Bug 1246229 - Enable DCE to remove OSR guards if their values are optimized-out. r=h4writer

js/src/jit-test/tests/ion/osr-with-optimized-out.js

@@ -0,0 +1,22 @@
+// |jit-test| --ion-offthread-compile=off;
+
+// We disable any off-main thread compilation, and set a definite trigger for
+// Ion compilation, such that we can garantee that we would OSR into the inner
+// loop before we reach the end of the loop.
+setJitCompilerOption("ion.warmup.trigger", 30);
+
+function f (n) {
+    while (!inIon()) {
+        var inner = 0;
+        let x = {};
+        for (var i = 0; i < n; i++) {
+            inner += inIon() == true ? 1 : 0;
+            if (inner <= 1)
+                bailout();
+        }
+        assertEq(inner != 1, true);
+    }
+}
+
+// Iterate enough to ensure that we OSR in this inner loop.
+f(300);

js/src/jit/IonAnalysis.cpp

@@ -950,7 +950,9 @@ jit::EliminateDeadResumePointOperands(MIRGenerator* mir, MIRGraph& graph)
 bool
 js::jit::DeadIfUnused(const MDefinition* def)
 {
-    return !def->isEffectful() && !def->isGuard() && !def->isGuardRangeBailouts() &&
+    return !def->isEffectful() &&
+           (!def->isGuard() || def->block() == def->block()->graph().osrBlock()) &&
+           !def->isGuardRangeBailouts() &&
            !def->isControlInstruction() &&
            (!def->isInstruction() || !def->toInstruction()->resumePoint());
 }