[PATCH] D152275: Use memory region declaration intrinsic when generating code for array subscripts

[Simeon Krastnikov via Phabricator via cfe-commits]

simeon created this revision.
simeon added reviewers: efriedma, lebedev.ri, arichardson.
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As an alternative to https://reviews.llvm.org/D150192, we consider using the memory region declaration intrinsic, introduced in https://reviews.llvm.org/D115274, as another way to propagate the semantics of C/C++ array-indexing rules to LLVM IR.

This patch is only only a proof-of-concept at this point, intended to gather feedback and bring https://reviews.llvm.org/D115274 back into focus. As was mentioned there (and as seen from test results), completing this patch would require patching a lot of optimizations. For example, `InstructionCombiningPass` works with `GetElementPtrInst`s directly and will not recognize an intrinsic call as such. Many other passes such as `IndVarSimplifyPass`, `EarlyCSE`, etc., routinely use a switch case or call `isa<GetElementPtrInst>()` to detect `GEP`s.

We'd be fine with doing that sort of work, if we get some initial thumbs up from the community that this is the way to go. Of course, we're also open to trying a different approach if it avoids the apparently unavoidable complexity of this one.

Depends on D115274 <https://reviews.llvm.org/D115274>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D152275

Files:
  clang/lib/CodeGen/CGExpr.cpp


Index: clang/lib/CodeGen/CGExpr.cpp
===================================================================
--- clang/lib/CodeGen/CGExpr.cpp
+++ clang/lib/CodeGen/CGExpr.cpp
@@ -3936,10 +3936,44 @@
 
     // Propagate the alignment from the array itself to the result.
     QualType arrayType = Array->getType();
-    Addr = emitArraySubscriptGEP(
-        *this, ArrayLV.getAddress(*this), {CGM.getSize(CharUnits::Zero()), Idx},
-        E->getType(), !getLangOpts().isSignedOverflowDefined(), SignedIndices,
-        E->getExprLoc(), &arrayType, E->getBase());
+
+    Address ArrayLVAddr = ArrayLV.getAddress(*this);
+
+    if (!getLangOpts().isSignedOverflowDefined() && 
+        // ISO/IEC 9899:TC3, 6.5.6.8
+        (getLangOpts().C99 || getLangOpts().CPlusPlus) &&
+        getContext().getAsConstantArrayType(arrayType)) {
+      auto *CAT = getContext().getAsConstantArrayType(arrayType);
+      uint64_t BoundedRegionSize = (Idx->getType()->getScalarSizeInBits() / 8) * 
+          CAT->getSize().getZExtValue();
+
+      Address BeginOff = emitArraySubscriptGEP(
+          *this, ArrayLVAddr, 
+          {CGM.getSize(CharUnits::Zero()), CGM.getSize(CharUnits::Zero())},
+          E->getType(), !getLangOpts().isSignedOverflowDefined(), SignedIndices,
+          E->getExprLoc(), &arrayType, E->getBase());
+
+      llvm::Function *F = CGM.getIntrinsic(llvm::Intrinsic::memory_region_decl, 
+          BeginOff.getPointer()->getType());
+      llvm::Value *Call = Builder.CreateCall(F,
+          {BeginOff.getPointer(), 
+          llvm::ConstantInt::get(Int64Ty, 0), 
+          llvm::ConstantInt::get(Int64Ty, BoundedRegionSize)},
+          "arrayidx.bounded");
+      Address RetAddr(Call, BeginOff.getElementType(), 
+          ArrayLVAddr.getAlignment());
+
+      Addr = emitArraySubscriptGEP(
+          *this, RetAddr, {Idx},
+          E->getType(), !getLangOpts().isSignedOverflowDefined(), SignedIndices,
+          E->getExprLoc(), &arrayType, E->getBase());
+    } else {
+      Addr = emitArraySubscriptGEP(
+          *this, ArrayLVAddr, {CGM.getSize(CharUnits::Zero()), Idx},
+          E->getType(), !getLangOpts().isSignedOverflowDefined(), SignedIndices,
+          E->getExprLoc(), &arrayType, E->getBase());
+    }
+        
     EltBaseInfo = ArrayLV.getBaseInfo();
     EltTBAAInfo = CGM.getTBAAInfoForSubobject(ArrayLV, E->getType());
   } else {


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