Fwd: [llvm] r194220 - IR: Do not canonicalize constant GEPs into an out-of-bounds array access
David Blaikie via llvm-commits
llvm-commits at lists.llvm.org
Thu Aug 20 22:39:39 PDT 2015
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---------- Forwarded message ----------
From: David Blaikie <dblaikie at gmail.com>
Date: Thu, Aug 20, 2015 at 10:35 PM
Subject: Re: [llvm] r194220 - IR: Do not canonicalize constant GEPs into an
out-of-bounds array access
To: David Majnemer <david.majnemer at gmail.com>
Cc: "llvm-commits at cs.uiuc.edu" <llvm-commits at cs.uiuc.edu>
On Thu, Nov 7, 2013 at 2:15 PM, David Majnemer <david.majnemer at gmail.com>
wrote:
> Author: majnemer
> Date: Thu Nov 7 16:15:53 2013
> New Revision: 194220
>
> URL: http://llvm.org/viewvc/llvm-project?rev=194220&view=rev
> Log:
> IR: Do not canonicalize constant GEPs into an out-of-bounds array access
>
> Summary:
> Consider a GEP of:
> i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c, i32 0, i32 0,
> i64 0)
>
> If we proceeded to GEP the aforementioned object by 8, would form a GEP of:
> i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c, i32 0, i32 0,
> i64 8)
>
> Note that we would go through the first array member, causing an
> out-of-bounds accesses. This is problematic because we might get fooled
> if we are trying to evaluate loads using this GEP, for example, based
> off of an object with a constant initializer where the array is zero.
>
> This fixes PR17732.
>
> Reviewers: nicholas, chandlerc, void
>
> Reviewed By: void
>
> CC: llvm-commits, echristo, void, aemerson
>
> Differential Revision: http://llvm-reviews.chandlerc.com/D2093
>
> Added:
> llvm/trunk/test/Transforms/GVN/pr17732.ll
> Modified:
> llvm/trunk/lib/IR/ConstantFold.cpp
>
> Modified: llvm/trunk/lib/IR/ConstantFold.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/ConstantFold.cpp?rev=194220&r1=194219&r2=194220&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/IR/ConstantFold.cpp (original)
> +++ llvm/trunk/lib/IR/ConstantFold.cpp Thu Nov 7 16:15:53 2013
> @@ -1940,7 +1940,43 @@ static Constant *ConstantFoldGetElementP
> I != E; ++I)
> LastTy = *I;
>
> - if ((LastTy && isa<SequentialType>(LastTy)) || Idx0->isNullValue())
> {
> + // We cannot combine indices if doing so would take us outside of an
> + // array or vector. Doing otherwise could trick us if we evaluated
> such a
> + // GEP as part of a load.
> + //
> + // e.g. Consider if the original GEP was:
> + // i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c,
> + // i32 0, i32 0, i64 0)
> + //
> + // If we then tried to offset it by '8' to get to the third element,
> + // an i8, we should *not* get:
> + // i8* getelementptr ({ [2 x i8], i32, i8, [3 x i8] }* @main.c,
> + // i32 0, i32 0, i64 8)
> + //
> + // This GEP tries to index array element '8 which runs
> out-of-bounds.
> + // Subsequent evaluation would get confused and produce erroneous
> results.
> + //
> + // The following prohibits such a GEP from being formed by checking
> to see
> + // if the index is in-range with respect to an array or vector.
> + bool IsSequentialAccessInRange = false;
> + if (LastTy && isa<SequentialType>(LastTy)) {
> + uint64_t NumElements = 0;
> + if (ArrayType *ATy = dyn_cast<ArrayType>(LastTy))
> + NumElements = ATy->getNumElements();
> + else if (VectorType *VTy = dyn_cast<VectorType>(LastTy))
> + NumElements = VTy->getNumElements();
> +
> + if (ConstantInt *CI = dyn_cast<ConstantInt>(Idx0)) {
> + int64_t Idx0Val = CI->getSExtValue();
> + if (NumElements > 0 && Idx0Val >= 0 &&
> + (uint64_t)Idx0Val < NumElements)
> + IsSequentialAccessInRange = true;
> + } else if (PointerType *PTy = dyn_cast<PointerType>(LastTy))
> + // Only handle pointers to sized types, not pointers to
> functions.
> + if (PTy->getElementType()->isSized())
>
While going about my business removing access to pointee types, I came
across this code. Removing this conditional ^ doesn't result in any tests
failing. Any idea if it's necessary/useful and could/should be tested? (&
if it's necessary, how to implement it in a world involving opaque pointer
types)
> + IsSequentialAccessInRange = true;
> + }
> + if (IsSequentialAccessInRange || Idx0->isNullValue()) {
> SmallVector<Value*, 16> NewIndices;
> NewIndices.reserve(Idxs.size() + CE->getNumOperands());
> for (unsigned i = 1, e = CE->getNumOperands()-1; i != e; ++i)
>
> Added: llvm/trunk/test/Transforms/GVN/pr17732.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/GVN/pr17732.ll?rev=194220&view=auto
>
> ==============================================================================
> --- llvm/trunk/test/Transforms/GVN/pr17732.ll (added)
> +++ llvm/trunk/test/Transforms/GVN/pr17732.ll Thu Nov 7 16:15:53 2013
> @@ -0,0 +1,30 @@
> +; RUN: opt -gvn -S -o - < %s | FileCheck %s
> +
> +target datalayout =
> "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> +target triple = "x86_64-unknown-linux-gnu"
> +
> +%struct.with_array = type { [2 x i8], i32, i8 }
> +%struct.with_vector = type { <2 x i8>, i32, i8 }
> +
> + at main.obj_with_array = private unnamed_addr constant { [2 x i8], i32, i8,
> [3 x i8] } { [2 x i8] zeroinitializer, i32 0, i8 1, [3 x i8] undef }, align
> 4
> + at array_with_zeroinit = common global %struct.with_array zeroinitializer,
> align 4
> +
> + at main.obj_with_vector = private unnamed_addr constant { <2 x i8>, i32,
> i8, [3 x i8] } { <2 x i8> zeroinitializer, i32 0, i8 1, [3 x i8] undef },
> align 4
> + at vector_with_zeroinit = common global %struct.with_vector
> zeroinitializer, align 4
> +
> +define i32 @main() {
> +entry:
> + tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds
> (%struct.with_array* @array_with_zeroinit, i64 0, i32 0, i64 0), i8*
> getelementptr inbounds ({ [2 x i8], i32, i8, [3 x i8] }*
> @main.obj_with_array, i64 0, i32 0, i64 0), i64 12, i32 4, i1 false)
> + %0 = load i8* getelementptr inbounds (%struct.with_array*
> @array_with_zeroinit, i64 0, i32 2), align 4
> +
> + tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds
> (%struct.with_vector* @vector_with_zeroinit, i64 0, i32 0, i64 0), i8*
> getelementptr inbounds ({ <2 x i8>, i32, i8, [3 x i8] }*
> @main.obj_with_vector, i64 0, i32 0, i64 0), i64 12, i32 4, i1 false)
> + %1 = load i8* getelementptr inbounds (%struct.with_vector*
> @vector_with_zeroinit, i64 0, i32 2), align 4
> + %conv0 = sext i8 %0 to i32
> + %conv1 = sext i8 %1 to i32
> + %and = and i32 %conv0, %conv1
> + ret i32 %and
> +; CHECK-LABEL: define i32 @main(
> +; CHECK: ret i32 1
> +}
> +
> +declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture
> readonly, i64, i32, i1)
>
>
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