[SimplifyCFG] Fold `llvm.guard(false)` to unreachable

Summary:
`llvm.guard(false)` always bails out of the current compilation unit, so
we can prune any control flow following it.

Reviewers: hfinkel, pcc, reames

Subscribers: majnemer, reames, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19245

llvm-svn: 266955

1 files changed, 19 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index bd8b7c45d1a..75eda97d0a3 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1310,7 +1310,7 @@ static bool markAliveBlocks(Function &F,
       // Assumptions that are known to be false are equivalent to unreachable.
       // Also, if the condition is undefined, then we make the choice most
       // beneficial to the optimizer, and choose that to also be unreachable.
-      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI))
+      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI)) {
         if (II->getIntrinsicID() == Intrinsic::assume) {
           bool MakeUnreachable = false;
           if (isa<UndefValue>(II->getArgOperand(0)))
@@ -1327,6 +1327,24 @@ static bool markAliveBlocks(Function &F,
           }
         }
 
+        if (II->getIntrinsicID() == Intrinsic::experimental_guard) {
+          // A call to the guard intrinsic bails out of the current compilation
+          // unit if the predicate passed to it is false.  If the predicate is a
+          // constant false, then we know the guard will bail out of the current
+          // compile unconditionally, so all code following it is dead.
+          //
+          // Note: unlike in llvm.assume, it is not "obviously profitable" for
+          // guards to treat `undef` as `false` since a guard on `undef` can
+          // still be useful for widening.
+          if (auto *CI = dyn_cast<ConstantInt>(II->getArgOperand(0)))
+            if (CI->isZero() && !isa<UnreachableInst>(II->getNextNode())) {
+              changeToUnreachable(II->getNextNode(), /*UseLLVMTrap=*/ false);
+              Changed = true;
+              break;
+            }
+        }
+      }
+
       if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
         if (CI->doesNotReturn()) {
           // If we found a call to a no-return function, insert an unreachable