Reapply [GlobalOpt] Remove unreachable blocks before optimizing a function.

This commit reapplies r307215 now that we found out and fixed
the cause of the cfi test failure (in r307871).

llvm-svn: 307920

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index 3d57acf06e7..93eab680ca6 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -2026,6 +2026,24 @@ OptimizeFunctions(Module &M, TargetLibraryInfo *TLI,
       continue;
     }
 
+    // LLVM's definition of dominance allows instructions that are cyclic
+    // in unreachable blocks, e.g.:
+    // %pat = select i1 %condition, @global, i16* %pat
+    // because any instruction dominates an instruction in a block that's
+    // not reachable from entry.
+    // So, remove unreachable blocks from the function, because a) there's
+    // no point in analyzing them and b) GlobalOpt should otherwise grow
+    // some more complicated logic to break these cycles.
+    // Removing unreachable blocks might invalidate the dominator so we
+    // recalculate it.
+    if (!F->isDeclaration()) {
+      if (removeUnreachableBlocks(*F)) {
+        auto &DT = LookupDomTree(*F);
+        DT.recalculate(*F);
+        Changed = true;
+      }
+    }
+
     Changed |= processGlobal(*F, TLI, LookupDomTree);
 
     if (!F->hasLocalLinkage())