Mark guards on true as "trivially dead"

This moves some logic added to EarlyCSE in rL268120 into
`llvm::isInstructionTriviallyDead`.  Adds a test case for DCE to
demonstrate that passes other than EarlyCSE can now pick up on the new
information.

llvm-svn: 268126

1 files changed, 2 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index c453c843bfc..2e00d676f15 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -555,15 +555,8 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
     }
 
     if (match(Inst, m_Intrinsic<Intrinsic::experimental_guard>())) {
-      Value *Cond = cast<CallInst>(Inst)->getArgOperand(0);
-
-      if (match(Cond, m_One())) {
-        // Elide guards on true, since operationally they're no-ops.  In the
-        // future we can consider more sophisticated tradeoffs here with
-        // consideration to potential for check widening, but for now we keep
-        // things simple.
-        Inst->eraseFromParent();
-      } else if (auto *CondI = dyn_cast<Instruction>(Cond)) {
+      if (auto *CondI =
+              dyn_cast<Instruction>(cast<CallInst>(Inst)->getArgOperand(0))) {
         // The condition we're on guarding here is true for all dominated
         // locations.
         if (SimpleValue::canHandle(CondI))