[SimplifyIndVars] Ignore dead users

IndVarSimplify sometimes makes transforms basing on users that are trivially dead. In particular,
if DCE wasn't run before it, there may be a dead `sext/zext` in loop that will trigger widening
transforms, however it makes no sense to do it.

This patch teaches IndVarsSimplify ignore the mist trivial cases of that.

Differential Revision: https://reviews.llvm.org/D47974
Reviewed By: sanjoy

llvm-svn: 334567

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index e72602d016f..a417b037314 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -26,6 +26,7 @@
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/Local.h"
 
 using namespace llvm;
 
@@ -772,6 +773,15 @@ void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
       SimpleIVUsers.pop_back_val();
     Instruction *UseInst = UseOper.first;
 
+    // If a user of the IndVar is trivially dead, we prefer just to mark it dead
+    // rather than try to do some complex analysis or transformation (such as
+    // widening) basing on it.
+    // TODO: Propagate TLI and pass it here to handle more cases.
+    if (isInstructionTriviallyDead(UseInst, /* TLI */ nullptr)) {
+      DeadInsts.emplace_back(UseInst);
+      continue;
+    }
+
     // Bypass back edges to avoid extra work.
     if (UseInst == CurrIV) continue;