SimplifyCFG: don't remove unreachable default switch destinations

An unreachable default destination can be exploited by other optimizations, and
SDag lowering is now prepared to handle them efficiently.

For example, branches to the unreachable destination will be optimized away,
such as in the case of range checks for switch lookup tables.

On 64-bit Linux, this reduces the size of a clang bootstrap by 80 kB (and
Chromium by 30 kB).

llvm-svn: 223050

1 files changed, 0 insertions, 49 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index daa576cfbdc..48d6a81364f 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3120,55 +3120,6 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
           --i; --e;
           Changed = true;
         }
-      // If the default value is unreachable, figure out the most popular
-      // destination and make it the default.
-      if (SI->getDefaultDest() == BB) {
-        std::map<BasicBlock*, std::pair<unsigned, unsigned> > Popularity;
-        for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
-             i != e; ++i) {
-          std::pair<unsigned, unsigned> &entry =
-              Popularity[i.getCaseSuccessor()];
-          if (entry.first == 0) {
-            entry.first = 1;
-            entry.second = i.getCaseIndex();
-          } else {
-            entry.first++;
-          }
-        }
-
-        // Find the most popular block.
-        unsigned MaxPop = 0;
-        unsigned MaxIndex = 0;
-        BasicBlock *MaxBlock = nullptr;
-        for (std::map<BasicBlock*, std::pair<unsigned, unsigned> >::iterator
-             I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
-          if (I->second.first > MaxPop ||
-              (I->second.first == MaxPop && MaxIndex > I->second.second)) {
-            MaxPop = I->second.first;
-            MaxIndex = I->second.second;
-            MaxBlock = I->first;
-          }
-        }
-        if (MaxBlock) {
-          // Make this the new default, allowing us to delete any explicit
-          // edges to it.
-          SI->setDefaultDest(MaxBlock);
-          Changed = true;
-
-          // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
-          // it.
-          if (isa<PHINode>(MaxBlock->begin()))
-            for (unsigned i = 0; i != MaxPop-1; ++i)
-              MaxBlock->removePredecessor(SI->getParent());
-
-          for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
-               i != e; ++i)
-            if (i.getCaseSuccessor() == MaxBlock) {
-              SI->removeCase(i);
-              --i; --e;
-            }
-        }
-      }
     } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
       if (II->getUnwindDest() == BB) {
         // Convert the invoke to a call instruction.  This would be a good