[PATCH] D140347: TargetLowering: Teach DemandedBits about VSCALE
Saleem Abdulrasool via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Dec 20 09:58:00 PST 2022
compnerd added inline comments.
================
Comment at: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp:1136
+ return false;
+ APInt VScaleResultUpperbound(64, *MaxVScale);
+ VScaleResultUpperbound *= Op.getConstantOperandAPInt(0).sextOrTrunc(64);
----------------
craig.topper wrote:
> MacDue wrote:
> > compnerd wrote:
> > > craig.topper wrote:
> > > > I'm not sure that hardcoding 64 here is the right fix. I think what we need to check is that the multiply doesn't overflow.
> > > @craig.topper I absolutely agree with you. I call this out in the commit message, that I'm just assuming that this is wide enough. This can potentially overflow still. Is there a good way to ensure that we have the proper width?
> > My thought was to do:
> >
> > ```
> > APInt Multiplier = Op.getConstantOperandAPInt(0);
> > unsigned MultiplyBits = Log2_32(*MaxVScale) + 1 + Multiplier.getActiveBits();
> > APInt VScaleResultUpperbound = APInt(MultiplyBits, *MaxVScale) * Multiplier.sextOrTrunc(MultiplyBits);
> > ```
> > Which should always have enough bits to store the full result.
> Here's what I was playing with
>
> ```
> APInt VScaleResultUpperbound(BitWidth, *MaxVScale);
> // TODO: Vscale max with no leading zeros requires special handling.
> if (VScaleResultUpperbound.isNegative())
> return false;
> bool Overflow;
> VScaleResultUpperbound =
> VScaleResultUpperbound.smul_ov(Op.getConstantOperandAPInt(0), Overflow);
> if (Overflow)
> return false;
> if (VScaleResultUpperbound.isNegative())
> Known.One.setHighBits(VScaleResultUpperbound.countLeadingOnes());
> else
> Known.Zero.setHighBits(VScaleResultUpperbound.countLeadingZeros());
> ```
I didn't know about the `smul_ov` function, that does seem better. The use of `BitWidth` is still a problem. The `BitWidth` is computed from the width of the `DemandedBits` which is computed from the type of the result of the operation. In this particular test (luckily identifies the issue!) the result type is `i8`, which results in the `BitWidth` being 8, but the vrange is 2,1024, which exceeds the range of i8, and thus when we do scale * multiplier and overflow, we end up with the wrong result. I think that we should either go with the wider bit width here.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D140347/new/
https://reviews.llvm.org/D140347
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