SSA: Add data flow integration layer

[hvitved]

This PR adds a new module, DataFlowIntegration, to the shared SSA library, which makes it much easier to integrate SSA into the shared data flow library. The integration layer gives you all the relevant flow steps (such as use-use), but also gives you phi-reads and phi-input barriers without having to worry about how they are implemented.

The interface is inspired by the variable capture library, meaning that we construct a Node type where some branches should map to existing branches in the DataFlow::Node type, while the SsaNode sub class should be added as a new branch to DataFlow::Node.

This PR only adds the new integration layer; follow-up PRs will adopt it for C#, Ruby, and Java.

Examples

The examples below show which local flow steps are generated for a particular code snippet.

Example 1

    void M1()
    {
        string s = "taint";
        Use(s);
        s = "safe";
        Use(s);
    }

flowchart TD
929["SSA def(s)"]
931[""taint""]
934["access to local variable s"]
937["SSA def(s)"]
938[""safe""]
941["access to local variable s"]

929 --> 934
931 --> 929
937 --> 941
938 --> 937

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Example 2

    void M2()
    {
        SsaFlow s = new();
        s.Field = "taint";
        Use(s.Field);
    }

flowchart TD
944["SSA def(s)"]
946["(...) ..."]
949["[post] access to local variable s"]
950["access to local variable s"]
957["access to local variable s"]

944 --> 950
946 --> 944
949 --> 957
950 --> 957

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Example 3

    void M3(bool b1, bool b2)
    {
        string s = "a";

        if (b1)
        {
            s = "b"; // s1
        }
        else
        {
            s = "c"; // s2
        }

        // s3 = phi(s1, s2)

        if (b2)
        {
            Use(s); // s3
        }
        else
        {
            Use(s); // s3
        }
    }

flowchart TD
971["SSA def(s)"]
972["[input] SSA phi(s)"]
973[""b""]
976["SSA def(s)"]
977["[input] SSA phi(s)"]
978[""c""]
979["SSA phi(s)"]
983["access to local variable s"]
986["access to local variable s"]

971 --> 972
972 --> 979
973 --> 971
976 --> 977
977 --> 979
978 --> 976
979 --> 983
979 --> 986

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Example 4

    void M4(bool b1, bool b2)
    {
        string s = "a";

        if (b1)
        {
            s = "b"; // s1
            Use(s); // s1
        }
        else
        {
            s = "c"; // s2
        }

        // s3 = phi(s1, s2)

        if (b2)
        {
            Use(s); // s3
        }
        else
        {
            s = "d"; // s4
            Use(s); // s4
        }

        // s5 = phi(s3, s4)

        Use(s); // s5
    }

flowchart TD
1000["[input] SSA phi(s)"]
1002["[post] access to local variable s"]
1003["access to local variable s"]
1006["SSA def(s)"]
1007["[input] SSA phi(s)"]
1008[""c""]
1009["SSA phi(s)"]
1011["[input] SSA phi(s)"]
1013["[post] access to local variable s"]
1014["access to local variable s"]
1017["SSA def(s)"]
1018[""d""]
1019["[input] SSA phi(s)"]
1021["[post] access to local variable s"]
1022["access to local variable s"]
1024["SSA phi(s)"]
1026["access to local variable s"]
998["SSA def(s)"]
999[""b""]

1000 --> 1009
1002 --> 1000
1003 --> 1000
1006 --> 1007
1007 --> 1009
1008 --> 1006
1009 --> 1014
1011 --> 1024
1013 --> 1011
1014 --> 1011
1017 --> 1022
1018 --> 1017
1019 --> 1024
1021 --> 1019
1022 --> 1019
1024 --> 1026
998 --> 1003
999 --> 998

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Example 5

    void M5(bool b1, bool b2)
    {
        string s = "taint"; // s1

        if (b1)
        {
            Use(s); // s1
        }
        else
        {
            Use(s); // s1
        }

        // s2 = phi_read(s1)

        if (b2)
        {
            Use(s); // s2
        }
        else
        {
            Use(s); // s2
        }
    }

flowchart TD
1033["SSA def(s)"]
1035[""taint""]
1037["[input] SSA phi read(s)"]
1039["[post] access to local variable s"]
1040["access to local variable s"]
1041["[input] SSA phi read(s)"]
1043["[post] access to local variable s"]
1044["access to local variable s"]
1045["SSA phi read(s)"]
1049["access to local variable s"]
1052["access to local variable s"]

1033 --> 1040
1033 --> 1044
1035 --> 1033
1037 --> 1045
1039 --> 1037
1040 --> 1037
1041 --> 1045
1043 --> 1041
1044 --> 1041
1045 --> 1049
1045 --> 1052

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[MathiasVP]

Only a few small comments, but otherwise this LGTM!

[MathiasVP]

Should probably also mention isUseStep in the QLDoc.

[MathiasVP]

Is the missing space intentional?

Suggested change

	override string toString() { result = e.toString() + " [postupdate]" }
	override string toString() { result = e.toString() + " [post update]" }

[hvitved]

I simply copied it from the variable capture library. However, these nodes should never actually be rendered, because they will be mapped to existing DataFlow::Nodes with appropriate toStrings.

[MathiasVP]

LGTM!

[hvitved]

I'm going to ignore the failing C++ test, as it cannot possibly be caused by this PR (which adds, for now, dead code).