Fix non-determinism introduced in r168970 and pointed out by Duncan.

We're iterating over a non-deterministically ordered container looking
for two saturating flags. To do this correctly, we have to saturate
both, and only stop looping if both saturate to their final value.
Otherwise, which flag we see first changes the result.

This is also a micro-optimization of the previous version as now we
don't go into the (possibly expensive) test logic once the first
violation of either constraint is detected.

llvm-svn: 168989

1 files changed, 13 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 5a1b21025cc..f6d43a9e6ce 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3521,12 +3521,20 @@ static bool ShouldBuildLookupTable(SwitchInst *SI,
   for (SmallDenseMap<PHINode*, Type*>::const_iterator I = ResultTypes.begin(),
        E = ResultTypes.end(); I != E; ++I) {
     Type *Ty = I->second;
-    if (!TTI->getScalarTargetTransformInfo()->isTypeLegal(Ty))
-      HasIllegalType = true;
-    if (!SwitchLookupTable::WouldFitInRegister(TD, TableSize, Ty)) {
-      AllTablesFitInRegister = false;
+
+    // Saturate this flag to true.
+    HasIllegalType = HasIllegalType ||
+      !TTI->getScalarTargetTransformInfo()->isTypeLegal(Ty);
+
+    // Saturate this flag to false.
+    AllTablesFitInRegister = AllTablesFitInRegister &&
+      SwitchLookupTable::WouldFitInRegister(TD, TableSize, Ty);
+
+    // If both flags saturate, we're done. NOTE: This *only* works with
+    // saturating flags, and all flags have to saturate first due to the
+    // non-deterministic behavior of iterating over a dense map.
+    if (HasIllegalType && !AllTablesFitInRegister)
       break;
-    }
   }
 
   // If each table would fit in a register, we should build it anyway.