[PATCH] D75407: [InstSimplify] Constant fold icmp of gep

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
nikic added reviewers: spatel, lebedev.ri, efriedma.
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InstSimplify can fold icmps of gep where the base pointers are the same and the offsets are constant. It does so by constructing a constant expression icmp and assumes that it gets folded -- but this doesn't actually happen, because GEP expressions can usually only be folded by the target-dependent constant folding layer. As such, we need to explicitly invoke it here.

PS: Does someone know why we constant folding is separated in this way? I understand that we don't want to have ConstantExpr depend on TLI, but why can't it depend on DL?


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75407

Files:
  lib/Analysis/InstructionSimplify.cpp
  test/Transforms/InstSimplify/compare.ll


Index: test/Transforms/InstSimplify/compare.ll
===================================================================
--- test/Transforms/InstSimplify/compare.ll
+++ test/Transforms/InstSimplify/compare.ll
@@ -301,7 +301,7 @@
 
 define i1 @gep_same_base_constant_indices(i8* %a) {
 ; CHECK-LABEL: @gep_same_base_constant_indices(
-; CHECK-NEXT:    ret i1 icmp slt (i8* getelementptr (i8, i8* null, i64 1), i8* getelementptr (i8, i8* null, i64 10))
+; CHECK-NEXT:    ret i1 true
 ;
   %arrayidx1 = getelementptr inbounds i8, i8* %a, i64 1
   %arrayidx2 = getelementptr inbounds i8, i8* %a, i64 10
Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -3481,7 +3481,10 @@
         SmallVector<Value *, 4> IndicesRHS(GRHS->idx_begin(), GRHS->idx_end());
         Constant *NewRHS = ConstantExpr::getGetElementPtr(
             GLHS->getSourceElementType(), Null, IndicesRHS);
-        return ConstantExpr::getICmp(Pred, NewLHS, NewRHS);
+        Constant *NewICmp = ConstantExpr::getICmp(Pred, NewLHS, NewRHS);
+        if (auto *FoldedICmp = ConstantFoldConstant(NewICmp, Q.DL))
+          return FoldedICmp;
+        return NewICmp;
       }
     }
   }


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