[llvm] r191200 - InstSimplify: Fold equality comparisons between non-inbounds GEPs.

[Benjamin Kramer]

Author: d0k
Date: Mon Sep 23 09:16:38 2013
New Revision: 191200

URL: http://llvm.org/viewvc/llvm-project?rev=191200&view=rev
Log:
InstSimplify: Fold equality comparisons between non-inbounds GEPs.

Overflow doesn't affect the correctness of equalities. Computing this is cheap,
we just reuse the computation for the inbounds case and try to peel of more
non-inbounds GEPs. This pattern is unlikely to ever appear in code generated by
Clang, but SCEV occasionally produces it.

Modified:
    llvm/trunk/lib/Analysis/InstructionSimplify.cpp
    llvm/trunk/test/Transforms/InstSimplify/compare.ll

Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=191200&r1=191199&r2=191200&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Mon Sep 23 09:16:38 2013
@@ -668,7 +668,8 @@ Value *llvm::SimplifyAddInst(Value *Op0,
 /// follow non-inbounds geps. This allows it to remain usable for icmp ult/etc.
 /// folding.
 static Constant *stripAndComputeConstantOffsets(const DataLayout *TD,
-                                                Value *&V) {
+                                                Value *&V,
+                                                bool AllowNonInbounds = false) {
   assert(V->getType()->getScalarType()->isPointerTy());
 
   // Without DataLayout, just be conservative for now. Theoretically, more could
@@ -685,7 +686,8 @@ static Constant *stripAndComputeConstant
   Visited.insert(V);
   do {
     if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
-      if (!GEP->isInBounds() || !GEP->accumulateConstantOffset(*TD, Offset))
+      if ((!AllowNonInbounds && !GEP->isInBounds()) ||
+          !GEP->accumulateConstantOffset(*TD, Offset))
         break;
       V = GEP->getPointerOperand();
     } else if (Operator::getOpcode(V) == Instruction::BitCast) {
@@ -1837,6 +1839,17 @@ static Constant *computePointerICmp(cons
         return ConstantInt::get(GetCompareTy(LHS),
                                 !CmpInst::isTrueWhenEqual(Pred));
     }
+
+    // Even if an non-inbounds GEP occurs along the path we can still optimize
+    // equality comparisons concerning the result. We avoid walking the whole
+    // chain again by starting where the last calls to
+    // stripAndComputeConstantOffsets left off and accumulate the offsets.
+    Constant *LHSNoBound = stripAndComputeConstantOffsets(TD, LHS, true);
+    Constant *RHSNoBound = stripAndComputeConstantOffsets(TD, RHS, true);
+    if (LHS == RHS)
+      return ConstantExpr::getICmp(Pred,
+                                   ConstantExpr::getAdd(LHSOffset, LHSNoBound),
+                                   ConstantExpr::getAdd(RHSOffset, RHSNoBound));
   }
 
   // Otherwise, fail.

Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=191200&r1=191199&r2=191200&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Mon Sep 23 09:16:38 2013
@@ -717,3 +717,12 @@ define i1 @alloca_gep(i64 %a, i64 %b) {
   ret i1 %cmp
 ; CHECK-NEXT: ret i1 false
 }
+
+define i1 @non_inbounds_gep_compare(i64* %a) {
+; CHECK-LABEL: @non_inbounds_gep_compare(
+; Equality compares with non-inbounds GEPs can be folded.
+  %x = getelementptr i64* %a, i64 42
+  %cmp = icmp eq i64* %a, %x
+  ret i1 %cmp
+; CHECK-NEXT: ret i1 false
+}