[llvm-commits] [llvm] r160589 - in /llvm/trunk: include/llvm/Transforms/Utils/Local.h lib/Analysis/MemoryBuiltins.cpp test/Instrumentation/BoundsChecking/simple.ll

[Duncan Sands]

Hi Nuno,

> baby steps toward fixing some problems with inbound GEPs that overflow, as discussed 2 months ago or so.
> Make sure we do not emit index computations with NSW flags so that we dont get an undef value if the GEP overflows

does this really help?  I mean, the optimizers may have exploited nsw, undef and
inbounds before this point to produce really weird bitcode, and may exploit them
afterwards, eg because after inlining they see that your computation is fed by
undef.  I'm worried that this appears to fix something, but doesn't really.  I
don't think this kind of issue can be fixed without some kind of compiler
barrier like the llvm.identity intrinsic I mentioned a while ago.

Ciao, Duncan.

>
> Modified:
>      llvm/trunk/include/llvm/Transforms/Utils/Local.h
>      llvm/trunk/lib/Analysis/MemoryBuiltins.cpp
>      llvm/trunk/test/Instrumentation/BoundsChecking/simple.ll
>
> Modified: llvm/trunk/include/llvm/Transforms/Utils/Local.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Transforms/Utils/Local.h?rev=160589&r1=160588&r2=160589&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Transforms/Utils/Local.h (original)
> +++ llvm/trunk/include/llvm/Transforms/Utils/Local.h Fri Jul 20 18:07:40 2012
> @@ -168,15 +168,18 @@
>   /// EmitGEPOffset - Given a getelementptr instruction/constantexpr, emit the
>   /// code necessary to compute the offset from the base pointer (without adding
>   /// in the base pointer).  Return the result as a signed integer of intptr size.
> +/// When NoAssumptions is true, no assumptions about index computation not
> +/// overflowing is made.
>   template<typename IRBuilderTy>
> -Value *EmitGEPOffset(IRBuilderTy *Builder, const TargetData &TD, User *GEP) {
> +Value *EmitGEPOffset(IRBuilderTy *Builder, const TargetData &TD, User *GEP,
> +                     bool NoAssumptions = false) {
>     gep_type_iterator GTI = gep_type_begin(GEP);
>     Type *IntPtrTy = TD.getIntPtrType(GEP->getContext());
>     Value *Result = Constant::getNullValue(IntPtrTy);
>
>     // If the GEP is inbounds, we know that none of the addressing operations will
>     // overflow in an unsigned sense.
> -  bool isInBounds = cast<GEPOperator>(GEP)->isInBounds();
> +  bool isInBounds = cast<GEPOperator>(GEP)->isInBounds() && !NoAssumptions;
>
>     // Build a mask for high order bits.
>     unsigned IntPtrWidth = TD.getPointerSizeInBits();
>
> Modified: llvm/trunk/lib/Analysis/MemoryBuiltins.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/MemoryBuiltins.cpp?rev=160589&r1=160588&r2=160589&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Analysis/MemoryBuiltins.cpp (original)
> +++ llvm/trunk/lib/Analysis/MemoryBuiltins.cpp Fri Jul 20 18:07:40 2012
> @@ -645,7 +645,7 @@
>     if (!bothKnown(PtrData))
>       return unknown();
>
> -  Value *Offset = EmitGEPOffset(&Builder, *TD, &GEP);
> +  Value *Offset = EmitGEPOffset(&Builder, *TD, &GEP, /*NoAssumptions=*/true);
>     Offset = Builder.CreateAdd(PtrData.second, Offset);
>     return std::make_pair(PtrData.first, Offset);
>   }
>
> Modified: llvm/trunk/test/Instrumentation/BoundsChecking/simple.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Instrumentation/BoundsChecking/simple.ll?rev=160589&r1=160588&r2=160589&view=diff
> ==============================================================================
> --- llvm/trunk/test/Instrumentation/BoundsChecking/simple.ll (original)
> +++ llvm/trunk/test/Instrumentation/BoundsChecking/simple.ll Fri Jul 20 18:07:40 2012
> @@ -116,3 +116,13 @@
>     %3 = load i8* %2, align 4
>     ret void
>   }
> +
> +; CHECK: @f12
> +define i64 @f12(i64 %x, i64 %y) nounwind {
> +  %1 = tail call i8* @calloc(i64 1, i64 %x)
> +; CHECK: mul i64 %y, 8
> +  %2 = bitcast i8* %1 to i64*
> +  %3 = getelementptr inbounds i64* %2, i64 %y
> +  %4 = load i64* %3, align 8
> +  ret i64 %4
> +}
>
>
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