[LLVMdev] SimplifyLibCalls doesn't check TLI for LibFunc availability

[Eli Friedman]

On Tue, Aug 13, 2013 at 5:58 AM, Kuperstein, Michael M <
michael.m.kuperstein at intel.com> wrote:

>  Hi,****
>
> ** **
>
> It looks like SimplifyLibCalls has a tendency to emit calls to libm
> functions without checking with TLI whether these calls are available.****
>
> For example, PowOpt has this code:****
>
> ** **
>
> struct PowOpt : public UnsafeFPLibCallOptimization {****
>
>   PowOpt(bool UnsafeFPShrink) :
> UnsafeFPLibCallOptimization(UnsafeFPShrink) {}****
>
>   virtual Value *callOptimizer(Function *Callee, CallInst *CI, IRBuilder<>
> &B) {****
>
>     Value *Ret = NULL;****
>
>     if (UnsafeFPShrink && Callee->getName() == "pow" &&****
>
>         TLI->has(LibFunc::powf)) {****
>
>       UnaryDoubleFPOpt UnsafeUnaryDoubleFP(true);****
>
>       Ret = UnsafeUnaryDoubleFP.callOptimizer(Callee, CI, B);****
>
>     }****
>
> ** **
>
> [...]****
>
> ** **
>
>     if (Op2C->isExactlyValue(0.5)) {****
>
>       // Expand pow(x, 0.5) to (x == -infinity ? +infinity :
> fabs(sqrt(x))).****
>
>       // This is faster than calling pow, and still handles negative zero*
> ***
>
>       // and negative infinity correctly.****
>
>       // TODO: In fast-math mode, this could be just sqrt(x).****
>
>       // TODO: In finite-only mode, this could be just fabs(sqrt(x)).****
>
>       Value *Inf = ConstantFP::getInfinity(CI->getType());****
>
>       Value *NegInf = ConstantFP::getInfinity(CI->getType(), true);****
>
>       Value *Sqrt = EmitUnaryFloatFnCall(Op1, "sqrt", B,****
>
>                                          Callee->getAttributes());****
>
>       Value *FAbs = EmitUnaryFloatFnCall(Sqrt, "fabs", B,****
>
>                                          Callee->getAttributes());****
>
>       Value *FCmp = B.CreateFCmpOEQ(Op1, NegInf);****
>
>       Value *Sel = B.CreateSelect(FCmp, Inf, FAbs);****
>
>       return Sel;****
>
>     }****
>
> ** **
>
> [...]****
>
> }****
>
> ** **
>
> So, before the callOptimizer call, it actually does check TLI to see that
> powf is available. However, the code below (that expands pow(x, 0.5) )
> emits sqrt and fabs without making any checks.****
>
> ** **
>
> This may be problematic in two cases:****
>
> **1)      **If LibFunc::pow is available, but LibFunc::sqrt and
> LibFunc::fabs are not. This is not very likely, but I guess it is
> technically possible.****
>
> **2)      **The simplification functions are also called for intrinsics
> (pow and exp2). In this case, it’s possible that libm is not available at
> all.****
>
> ** **
>
> Is this a bug, or am I missing something here? Those unchecked
> EmitnaryFloatFnCalls are all over the pass.****
>
>
>
>
>
Bug.

-Eli
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130813/18b8bb08/attachment.html>