SimplifyCFG now predicts some conditional branches to true or false depending on previous branch on same comparison operands.

For example, 

if (a == b) {
    if (a > b) // this is false
    
Fixes some of the issues on <rdar://problem/10554090>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146822 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 42 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index bf2cb49..d20e5a5 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "simplifycfg"
+#include "llvm/Transforms/Utils/CmpInstAnalysis.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Constants.h"
 #include "llvm/Instructions.h"
@@ -1702,6 +1703,47 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
     }
   }
   
+  // Treat "if (cond1) { if (cond2) {} }" as "cond1 & cond2" and fold.
+  // This gives us the value of what cond2 is given cond1 is already known to
+  // be true.
+  if (ICmpInst *LHS = dyn_cast<ICmpInst>(PBI->getCondition())) {
+    if (ICmpInst *RHS = dyn_cast<ICmpInst>(BI->getCondition())) {
+      ICmpInst::Predicate LHSCC = LHS->getPredicate(),
+      RHSCC = RHS->getPredicate();
+      if (PredicatesFoldable(LHSCC, RHSCC)) {
+        if (LHS->getOperand(0) == RHS->getOperand(1) &&
+            LHS->getOperand(1) == RHS->getOperand(0))
+          LHS->swapOperands();
+        if (LHS->getOperand(0) == RHS->getOperand(0) &&
+            LHS->getOperand(1) == RHS->getOperand(1) &&
+            BB->getSinglePredecessor()) {
+          Value *Op0 = LHS->getOperand(0), *Op1 = LHS->getOperand(1);
+          bool CondIsTrue = PBI->getSuccessor(0) == BB;
+          unsigned LHSCode = getICmpCode(LHS, !CondIsTrue);
+          unsigned RHSCode = getICmpCode(RHS);
+          unsigned Code = LHSCode & RHSCode;
+          
+          Value *ConstantCondition = NULL;
+          // If the resultant code is the same as the LHS code then as that
+          // code is known to be true we can make RHS now be true.
+          if (Code == LHSCode)
+            ConstantCondition = ConstantInt::get(
+                                 CmpInst::makeCmpResultType(LHS->getType()), 1);
+          else {
+            bool isSigned = LHS->isSigned() || RHS->isSigned();
+            CmpInst::Predicate IgnoredNewPred;
+            ConstantCondition = getICmpValue(isSigned, Code, Op0, Op1,
+                                             IgnoredNewPred);
+          }
+          if (ConstantCondition) {
+            RHS->replaceAllUsesWith(ConstantCondition);
+            return true;
+          }
+        }
+      }
+    }
+  }
+  
   // If this is a conditional branch in an empty block, and if any
   // predecessors is a conditional branch to one of our destinations,
   // fold the conditions into logical ops and one cond br.