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

3 files changed, 19 insertions, 11 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))
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 82961a678be..5af3712a3ab 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -321,8 +321,12 @@ bool llvm::isInstructionTriviallyDead(Instruction *I,
         II->getIntrinsicID() == Intrinsic::lifetime_end)
       return isa<UndefValue>(II->getArgOperand(1));
 
-    // Assumptions are dead if their condition is trivially true.
-    if (II->getIntrinsicID() == Intrinsic::assume) {
+    // Assumptions are dead if their condition is trivially true.  Guards on
+    // true are operationally no-ops.  In the future we can consider more
+    // sophisticated tradeoffs for guards considering potential for check
+    // widening, but for now we keep things simple.
+    if (II->getIntrinsicID() == Intrinsic::assume ||
+        II->getIntrinsicID() == Intrinsic::experimental_guard) {
       if (ConstantInt *Cond = dyn_cast<ConstantInt>(II->getArgOperand(0)))
         return !Cond->isZero();
 
diff --git a/llvm/test/Transforms/DCE/guards.ll b/llvm/test/Transforms/DCE/guards.ll
new file mode 100644
index 00000000000..d39c44058a7
--- /dev/null
+++ b/llvm/test/Transforms/DCE/guards.ll
@@ -0,0 +1,11 @@
+; RUN: opt -dce -S < %s | FileCheck %s
+
+declare void @llvm.experimental.guard(i1,...)
+
+define void @f(i32 %val) {
+; CHECK-LABEL: @f(
+; CHECK-NEXT: ret void
+  %val2 = add i32 %val, 1
+  call void(i1, ...) @llvm.experimental.guard(i1 true) [ "deopt"(i32 %val2) ]
+  ret void
+}