[PATCH] D153700: [InstSimplify] Fold icmp comparing GEPs with global values

[hanbum via Phabricator via llvm-commits]

ParkHanbum updated this revision to Diff 534924.
ParkHanbum added a comment.

updated the code according to nikic's comment.

> This condition isn't wrong, but it's also not as accurate as it could be.
>
> Let's say `LHSSize = 4`, `LHSOffset = 0`, `RHSSize = 2`, `RHSOffset = 2` with `LHSOffset - RHSOffset = -2`. These can be equal.
>
> Then consider `LHSSize = 4`, `LHSOffset = 2`, `RHSSize = 2`, `RHSOffset = 0` with `LHSOffset - RHSOffset = 2`. These cannot be equal.
>
> As you can see, the sign of the result matters and we should not just take the absolute value.
>
> I believe the condition should be `Dist.isNonNegative() ? Dist.ult(LHSSize) : (-Dist).ult(RHSSize)`.




CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D153700/new/

https://reviews.llvm.org/D153700

Files:
  llvm/lib/IR/ConstantFold.cpp
  llvm/test/Transforms/InstSimplify/compare.ll
  llvm/test/Transforms/InstSimplify/past-the-end.ll


Index: llvm/test/Transforms/InstSimplify/past-the-end.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/past-the-end.ll
+++ llvm/test/Transforms/InstSimplify/past-the-end.ll
@@ -21,13 +21,12 @@
 
 define zeroext i1 @both_past_the_end() {
 ; CHECK-LABEL: @both_past_the_end(
-; CHECK:         ret i1 icmp eq (ptr getelementptr inbounds (i32, ptr @opte_a, i32 1), ptr getelementptr inbounds (i32, ptr @opte_b, i32 1))
+; CHECK:         ret i1 false
 ;
   %x = getelementptr i32, ptr @opte_a, i32 1
   %y = getelementptr i32, ptr @opte_b, i32 1
   %t = icmp eq ptr %x, %y
   ret i1 %t
-  ; TODO: refine this
 }
 
 ; Comparing past-the-end addresses of one global to the base address
Index: llvm/test/Transforms/InstSimplify/compare.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/compare.ll
+++ llvm/test/Transforms/InstSimplify/compare.ll
@@ -2778,10 +2778,9 @@
   ret i1 %res
 }
 
-; TODO: Never equal
 define i1 @globals_offset_inequal() {
 ; CHECK-LABEL: @globals_offset_inequal(
-; CHECK-NEXT:    ret i1 icmp ne (ptr getelementptr (i8, ptr @A, i32 1), ptr getelementptr (i8, ptr @B, i32 1))
+; CHECK-NEXT:    ret i1 true
 ;
   %a.off = getelementptr i8, ptr @A, i32 1
   %b.off = getelementptr i8, ptr @B, i32 1
Index: llvm/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/lib/IR/ConstantFold.cpp
+++ llvm/lib/IR/ConstantFold.cpp
@@ -1547,9 +1547,26 @@
         if (isa<GlobalValue>(CE1Op0) && isa<GlobalValue>(CE2Op0)) {
           // Don't know relative ordering, but check for inequality.
           if (CE1Op0 != CE2Op0) {
-            if (CE1GEP->hasAllZeroIndices() && CE2GEP->hasAllZeroIndices())
+            DataLayout DL(cast<GlobalValue>(CE1Op0)->getParent());
+            Type *GVT;
+            unsigned BitWidth;
+            APInt Dist;
+            GVT = CE1Op0->getType();
+            auto CE1Op0Size = DL.getTypeAllocSize(GVT);
+            GVT = CE2Op0->getType();
+            auto CE2Op0Size = DL.getTypeAllocSize(GVT);
+            BitWidth = DL.getIndexTypeSizeInBits(CE1Op0->getType());
+            APInt CE1GEPOffset(BitWidth, 0);
+            BitWidth = DL.getIndexTypeSizeInBits(CE2Op0->getType());
+            APInt CE2GEPOffset(BitWidth, 0);
+            CE1GEP->accumulateConstantOffset(DL, CE1GEPOffset);
+            CE2GEP->accumulateConstantOffset(DL, CE2GEPOffset);
+
+            Dist = CE1GEPOffset - CE2GEPOffset;
+            if (Dist.isNonNegative() ? Dist.ult(CE1Op0Size) : (-Dist).ult(CE2Op0Size))
               return areGlobalsPotentiallyEqual(cast<GlobalValue>(CE1Op0),
                                                 cast<GlobalValue>(CE2Op0));
+
             return ICmpInst::BAD_ICMP_PREDICATE;
           }
         }


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