[PATCH] D18230: [SimplifyLibCalls] Simplify strlen to a subtraction for certain cases

[Daniel Berlin via llvm-commits]

So, uh, why not make getConstantStringInfo work for this case too, rather
than duplicate it?


On Tue, Apr 5, 2016 at 12:18 PM, Li Huang via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> lihuang added a comment.
>
> Sure, I will rebase the patch after 265431.
>
>
> ================
> Comment at: lib/Transforms/Utils/SimplifyLibCalls.cpp:541-558
> @@ +540,20 @@
> +  // [0, strlen(s)] or the string has a single null terminator '\0' at
> the end.
> +  if (GEPOperator *GEP = dyn_cast<GEPOperator>(Src)) {
> +    if (GEP->getNumOperands() != 3)
> +      return nullptr;
> +
> +    // We only try to simplify strlen when the pointer s points to an
> array
> +    // of i8. Otherwise, we would need to scale the offset x before doing
> the
> +    // subtraction. This will make the optimization more complex, and
> it's not
> +    // very useful because calling strlen for a pointer of other types is
> +    // very uncommon.
> +    PointerType *PT = cast<PointerType>(GEP->getOperand(0)->getType());
> +    ArrayType *AT = dyn_cast<ArrayType>(PT->getElementType());
> +    if (!AT || !AT->getElementType()->isIntegerTy(8))
> +      return nullptr;
> +
> +    // First index should be 0.
> +    const ConstantInt *FirstIdx =
> dyn_cast<ConstantInt>(GEP->getOperand(1));
> +    if (!FirstIdx || !FirstIdx->isZero())
> +      return nullptr;
> +
> ----------------
> spatel wrote:
> > These checks are included in getConstantStringInfo(). You do not need to
> repeat them here?
> Yes these lines are the same lines in getConstantStringInfo(). The reason
> I didn't use getConstantStringInfo() here is it only works for GEP with a
> constant offset. In this case, the offset is a variable, and we need the
> same checks.
>
>
> http://reviews.llvm.org/D18230
>
>
>
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