[SimlifyCFG] Prevent passes from destroying canonical loop structure, especially for nested loops

    
When eliminating or merging almost empty basic blocks, the existence of non-trivial PHI nodes
is currently used to recognize potential loops of which the block is the header and keep the block.
However, the current algorithm fails if the loops' exit condition is evaluated only with volatile
values hence no PHI nodes in the header. Especially when such a loop is an outer loop of a nested
loop, the loop is collapsed into a single loop which prevent later optimizations from being
applied (e.g., transforming nested loops into simplified forms and loop vectorization).
    
The patch augments the existing PHI node-based check by adding a pre-test if the BB actually
belongs to a set of loop headers and not eliminating it if yes.

llvm-svn: 264697

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
index 687d388701e..30fbe047190 100644
--- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
+++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
@@ -28,6 +28,7 @@
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
@@ -131,12 +132,19 @@ static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
                                    unsigned BonusInstThreshold) {
   bool Changed = false;
   bool LocalChange = true;
+
+  SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
+  FindFunctionBackedges(F, Edges);
+  SmallPtrSet<BasicBlock *, 16> LoopHeaders;
+  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
+    LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
+
   while (LocalChange) {
     LocalChange = false;
 
     // Loop over all of the basic blocks and remove them if they are unneeded.
     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
-      if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC)) {
+      if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC, &LoopHeaders)) {
         LocalChange = true;
         ++NumSimpl;
       }