[llvm-bugs] [Bug 45315] New: TargetLowering::computeKnownBitsForTargetNode - constant pool extraction

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https://bugs.llvm.org/show_bug.cgi?id=45315

            Bug ID: 45315
           Summary: TargetLowering::computeKnownBitsForTargetNode -
                    constant pool extraction
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: craig.topper at gmail.com, efriedma at quicinc.com,
                    llvm-bugs at lists.llvm.org, spatel+llvm at rotateright.com

We can't safely extract the knownbits of target constant pools (e.g.
X86ISD::VBROADCAST_LOAD) because SimplifyDemandedBits may then attempt to
constant fold it, which then infinite loops as it gets relowered to
X86ISD::VBROADCAST_LOAD again......

As an initial stopgap, we could just not constant fold if SimplifyDemandedBits
encounters an opcode above FIRST_TARGET_MEMORY_OPCODE (or is a MemSDNode?) but
this doesn't address non-loading target patterns that might be used for
constant creation, so maybe we need some way to flag that a
computeKnownBitsForTargetNode call came from a lowered constant?

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