[PATCH] D106053: [CodeGen] Remove pending AssertZext AssertSext in promoting FP_TO_INT

[Xiang Zhang via Phabricator via llvm-commits]

xiangzhangllvm added a comment.

> I'm working on assumption that AssertZext means "if the operand is not poison, the high N bits are not set"

And In fact, current optimizations on AssertZext don't care about its operand's poison or other details, it just thought its hight N bits are 0.
for example, in

  "SelectionDAG::computeKnownBits" 
   3297   case ISD::AssertZext: {
   3298     EVT VT = cast<VTSDNode>(Op.getOperand(1))->getVT();
   3299     APInt InMask = APInt::getLowBitsSet(BitWidth, VT.getSizeInBits());
   3300     Known = computeKnownBits(Op.getOperand(0), Depth+1);
   3301     Known.Zero |= (~InMask);
   3302     Known.One  &= (~Known.Zero);
   3303     break;

You can see the high N bits directly be mark as zero.


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  https://reviews.llvm.org/D106053/new/

https://reviews.llvm.org/D106053