From 1e4e1c768bcbc548345348c062f402967c78c48a Mon Sep 17 00:00:00 2001
From: Nick Lewycky <nicholas@mxc.ca>
Date: Sat, 25 Feb 2012 19:07:42 +0000
Subject: Teach instsimplify to be more aggressive when analyzing comparisons
 of pointers by using llvm::isIdentifiedObject. Also teach it to handle GEPs
 that have the same base pointer and constant operands. Fixes PR11238!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151449 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/InstructionSimplify.cpp | 51 +++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 10 deletions(-)

(limited to 'lib/Analysis')

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 37253f7..a6a080e 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -21,6 +21,7 @@
 #include "llvm/Operator.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/ValueTracking.h"
@@ -1608,26 +1609,37 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
     }
   }
 
-  // icmp <alloca*>, <global/alloca*/null> - Different stack variables have
+  // icmp <object*>, <object*/null> - Different identified objects have
   // different addresses, and what's more the address of a stack variable is
-  // never null or equal to the address of a global.  Note that generalizing
-  // to the case where LHS is a global variable address or null is pointless,
-  // since if both LHS and RHS are constants then we already constant folded
-  // the compare, and if only one of them is then we moved it to RHS already.
+  // never equal to another argument.  Note that generalizing to the case where
+  // LHS is a global variable address or null is pointless, since if both LHS
+  // and RHS are constants then we already constant folded the compare, and if
+  // only one of them is then we moved it to RHS already.
   Value *LHSPtr = LHS->stripPointerCasts();
   Value *RHSPtr = RHS->stripPointerCasts();
   if (LHSPtr == RHSPtr)
     return ConstantInt::get(ITy, CmpInst::isTrueWhenEqual(Pred));
-  
+
   // Be more aggressive about stripping pointer adjustments when checking a
   // comparison of an alloca address to another object.  We can rip off all
   // inbounds GEP operations, even if they are variable.
   LHSPtr = stripPointerAdjustments(LHSPtr);
-  if (isa<AllocaInst>(LHSPtr)) {
+  if (llvm::isIdentifiedObject(LHSPtr)) {
     RHSPtr = stripPointerAdjustments(RHSPtr);
-    if (LHSPtr != RHSPtr &&
-        (isa<GlobalValue>(RHSPtr) || isa<AllocaInst>(RHSPtr)  ||
-         isa<ConstantPointerNull>(RHSPtr)))
+    // If both sides are different identified objects, they aren't equal unless
+    // they're null.
+    if (LHSPtr != RHSPtr && llvm::isIdentifiedObject(RHSPtr) &&
+        (llvm::isKnownNonNull(LHSPtr) || llvm::isKnownNonNull(RHSPtr)))
+      return ConstantInt::get(ITy, CmpInst::isFalseWhenEqual(Pred));
+
+    // Assume that the null is on the right.
+    if (llvm::isKnownNonNull(LHSPtr) && isa<ConstantPointerNull>(RHSPtr))
+      return ConstantInt::get(ITy, CmpInst::isFalseWhenEqual(Pred));
+
+    // A local instruction (alloca or noalias call) can't alias any incoming
+    // argument.
+    if ((isa<Instruction>(LHSPtr) && isa<Argument>(RHSPtr)) ||
+        (isa<Instruction>(RHSPtr) && isa<Argument>(LHSPtr)))
       return ConstantInt::get(ITy, CmpInst::isFalseWhenEqual(Pred));
   }
 
@@ -2239,6 +2251,25 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
       return getFalse(ITy);
   }
 
+  // Simplify comparisons of GEPs.
+  if (GetElementPtrInst *GLHS = dyn_cast<GetElementPtrInst>(LHS)) {
+    if (GEPOperator *GRHS = dyn_cast<GEPOperator>(RHS)) {
+      if (GLHS->getPointerOperand() == GRHS->getPointerOperand() &&
+          GLHS->hasAllConstantIndices() && GRHS->hasAllConstantIndices()) {
+        // The bases are equal and the indices are constant.  Build a constant
+        // expression GEP with the same indices and a null base pointer to see
+        // what constant folding can make out of it.
+        Constant *Null = Constant::getNullValue(GLHS->getPointerOperandType());
+        SmallVector<Value *, 4> IndicesLHS(GLHS->idx_begin(), GLHS->idx_end());
+        Constant *NewLHS = ConstantExpr::getGetElementPtr(Null, IndicesLHS);
+
+        SmallVector<Value *, 4> IndicesRHS(GRHS->idx_begin(), GRHS->idx_end());
+        Constant *NewRHS = ConstantExpr::getGetElementPtr(Null, IndicesRHS);
+        return ConstantExpr::getICmp(Pred, NewLHS, NewRHS);
+      }
+    }
+  }
+
   // If the comparison is with the result of a select instruction, check whether
   // comparing with either branch of the select always yields the same value.
   if (isa<SelectInst>(LHS) || isa<SelectInst>(RHS))
-- 
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