[llvm-commits] [Patch] Make GlobalOpt conservative with TLS vars (PR14309, 3.2-blocker)

[Eli Friedman]

On Mon, Nov 12, 2012 at 8:20 AM, Hans Wennborg <hans at chromium.org> wrote:
> Hi all,
>
> The attached patch takes a stab at fixing PR14309 which was marked as
> a 3.2-blocker.
>
> For global variables which get the same value stored into them
> everywhere, GlobalOpt will replace them with a constant. The problem
> is that a thread-local GlobalVariable looks like one value, but is
> different between threads.
>
> My patch introduces Constant::isThreadDependent() which returns true
> for thread-local variables and constants which depend on them (e.g. a
> GEP into a thread-local array), and teaches GlobalOpt not to track
> such values.
>
> Please take a look!
>
> Also, if anyone thinks there are more passes which assume that
> thread-local GlobalVariables have the same value everywhere, please
> let me know.


+/// isThreadDependent - Return true if the value can vary between threads.
+bool Constant::isThreadDependent() const {
+  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(this))
+    return GV->isThreadLocal();
+
+  for (unsigned I = 0, E = getNumOperands(); I != E; ++I) {
+    if (const Constant *C = dyn_cast<Constant>(getOperand(I))) {
+      if (C->isThreadDependent())
+        return true;
+    }
+  }
+
+  return false;
+}

Dumb recursion is exponential time over an arbitrary ConstantExpr.

-Eli