[SelectionDAG] Don't inspect !range metadata for extended loads

[sanjoy]

Summary

Don't call computeKnownBitsFromRangeMetadata for extended loads --
this can cause a mismatch between the width of the !range metadata and
the width of the APInt's accumulating KnownZero (and KnownOne in the
future). This isn't a problem now, but will be after a future change.

Note: this can be made more aggressive in the future.

[nlewycky]

That is one big function.

No testsuite changes at all? It seems like this could earn itself a test for the miscompile that it fixes which was asymptomatic before D14100?

[sanjoy]

@nlewycky after this and D14100, CodeGen/PowerPC/glob-comp-aa-crash.ll crashes. I can make a separate test out of that if you like.

[nlewycky]

I'm okay with either or both of changing existing testcases because of this change (ie., to represent the slightly weaker optimization) and/or adding a new testcase to ensure we don't get this exact thing wrong. I just didn't want it to go in with no change to the tests at all.

[sanjoy]

• add a test case

[hfinkel]

LGTM.

lib/CodeGen/SelectionDAG/SelectionDAG.cpp

void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
   case ISD::LOAD: {
     LoadSDNode *LD = cast<LoadSDNode>(Op);
     // If this is a ZEXTLoad and we are looking at the loaded value.
     if (ISD::isZEXTLoad(Op.getNode()) && Op.getResNo() == 0) {
       EVT VT = LD->getMemoryVT();
       unsigned MemBits = VT.getScalarType().getSizeInBits();
       KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - MemBits);
     } else if (const MDNode *Ranges = LD->getRanges()) {
+      if (LD->getExtensionType() == ISD::NON_EXTLOAD)
         computeKnownBitsFromRangeMetadata(*Ranges, KnownZero);
     }
     break;
   }
   case ISD::ZERO_EXTEND: {
     EVT InVT = Op.getOperand(0).getValueType();
     unsigned InBits = InVT.getScalarType().getSizeInBits();
     APInt NewBits   = APInt::getHighBitsSet(BitWidth, BitWidth - InBits);
     KnownZero = KnownZero.trunc(InBits);

test/CodeGen/PowerPC/selectiondag-extload-computeknownbits.ll

+; RUN: llc -mtriple=powerpc64-bgq-linux < %s
+
+; Check that llc does not crash due to an illegal APInt operation
+
+define i1 @f(i8* %ptr) {
+ entry:
+  %val = load i8, i8* %ptr, align 8, !range !0
+  %tobool = icmp eq i8 %val, 0
+  ret i1 %tobool
+}
+
+!0 = !{i8 0, i8 2}