVar(i)) and
-          tag = "result-init:" + i.toString()
-        )
-        or
-        // Result variable zero init (on the function body)
+        // Result variable zero init + init (on the function body). This single
+        // node computes the zero value and writes it to the result variable (see
+        // `IR::InitResultInstruction`).
         exists(int i, Go::FuncDef fd |
           n = fd.getBody() and
           exists(fd.getBody()) and
@@ -1731,7 +1725,7 @@ module GoCfg {
      * Function definition prologue and epilogue:
      * - Prologue: Before(body) → param-init:-1 → param-init:0 → ... when a
      *             receiver exists; otherwise it starts at param-init:0. Then
-     *             result-zero-init:0 → result-init:0 → ... → first statement
+     *             result-zero-init:0 → result-zero-init:1 → ... → first statement
      * - Epilogue: return → result-read:0 → result-read:1 → ... → result-read:last
      *
      * The last result-read node goes to `NormalExit(fd)` via the shared
@@ -1812,15 +1806,11 @@ module GoCfg {
           )
         )
         or
-        // result-zero-init:j → result-init:j (for each result variable)
+        // result-zero-init:j → next: result-zero-init:(j+1), or Before(body).
+        // The zero-init node also writes the result variable (see
+        // `IR::InitResultInstruction`), so there is no separate result-init node.
         exists(int j | exists(fd.getResultVar(j)) |
           n1.isAdditional(fd.getBody(), "result-zero-init:" + j.toString()) and
-          n2.isAdditional(fd.getBody(), "result-init:" + j.toString())
-        )
-        or
-        // result-init:j → next: result-zero-init:(j+1), or Before(body)
-        exists(int j | exists(fd.getResultVar(j)) |
-          n1.isAdditional(fd.getBody(), "result-init:" + j.toString()) and
           (
             // Next result var exists
             exists(fd.getResultVar(j + 1)) and

go/ql/lib/semmle/go/controlflow/IR.qll

@@ -199,8 +199,6 @@ module IR {
       or
       this instanceof InitParameterInstruction and result = "parameter initialization"
       or
-      this instanceof InitResultInstruction and result = "result initialization"
-      or
       this instanceof GetNextEntryInstruction and result = "next key-value pair"
       or
       this instanceof EvalImplicitTrueInstruction and result = "implicit true"
@@ -1038,22 +1036,24 @@ module IR {
     override ControlFlow::Root getRoot() { result = parm.getFunction() }
   }
 
-  /** An instruction initializing a result variable to its zero value. */
+  /**
+   * An instruction initializing a result variable to its zero value.
+   *
+   * This is the same node as the `EvalImplicitInitInstruction` that computes the
+   * zero value: the zero-value instruction performs the write of the result
+   * variable directly, rather than feeding a separate write node.
+   */
   class InitResultInstruction extends WriteInstruction {
     ResultVariable res;
     int idx;
     FuncDef fd;
 
     InitResultInstruction() {
-      this.isAdditional(fd.getBody(), "result-init:" + idx.toString()) and
+      this.isAdditional(fd.getBody(), "result-zero-init:" + idx.toString()) and
       res = fd.getResultVar(idx)
     }
 
-    override Instruction getRhs() {
-      result
-          .(EvalImplicitInitInstruction)
-          .isAdditional(fd.getBody(), "result-zero-init:" + idx.toString())
-    }
+    override Instruction getRhs() { result = this }
 
     override ControlFlow::Root getRoot() { result = res.getFunction() }
   }
@@ -1173,7 +1173,7 @@ module IR {
       )
       or
       exists(FuncDef fd, int idx |
-        write.isAdditional(fd.getBody(), "result-init:" + idx.toString()) and
+        write.isAdditional(fd.getBody(), "result-zero-init:" + idx.toString()) and
         lhs = fd.getResultVar(idx).getDeclaration()
       )
     } or

go/ql/test/library-tests/semmle/go/controlflow/ControlFlowGraph/ControlFlowNode_getASuccessor.expected

@@ -245,12 +245,10 @@
 | epilogues.go:42:1:48:1 | function declaration | epilogues.go:42:1:48:1 | After function declaration |
 | epilogues.go:42:51:48:1 | block statement | epilogues.go:42:51:48:1 | param-init:0 block statement |
 | epilogues.go:42:51:48:1 | param-init:0 block statement | epilogues.go:42:51:48:1 | result-zero-init:0 block statement |
-| epilogues.go:42:51:48:1 | result-init:0 block statement | epilogues.go:42:51:48:1 | result-zero-init:1 block statement |
-| epilogues.go:42:51:48:1 | result-init:1 block statement | epilogues.go:43:2:46:2 | if statement |
 | epilogues.go:42:51:48:1 | result-read:0 block statement | epilogues.go:42:51:48:1 | result-read:1 block statement |
 | epilogues.go:42:51:48:1 | result-read:1 block statement | epilogues.go:42:1:48:1 | Normal Exit |
-| epilogues.go:42:51:48:1 | result-zero-init:0 block statement | epilogues.go:42:51:48:1 | result-init:0 block statement |
-| epilogues.go:42:51:48:1 | result-zero-init:1 block statement | epilogues.go:42:51:48:1 | result-init:1 block statement |
+| epilogues.go:42:51:48:1 | result-zero-init:0 block statement | epilogues.go:42:51:48:1 | result-zero-init:1 block statement |
+| epilogues.go:42:51:48:1 | result-zero-init:1 block statement | epilogues.go:43:2:46:2 | if statement |
 | epilogues.go:43:2:46:2 | After if statement | epilogues.go:47:2:47:14 | Before return statement |
 | epilogues.go:43:2:46:2 | if statement | epilogues.go:43:5:43:9 | Before ...<... |
 | epilogues.go:43:5:43:5 | After x | epilogues.go:43:9:43:9 | Before 0 |
@@ -293,9 +291,8 @@
 | epilogues.go:51:1:54:1 | function declaration | epilogues.go:51:1:54:1 | After function declaration |
 | epilogues.go:51:38:54:1 | block statement | epilogues.go:51:38:54:1 | param-init:0 block statement |
 | epilogues.go:51:38:54:1 | param-init:0 block statement | epilogues.go:51:38:54:1 | result-zero-init:0 block statement |
-| epilogues.go:51:38:54:1 | result-init:0 block statement | epilogues.go:52:2:52:10 | ... = ... |
 | epilogues.go:51:38:54:1 | result-read:0 block statement | epilogues.go:51:1:54:1 | Normal Exit |
-| epilogues.go:51:38:54:1 | result-zero-init:0 block statement | epilogues.go:51:38:54:1 | result-init:0 block statement |
+| epilogues.go:51:38:54:1 | result-zero-init:0 block statement | epilogues.go:52:2:52:10 | ... = ... |
 | epilogues.go:52:2:52:10 | ... = ... | epilogues.go:52:6:52:10 | Before ...+... |
 | epilogues.go:52:2:52:10 | After ... = ... | epilogues.go:53:2:53:7 | Before return statement |
 | epilogues.go:52:2:52:10 | assign:0 ... = ... | epilogues.go:52:2:52:10 | After ... = ... |
@@ -512,9 +509,8 @@
 | epilogues.go:87:1:98:1 | function declaration | epilogues.go:87:1:98:1 | After function declaration |
 | epilogues.go:87:42:98:1 | block statement | epilogues.go:87:42:98:1 | param-init:0 block statement |
 | epilogues.go:87:42:98:1 | param-init:0 block statement | epilogues.go:87:42:98:1 | result-zero-init:0 block statement |
-| epilogues.go:87:42:98:1 | result-init:0 block statement | epilogues.go:88:2:92:4 | Before defer statement |
 | epilogues.go:87:42:98:1 | result-read:0 block statement | epilogues.go:87:1:98:1 | Normal Exit |
-| epilogues.go:87:42:98:1 | result-zero-init:0 block statement | epilogues.go:87:42:98:1 | result-init:0 block statement |
+| epilogues.go:87:42:98:1 | result-zero-init:0 block statement | epilogues.go:88:2:92:4 | Before defer statement |
 | epilogues.go:88:2:92:4 | After defer statement | epilogues.go:93:2:95:2 | if statement |
 | epilogues.go:88:2:92:4 | Before defer statement | epilogues.go:88:8:92:4 | function call |
 | epilogues.go:88:2:92:4 | defer statement | epilogues.go:88:2:92:4 | After defer statement |
@@ -610,12 +606,10 @@
 | epilogues.go:102:1:110:1 | function declaration | epilogues.go:102:1:110:1 | After function declaration |
 | epilogues.go:102:50:110:1 | block statement | epilogues.go:102:50:110:1 | param-init:0 block statement |
 | epilogues.go:102:50:110:1 | param-init:0 block statement | epilogues.go:102:50:110:1 | result-zero-init:0 block statement |
-| epilogues.go:102:50:110:1 | result-init:0 block statement | epilogues.go:102:50:110:1 | result-zero-init:1 block statement |
-| epilogues.go:102:50:110:1 | result-init:1 block statement | epilogues.go:103:2:103:19 | Before defer statement |
 | epilogues.go:102:50:110:1 | result-read:0 block statement | epilogues.go:102:50:110:1 | result-read:1 block statement |
 | epilogues.go:102:50:110:1 | result-read:1 block statement | epilogues.go:102:1:110:1 | Normal Exit |
-| epilogues.go:102:50:110:1 | result-zero-init:0 block statement | epilogues.go:102:50:110:1 | result-init:0 block statement |
-| epilogues.go:102:50:110:1 | result-zero-init:1 block statement | epilogues.go:102:50:110:1 | result-init:1 block statement |
+| epilogues.go:102:50:110:1 | result-zero-init:0 block statement | epilogues.go:102:50:110:1 | result-zero-init:1 block statement |
+| epilogues.go:102:50:110:1 | result-zero-init:1 block statement | epilogues.go:103:2:103:19 | Before defer statement |
 | epilogues.go:103:2:103:19 | After defer statement | epilogues.go:104:2:106:2 | if statement |
 | epilogues.go:103:2:103:19 | Before defer statement | epilogues.go:103:8:103:19 | call to epiRecover |
 | epilogues.go:103:2:103:19 | defer statement | epilogues.go:103:2:103:19 | After defer statement |
@@ -1292,9 +1286,8 @@
 | exprs.go:49:1:54:1 | function declaration | exprs.go:49:1:54:1 | After function declaration |
 | exprs.go:49:30:54:1 | block statement | exprs.go:49:30:54:1 | param-init:0 block statement |
 | exprs.go:49:30:54:1 | param-init:0 block statement | exprs.go:49:30:54:1 | result-zero-init:0 block statement |
-| exprs.go:49:30:54:1 | result-init:0 block statement | exprs.go:50:2:52:2 | for statement |
 | exprs.go:49:30:54:1 | result-read:0 block statement | exprs.go:49:1:54:1 | Normal Exit |
-| exprs.go:49:30:54:1 | result-zero-init:0 block statement | exprs.go:49:30:54:1 | result-init:0 block statement |
+| exprs.go:49:30:54:1 | result-zero-init:0 block statement | exprs.go:50:2:52:2 | for statement |
 | exprs.go:50:2:52:2 | After for statement | exprs.go:53:2:53:7 | Before return statement |
 | exprs.go:50:2:52:2 | [LoopHeader] for statement | exprs.go:50:27:50:29 | Before increment statement |
 | exprs.go:50:2:52:2 | for statement | exprs.go:50:6:50:11 | ... := ... |
@@ -2143,9 +2136,8 @@
 | main.go:47:1:50:1 | Normal Exit | main.go:47:1:50:1 | Exit |
 | main.go:47:1:50:1 | function declaration | main.go:47:1:50:1 | After function declaration |
 | main.go:47:25:50:1 | block statement | main.go:47:25:50:1 | result-zero-init:0 block statement |
-| main.go:47:25:50:1 | result-init:0 block statement | main.go:48:2:48:12 | ... = ... |
 | main.go:47:25:50:1 | result-read:0 block statement | main.go:47:1:50:1 | Normal Exit |
-| main.go:47:25:50:1 | result-zero-init:0 block statement | main.go:47:25:50:1 | result-init:0 block statement |
+| main.go:47:25:50:1 | result-zero-init:0 block statement | main.go:48:2:48:12 | ... = ... |
 | main.go:48:2:48:12 | ... = ... | main.go:48:11:48:12 | Before 42 |
 | main.go:48:2:48:12 | After ... = ... | main.go:49:2:49:7 | Before return statement |
 | main.go:48:2:48:12 | assign:0 ... = ... | main.go:48:2:48:12 | After ... = ... |
@@ -2160,9 +2152,8 @@
 | main.go:52:1:54:1 | Normal Exit | main.go:52:1:54:1 | Exit |
 | main.go:52:1:54:1 | function declaration | main.go:52:1:54:1 | After function declaration |
 | main.go:52:26:54:1 | block statement | main.go:52:26:54:1 | result-zero-init:0 block statement |
-| main.go:52:26:54:1 | result-init:0 block statement | main.go:53:2:53:7 | Before return statement |
 | main.go:52:26:54:1 | result-read:0 block statement | main.go:52:1:54:1 | Normal Exit |
-| main.go:52:26:54:1 | result-zero-init:0 block statement | main.go:52:26:54:1 | result-init:0 block statement |
+| main.go:52:26:54:1 | result-zero-init:0 block statement | main.go:53:2:53:7 | Before return statement |
 | main.go:53:2:53:7 | Before return statement | main.go:53:2:53:7 | return statement |
 | main.go:53:2:53:7 | return statement | main.go:52:26:54:1 | result-read:0 block statement |
 | main.go:56:1:64:1 | After function declaration | main.go:66:1:90:1 | Before function declaration |
@@ -2172,9 +2163,8 @@
 | main.go:56:1:64:1 | function declaration | main.go:56:1:64:1 | After function declaration |
 | main.go:56:38:64:1 | block statement | main.go:56:38:64:1 | param-init:0 block statement |
 | main.go:56:38:64:1 | param-init:0 block statement | main.go:56:38:64:1 | result-zero-init:0 block statement |
-| main.go:56:38:64:1 | result-init:0 block statement | main.go:57:2:57:11 | ... = ... |
 | main.go:56:38:64:1 | result-read:0 block statement | main.go:56:1:64:1 | Normal Exit |
-| main.go:56:38:64:1 | result-zero-init:0 block statement | main.go:56:38:64:1 | result-init:0 block statement |
+| main.go:56:38:64:1 | result-zero-init:0 block statement | main.go:57:2:57:11 | ... = ... |
 | main.go:57:2:57:11 | ... = ... | main.go:57:11:57:11 | Before 0 |
 | main.go:57:2:57:11 | After ... = ... | main.go:58:2:62:2 | if statement |
 | main.go:57:2:57:11 | assign:0 ... = ... | main.go:57:2:57:11 | After ... = ... |
@@ -2377,12 +2367,10 @@
 | main.go:92:1:96:1 | Normal Exit | main.go:92:1:96:1 | Exit |
 | main.go:92:1:96:1 | function declaration | main.go:92:1:96:1 | After function declaration |
 | main.go:92:36:96:1 | block statement | main.go:92:36:96:1 | result-zero-init:0 block statement |
-| main.go:92:36:96:1 | result-init:0 block statement | main.go:92:36:96:1 | result-zero-init:1 block statement |
-| main.go:92:36:96:1 | result-init:1 block statement | main.go:93:2:93:8 | ... := ... |
 | main.go:92:36:96:1 | result-read:0 block statement | main.go:92:36:96:1 | result-read:1 block statement |
 | main.go:92:36:96:1 | result-read:1 block statement | main.go:92:1:96:1 | Normal Exit |
-| main.go:92:36:96:1 | result-zero-init:0 block statement | main.go:92:36:96:1 | result-init:0 block statement |
-| main.go:92:36:96:1 | result-zero-init:1 block statement | main.go:92:36:96:1 | result-init:1 block statement |
+| main.go:92:36:96:1 | result-zero-init:0 block statement | main.go:92:36:96:1 | result-zero-init:1 block statement |
+| main.go:92:36:96:1 | result-zero-init:1 block statement | main.go:93:2:93:8 | ... := ... |
 | main.go:93:2:93:8 | ... := ... | main.go:93:7:93:8 | Before 23 |
 | main.go:93:2:93:8 | After ... := ... | main.go:94:2:94:15 | ... = ... |
 | main.go:93:2:93:8 | assign:0 ... := ... | main.go:93:2:93:8 | After ... := ... |

go/ql/test/library-tests/semmle/go/dataflow/SSA/VarDefs.expected

@@ -10,9 +10,9 @@
 | main.go:34:19:36:1 | param-init:0 block statement | main.go:34:11:34:11 | x | main.go:34:19:36:1 | param-init:0 block statement |
 | main.go:39:2:39:8 | assign:0 ... := ... | main.go:39:2:39:2 | x | main.go:39:7:39:8 | 23 |
 | main.go:40:2:40:10 | assign:0 ... := ... | main.go:40:2:40:4 | ptr | main.go:40:9:40:10 | &... |
-| main.go:47:25:50:1 | result-init:0 block statement | main.go:47:13:47:18 | result | main.go:47:25:50:1 | result-zero-init:0 block statement |
+| main.go:47:25:50:1 | result-zero-init:0 block statement | main.go:47:13:47:18 | result | main.go:47:25:50:1 | result-zero-init:0 block statement |
 | main.go:48:2:48:12 | assign:0 ... = ... | main.go:47:13:47:18 | result | main.go:48:11:48:12 | 42 |
-| main.go:52:26:54:1 | result-init:0 block statement | main.go:52:14:52:19 | result | main.go:52:26:54:1 | result-zero-init:0 block statement |
+| main.go:52:26:54:1 | result-zero-init:0 block statement | main.go:52:14:52:19 | result | main.go:52:26:54:1 | result-zero-init:0 block statement |
 | main.go:57:6:57:10 | assign:0 value declaration specifier | main.go:57:6:57:6 | x | main.go:57:6:57:10 | zero-init:0 value declaration specifier |
 | main.go:59:3:59:7 | assign:0 ... = ... | main.go:57:6:57:6 | x | main.go:59:7:59:7 | 2 |
 | main.go:63:2:63:7 | assign:0 ... := ... | main.go:63:2:63:2 | y | main.go:63:7:63:7 | 1 |
@@ -23,8 +23,8 @@
 | main.go:73:6:73:11 | assign:0 ... := ... | main.go:73:6:73:6 | i | main.go:73:11:73:11 | 0 |
 | main.go:73:16:73:18 | increment statement | main.go:73:6:73:6 | i | main.go:73:16:73:18 | incdec-rhs increment statement |
 | main.go:74:3:74:7 | assign:0 ... = ... | main.go:72:2:72:2 | z | main.go:74:7:74:7 | 2 |
-| main.go:82:36:86:1 | result-init:0 block statement | main.go:82:18:82:18 | a | main.go:82:36:86:1 | result-zero-init:0 block statement |
-| main.go:82:36:86:1 | result-init:1 block statement | main.go:82:25:82:25 | b | main.go:82:36:86:1 | result-zero-init:1 block statement |
+| main.go:82:36:86:1 | result-zero-init:0 block statement | main.go:82:18:82:18 | a | main.go:82:36:86:1 | result-zero-init:0 block statement |
+| main.go:82:36:86:1 | result-zero-init:1 block statement | main.go:82:25:82:25 | b | main.go:82:36:86:1 | result-zero-init:1 block statement |
 | main.go:83:2:83:8 | assign:0 ... := ... | main.go:83:2:83:2 | x | main.go:83:7:83:8 | 23 |
 | main.go:84:2:84:15 | assign:0 ... = ... | main.go:83:2:83:2 | x | main.go:84:9:84:12 | ...+... |
 | main.go:84:2:84:15 | assign:1 ... = ... | main.go:82:18:82:18 | a | main.go:84:15:84:15 | x |