Bug 1677417 - treat this as a parameter for purposes of live range …

…r=jonco

Differential Revision: https://phabricator.services.mozilla.com/D97198

js/src/devtools/rootAnalysis/analyzeRoots.js

@@ -631,7 +631,7 @@ function findGCBeforeValueUse(start_body, start_point, suppressed, variable)
                 // iteration.
                 worklist.push({body: body, ppoint: body.Index[1],
                                gcInfo: gcInfo, why: entry});
-            } else if (variable.Kind == "Arg" && gcInfo) {
+            } else if ((variable.Kind == "Arg" || variable.Kind == "This") && gcInfo) {
                 // The scope of arguments starts at the beginning of the
                 // function
                 return entry;

js/src/devtools/rootAnalysis/t/hazards/source.cpp

@@ -316,3 +316,11 @@ void safevals() {
     consume(std::move(unsafe10));
   }
 }
+
+// Make sure `this` is live at the beginning of a function.
+class Subcell : public Cell {
+  int method() {
+    GC();
+    return f; // this->f
+  }
+};

js/src/devtools/rootAnalysis/t/hazards/test.py

@@ -1,5 +1,7 @@
 # flake8: noqa: F821
 
+from collections import defaultdict
+
 test.compile("source.cpp")
 test.run_analysis_script("gcTypes")
 
@@ -10,6 +12,7 @@
 assert "void suppressedFunction()" not in gcFunctions
 assert "void halfSuppressedFunction()" in gcFunctions
 assert "void unsuppressedFunction()" in gcFunctions
+assert "int32 Subcell::method()" in gcFunctions
 assert "Cell* f()" in gcFunctions
 
 hazards = test.load_hazards()
@@ -21,11 +24,12 @@
 assert "cell5" not in hazmap
 assert "cell6" not in hazmap
 assert "<returnvalue>" in hazmap
+assert "this" in hazmap
 
-# All hazards should be in f(), loopy(), and safevals()
+# All hazards should be in f(), loopy(), safevals(), and method()
 assert hazmap["cell2"].function == "Cell* f()"
 print(len(set(haz.function for haz in hazards)))
-assert len(set(haz.function for haz in hazards)) == 3
+assert len(set(haz.function for haz in hazards)) == 4
 
 # Check that the correct GC call is reported for each hazard. (cell3 has a
 # hazard from two different GC calls; it doesn't really matter which is
@@ -43,6 +47,7 @@
 # Type names are handy to have in the report.
 assert hazmap["cell2"].type == "Cell*"
 assert hazmap["<returnvalue>"].type == "Cell*"
+assert hazmap["this"].type == "Subcell*"
 
 # loopy hazards. See comments in source.
 assert "haz1" not in hazmap
@@ -66,3 +71,13 @@
 assert "safe8" not in hazmap
 assert "safe9" not in hazmap
 assert "safe10" not in hazmap
+
+# method hazard.
+
+byfunc = defaultdict(lambda: defaultdict(dict))
+for haz in hazards:
+    byfunc[haz.function][haz.variable] = haz
+
+methhaz = byfunc["int32 Subcell::method()"]
+assert "this" in methhaz
+assert methhaz["this"].type == "Subcell*"