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!

llvm-svn: 151449

llvm/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))

llvm/test/Transforms/InstCombine/icmp.ll

@@ -634,8 +634,6 @@ define i1 @test62(i8* %a) {
   %arrayidx2 = getelementptr inbounds i8* %a, i64 10
   %cmp = icmp slt i8* %arrayidx1, %arrayidx2
   ret i1 %cmp
-; Don't turn a signed cmp of GEPs into an index compare.
 ; CHECK: @test62
-; CHECK: %cmp = icmp slt i8* %arrayidx1, %arrayidx2
-; CHECK-NEXT: ret i1 %cmp
+; CHECK-NEXT: ret i1 true
 }

llvm/test/Transforms/InstSimplify/compare.ll

@@ -40,6 +40,50 @@ define i1 @gep2() {
 ; CHECK-NEXT: ret i1 true
 }
 
+; PR11238
+%t = type { i32, i32 }
+@y = global %t zeroinitializer, align 8
+
+define i1 @gep3() {
+; CHECK: @gep3
+  %x = alloca %t, align 8
+  %a = getelementptr %t* %x, i64 0, i32 0
+  %b = getelementptr %t* %x, i64 0, i32 1
+  %equal = icmp eq i32* %a, %b
+  ret i1 %equal
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @gep4() {
+; CHECK: @gep4
+  %x = alloca %t, align 8
+  %a = getelementptr %t* @y, i64 0, i32 0
+  %b = getelementptr %t* @y, i64 0, i32 1
+  %equal = icmp eq i32* %a, %b
+  ret i1 %equal
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @gep5() {
+; CHECK: @gep5
+  %x = alloca %t, align 8
+  %a = getelementptr inbounds %t* %x, i64 0, i32 1
+  %b = getelementptr %t* @y, i64 0, i32 0
+  %equal = icmp eq i32* %a, %b
+  ret i1 %equal
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @gep6(%t* %x) {
+; Same as @gep3 but potentially null.
+; CHECK: @gep6
+  %a = getelementptr %t* %x, i64 0, i32 0
+  %b = getelementptr %t* %x, i64 0, i32 1
+  %equal = icmp eq i32* %a, %b
+  ret i1 %equal
+; CHECK-NEXT: ret i1 false
+}
+
 define i1 @zext(i32 %x) {
 ; CHECK: @zext
   %e1 = zext i32 %x to i64