Treat this as a parameter in IR generation

[rdmarsh2]

This stops generating InitalizeThis instructions for C++, in favor of treating this as a parameter, with corresponding InitializeParameter and InitializeIndirection instructions. This makes alias analysis handle the this pointer accurately. C# IR generation is not currently affected.

This closes https://github.com/github/codeql-c-analysis-team/issues/50

[dbartol]

Mostly LGTM. A few suggested tweaks.

Does this really not affect any dataflow results (yet)?

[dbartol]

Suggested change

	TIndirectParameterAllocation(IRVariable var) {
	TIndirectParameterAllocation(IRAutomaticVariable var) {

[dbartol]

Suggested change

	IRVariable var;
	IRAutomaticVariable var;

[dbartol]

You could probably just reuse the tags we already use when generating other InitializeParameter sequences. Not a big deal either way, though.

[rdmarsh2]

It does affect one result in dataflow, but this doesn't add the output side. I think it should affect a few more tests, though...

[MathiasVP]

Does this really not affect any dataflow results (yet)?

Apparently the reason we're not getting a lot of dataflow changes is because a setter like this:

struct A {
  int x; int y;
  void set_x(int val) {
    this->x = val;
  }
};

is not getting a ReturnIndirection from which the partial definition of this gets propagated out to the caller. With this PR I would expect that setter to behave as the following code, though:

void set_x(A* a, int val) {
  a->x = val;
}

But the IR for the first setter is:

#  118| void A::set_x(int)
#  118|   Block 0
#  118|     v118_1(void)           = EnterFunction                :
#  118|     m118_2(unknown)        = AliasedDefinition            :
#  118|     m118_3(unknown)        = InitializeNonLocal           :
#  118|     m118_4(unknown)        = Chi                          : total:m118_2, partial:m118_3
#  118|     r118_5(glval<unknown>) = VariableAddress[#this]       :
#  118|     m118_6(glval<A>)       = InitializeParameter[#this]   : &:r118_5
#  118|     r118_7(glval<A>)       = Load                         : &:r118_5, m118_6
#  118|     m118_8(A)              = InitializeIndirection[#this] : &:r118_7
#  118|     r118_9(glval<int>)     = VariableAddress[val]         :
#  118|     m118_10(int)           = InitializeParameter[val]     : &:r118_9
#  119|     r119_1(glval<int>)     = VariableAddress[val]         :
#  119|     r119_2(int)            = Load                         : &:r119_1, m118_10
#  119|     r119_3(glval<unknown>) = VariableAddress[#this]       :
#  119|     r119_4(A *)            = Load                         : &:r119_3, m118_6
#  119|     r119_5(glval<int>)     = FieldAddress[x]              : r119_4
#  119|     m119_6(int)            = Store                        : &:r119_5, r119_2
#  119|     m119_7(unknown)        = Chi                          : total:m118_8, partial:m119_6
#  120|     v120_1(void)           = NoOp                         :
#  118|     v118_11(void)          = ReturnVoid                   :
#  118|     v118_12(void)          = AliasedUse                   : m118_3
#  118|     v118_13(void)          = ExitFunction                 :

and for the second one it's:

#  124| void set_x(A*, int)
#  124|   Block 0
#  124|     v124_1(void)       = EnterFunction            :
#  124|     m124_2(unknown)    = AliasedDefinition        :
#  124|     m124_3(unknown)    = InitializeNonLocal       :
#  124|     m124_4(unknown)    = Chi                      : total:m124_2, partial:m124_3
#  124|     r124_5(glval<A *>) = VariableAddress[a]       :
#  124|     m124_6(A *)        = InitializeParameter[a]   : &:r124_5
#  124|     r124_7(A *)        = Load                     : &:r124_5, m124_6
#  124|     m124_8(unknown)    = InitializeIndirection[a] : &:r124_7
#  124|     r124_9(glval<int>) = VariableAddress[val]     :
#  124|     m124_10(int)       = InitializeParameter[val] : &:r124_9
#  125|     r125_1(glval<int>) = VariableAddress[val]     :
#  125|     r125_2(int)        = Load                     : &:r125_1, m124_10
#  125|     r125_3(glval<A *>) = VariableAddress[a]       :
#  125|     r125_4(A *)        = Load                     : &:r125_3, m124_6
#  125|     r125_5(glval<int>) = FieldAddress[x]          : r125_4
#  125|     m125_6(int)        = Store                    : &:r125_5, r125_2
#  125|     m125_7(unknown)    = Chi                      : total:m124_8, partial:m125_6
#  126|     v126_1(void)       = NoOp                     :
#  124|     v124_11(void)      = ReturnIndirection[a]     : &:r124_7, m125_7
#  124|     v124_12(void)      = ReturnVoid               :
#  124|     v124_13(void)      = AliasedUse               : m124_3
#  124|     v124_14(void)      = ExitFunction             :

@rdmarsh2 is that intentional?

[rdmarsh2]

Yes. I was planning to add the ReturnIndirection in a followup PR.

[dbartol]

LGTM
Take a quick look at the new consistency failure in fields/dataflow-ir-consistency.expected, but if it's just a dupe of the other two existing failures, feel free to merge.

[rdmarsh2]

It looks like the same issue as the existing two failures (Chi node associated with a call argument that's a pointer to a field having both the pointer type and an unknown type)