From 66ab0f045a37c25833dd22e0e725fe4833c92497 Mon Sep 17 00:00:00 2001
From: Philip Reames <listmail@philipreames.com>
Date: Tue, 16 Jun 2015 00:49:59 +0000
Subject: Move logic from JumpThreading into LazyValue info to simplify caller.

This change is hopefully NFC. The only tricky part is that I changed the context instruction being used to the branch rather than the comparison. I believe both to be correct, but the branch is strictly more powerful. With the moved code, using the branch instruction is required for the basic block comparison test to return the same result. The previous code was able to directly access both the branch and the comparison where the revised code is not.

Differential Revision: http://reviews.llvm.org/D9652

llvm-svn: 239797
---
 llvm/lib/Transforms/Scalar/JumpThreading.cpp | 68 +++++++---------------------
 1 file changed, 17 insertions(+), 51 deletions(-)

(limited to 'llvm/lib/Transforms/Scalar/JumpThreading.cpp')

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 711df417992..1ebc6a5b1aa 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -758,67 +758,33 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
 
 
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
-    // For a comparison where the LHS is outside this block, it's possible
-    // that we've branched on it before.  Used LVI to see if we can simplify
-    // the branch based on that.
+    // If we're branching on a conditional, LVI might be able to determine
+    // it's value at the the branch instruction.  We only handle comparisons
+    // against a constant at this time.
+    // TODO: This should be extended to handle switches as well.  
     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
-    pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
-    if (CondBr && CondConst && CondBr->isConditional() && PI != PE &&
-        (!isa<Instruction>(CondCmp->getOperand(0)) ||
-         cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
-      // For predecessor edge, determine if the comparison is true or false
-      // on that edge.  If they're all true or all false, we can simplify the
-      // branch.
-      // FIXME: We could handle mixed true/false by duplicating code.
-      LazyValueInfo::Tristate Baseline =
-        LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0),
-                                CondConst, *PI, BB, CondCmp);
-      if (Baseline != LazyValueInfo::Unknown) {
-        // Check that all remaining incoming values match the first one.
-        while (++PI != PE) {
-          LazyValueInfo::Tristate Ret =
-            LVI->getPredicateOnEdge(CondCmp->getPredicate(),
-                                    CondCmp->getOperand(0), CondConst, *PI, BB,
-                                    CondCmp);
-          if (Ret != Baseline) break;
-        }
-
-        // If we terminated early, then one of the values didn't match.
-        if (PI == PE) {
-          unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0;
-          unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1;
-          CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
-          BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
-          CondBr->eraseFromParent();
-          if (CondCmp->use_empty())
-            CondCmp->eraseFromParent();
-          else if (CondCmp->getParent() == BB) {
-            // If the fact we just learned is true for all uses of the
-            // condition, replace it with a constant value
-            auto *CI = Baseline == LazyValueInfo::True ?
-              ConstantInt::getTrue(CondCmp->getType()) :
-              ConstantInt::getFalse(CondCmp->getType());
-            CondCmp->replaceAllUsesWith(CI);
-            CondCmp->eraseFromParent();
-          }
-          return true;
-        }
-      }
-
-    } else if (CondBr && CondConst && CondBr->isConditional()) {
-      // There might be an invariant in the same block with the conditional
-      // that can determine the predicate.
-
+    if (CondBr && CondConst && CondBr->isConditional()) {
       LazyValueInfo::Tristate Ret =
         LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
-                            CondConst, CondCmp);
+                            CondConst, CondBr);
       if (Ret != LazyValueInfo::Unknown) {
         unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
         unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
         CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
         BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
         CondBr->eraseFromParent();
+        if (CondCmp->use_empty())
+          CondCmp->eraseFromParent();
+        else if (CondCmp->getParent() == BB) {
+          // If the fact we just learned is true for all uses of the
+          // condition, replace it with a constant value
+          auto *CI = Ret == LazyValueInfo::True ?
+            ConstantInt::getTrue(CondCmp->getType()) :
+            ConstantInt::getFalse(CondCmp->getType());
+          CondCmp->replaceAllUsesWith(CI);
+          CondCmp->eraseFromParent();
+        }
         return true;
       }
     }
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