StructurizeCFG: Test for branch divergence correctly

Summary:
This fixes cases like the new test @nonuniform. In that test, %cc itself
is a uniform value; however, when reading it after the end of the loop in
basic block %if, its value is effectively non-uniform.

This problem was encountered in
https://bugs.freedesktop.org/show_bug.cgi?id=103743; however, this change
in itself is not sufficient to fix that bug, as there is another issue
in the AMDGPU backend.

Change-Id: I32bbffece4a32f686fab54964dae1a5dd72949d4

Reviewers: arsenm, rampitec, jlebar

Subscribers: wdng, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D40546

llvm-svn: 325881

1 files changed, 12 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
index b8fb80b6cc2..dc4d1f73f87 100644
--- a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
@@ -55,6 +55,12 @@ static const char *const FlowBlockName = "Flow";
 
 namespace {
 
+static cl::opt<bool> ForceSkipUniformRegions(
+  "structurizecfg-skip-uniform-regions",
+  cl::Hidden,
+  cl::desc("Force whether the StructurizeCFG pass skips uniform regions"),
+  cl::init(false));
+
 // Definition of the complex types used in this pass.
 
 using BBValuePair = std::pair<BasicBlock *, Value *>;
@@ -242,8 +248,11 @@ class StructurizeCFG : public RegionPass {
 public:
   static char ID;
 
-  explicit StructurizeCFG(bool SkipUniformRegions = false)
-      : RegionPass(ID), SkipUniformRegions(SkipUniformRegions) {
+  explicit StructurizeCFG(bool SkipUniformRegions_ = false)
+      : RegionPass(ID),
+        SkipUniformRegions(SkipUniformRegions_) {
+    if (ForceSkipUniformRegions.getNumOccurrences())
+      SkipUniformRegions = ForceSkipUniformRegions.getValue();
     initializeStructurizeCFGPass(*PassRegistry::getPassRegistry());
   }
 
@@ -885,7 +894,7 @@ static bool hasOnlyUniformBranches(const Region *R,
     if (!Br || !Br->isConditional())
       continue;
 
-    if (!DA.isUniform(Br->getCondition()))
+    if (!DA.isUniform(Br))
       return false;
     DEBUG(dbgs() << "BB: " << BB->getName() << " has uniform terminator\n");
   }