mono struct return temporary variable not inlined

[arbruijn]

Mono mcs seems to create a temporary variable when accessing fields in structs returned from calls (e.g. getStruct().x). This temporary variable is not inlined with recent ILSpy versions. It used to be inlined with ILSpy 2, but the current IsGeneratedValueTypeTemporary check prevents this.

The variable is inlined when I add the following code at the top of IsGeneratedValueTypeTemporary, I don't know if that's a proper fix:

if (loadInst.Parent.OpCode == OpCode.LdFlda)
	return true;

For example for the following the class:

class Structret {
    struct S { public int x; }
    static S getS() { return new S(); }
    void Do(int x) { Do(getS().x); }
}

The Do method is compiled with mono to:

IL_0000: ldarg.0
IL_0001: call valuetype testvar/S testvar::getS()
IL_0006: stloc.0
IL_0007: ldloca.s 0
IL_0009: ldfld int32 testvar/S::x
IL_000e: call instance void testvar::Do(int32)

And this is currently decompiled to:

private void Do(int x)
{
	S s = getS();
	Do(s.x);
}

monostructret.zip

[tamlin-mike]

While that fixes the current scenario, I it could produce incorrect decompilation if the data in slot 0 is used (read) again later in scope.

Probably DFA - even if trivially simple - needs to be used, together with some heuristics to not generate excessively long expressions (in case access would be something like getS().foo.bar.longname.baz.x).
I believe this example could be a good starting point, to only inline 1st level of such chains.