[cfe-dev] Semantic checks and template instantiations

[Dario Domizioli]

Hello cfe-dev.

I am trying to perform additional checks on the alignment of C++ variables
(outputting diagnostic messages if the checks trigger), and the place where
it feels more natural to do so is in Sema::ActOnVariableDeclarator. There
is already a check for under-alignment there (a call to
Sema::CheckAlignasUnderalignment) so I thought I was on the right track.
However I am finding that my checks don't trigger when the variable being
declared/defined is a template instantiation.

Suppose for the sake of example that I wanted to add a check that warns if
the alignment exceeds a certain number of CharUnits, say 32. If I put the
check in ActOnVariableDeclarator right after the under-alignment check, it
does catch this case:

struct __attribute__(( aligned(64) )) my_aligned { int x; }
// ...
my_aligned my_variable; // Diagnostic here

However it does not catch this case:

template <typename T> class __attribute__(( aligned(64) ))
my_aligned_template {
    int x;
    void foo (T param) { /* some code */ }
}
// ...
my_aligned_template<float> my_variable; // No diagnostic here, but the IR
has the right alignment (64)

I am using ASTContext::getDeclAlign to get the alignment of the variable in
the declaration, as the documentation says that it returns a conservative
alignment for the variable, and it's actually the same function used at
CodeGen time to determine the alignment.
However I seem to understand that at that point (when
ActOnVariableDeclarator runs) the template instantiation hasn't been
processed yet and therefore the type of the variable is still a dependent
type (despite the fact that the layout of my_aligned_template does not in
fact depend on T, but I accept that in the general case it can do). This
means that the alignment is unknown at that stage and I can't really check
it.
In fact, I have noticed that Sema::CheckAlignasUnderalignment (which is my
reference for implementing the checks) also skips dependent and incomplete
types.

Of course when ASTContext::getDeclAlign is used later on in CodeGen
everything is known about variables and their types, so I could move my
checks there, but it would feel wrong to do the checks while IR is being
generated, I think Sema should be the right place to perform semantic
checks.

Can anyone point me in the right direction? Is there a more sensible place
than ActOnVariableDeclarator where I can put my additional alignment checks?

Thanks,
    Dario Domizioli
    SN Systems - Sony Computer Entertainment Group
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