[Lldb-commits] [PATCH] D98370: [lldb] Fix SBValue::Persist() for constant values

[Andy Yankovsky via Phabricator via lldb-commits]

werat added a comment.

In D98370#2705741 <https://reviews.llvm.org/D98370#2705741>, @jingham wrote:

> Sure.  But but when I was poking around at it a little bit, it seems like the other use cases already work, and the only one that was failing was the case where you call persist on a persistent variable.  If that is really true, then maybe we should fix the failing case directly.

Right now `Persist()` doesn't really work for values created via `CreateValueFromData`. You can read them, but can't modify:

  >>> data = lldb.SBData.CreateDataFromUInt64Array(lldb.process.GetByteOrder(), lldb.process.GetAddressByteSize(), [42])
  >>> v = lldb.target.CreateValueFromData('v', data, lldb.target.GetBasicType(lldb.eBasicTypeUnsignedLong))
  >>> v.value
  '42'
  >>> vp = v.Persist()
  >>> vp.name
  '$3'
  >>> lldb.frame.EvaluateExpression('$3').value
  '42'
  >>> lldb.frame.EvaluateExpression('++$3 + 1').value
  >>> lldb.frame.EvaluateExpression('++$3 + 1').error.GetCString()
  "error: supposed to interpret, but failed: Interpreter couldn't read from memory\n"

However I realized my patch doesn't completely fixes it either:

  >>> data = lldb.SBData.CreateDataFromUInt64Array(lldb.process.GetByteOrder(), lldb.process.GetAddressByteSize(), [42])
  >>> v = lldb.target.CreateValueFromData('v', data, lldb.target.GetBasicType(lldb.eBasicTypeUnsignedLong))
  >>> vp = v.Persist()
  >>> vp.name
  '$0'
  >>> lldb.frame.EvaluateExpression('$0').value
  '42'
  >>> lldb.frame.EvaluateExpression('++$0').value
  '43'
  >>> lldb.frame.EvaluateExpression('++$0').value
  '44'
  >>> vp.value
  '42'



In D98370#2705741 <https://reviews.llvm.org/D98370#2705741>, @jingham wrote:

> Not sure why?  The API is a request:  "I made a variable somehow, and I would like you to make it persist so I can use its value later on even if the underlying data has changed."  Why do you care whether you get a copy of an already persistent or just a shared value?

You're right, I got confused by something else. I don't care if I get a new name/copy, as long as I can use it by the returned name it's fine. However I want to point out that the current API does generate a new name every time (but the it points to the same data):

  >>> x = lldb.frame.FindVariable('x')
  >>> x.value
  '1'
  >>> xp1 = x.Persist()
  >>> xp1.name
  '$0'
  >>> xp2 = x.Persist()
  >>> xp2.name
  '$1'
  >>> lldb.frame.EvaluateExpression('++$0 + ++$1').value
  '3'
  >>> xp1.value
  '3'
  >>> xp2.value
  '3'




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  https://reviews.llvm.org/D98370/new/

https://reviews.llvm.org/D98370