CaptureTracking: Plug a loophole in the "too many uses" heuristic.

The heuristic was added to avoid spending too much compile time A specially
crafted test case (PR17461, PR16474) with many uses on a select or bitcast
instruction can still trigger the slow case. Add a check for that case.

This only affects compile time, don't have a good way to test it.

llvm-svn: 191896

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/CaptureTracking.cpp b/llvm/lib/Analysis/CaptureTracking.cpp
index 9eb76a8..79fab1b 100644
--- a/llvm/lib/Analysis/CaptureTracking.cpp
+++ b/llvm/lib/Analysis/CaptureTracking.cpp
@@ -146,8 +146,14 @@ void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
     case Instruction::PHI:
     case Instruction::Select:
       // The original value is not captured via this if the new value isn't.
+      Count = 0;
       for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
            UI != UE; ++UI) {
+        // If there are lots of uses, conservatively say that the value
+        // is captured to avoid taking too much compile time.
+        if (Count++ >= Threshold)
+          return Tracker->tooManyUses();
+
         Use *U = &UI.getUse();
         if (Visited.insert(U))
           if (Tracker->shouldExplore(U))