[PATCH] D71241: [OpenMP][WIP] Use overload centric declare variants

[Johannes Doerfert via Phabricator via cfe-commits]

jdoerfert added a comment.

In D71241#1782460 <https://reviews.llvm.org/D71241#1782460>, @JonChesterfield wrote:

> > https://clang.llvm.org/docs/InternalsManual.html#the-ast-library
> > 
> >   Faithfulness¶
> >   The AST intends to provide a representation of the program that is faithful to the original source. 
>
> That's pretty convincing.


So let's actually look at the AST instead of just talking about it:

We take the asm.cpp example from @ABataev that, as I argued earlier, shows nicely why the alias solution does not work at all once we start thinking about linking things.

Now here is the code with calls to make it actually interesting:

  static void cpu() { asm("nop"); }
  
  #pragma omp declare variant(cpu) match(device = {kind(cpu)})
  static __attribute__((used)) void wrong_asm() {
    asm ("xxx");
  }
  
  void foo() {
    cpu();
    wrong_asm();
  }

In the current approach the as of foo looks like this:

  `-FunctionDecl 0x563baf0af958 <line:8:1, line:11:1> line:8:6 foo 'void ()'
    `-CompoundStmt 0x563baf0afb38 <col:12, line:11:1>
      |-CallExpr 0x563baf0afa78 <line:9:3, col:7> 'void'
      | `-ImplicitCastExpr 0x563baf0afa60 <col:3> 'void (*)()' <FunctionToPointerDecay>
      |   `-DeclRefExpr 0x563baf0afa40 <col:3> 'void ()' lvalue Function 0x563baf0af458 'cpu' 'void ()'
      `-CallExpr 0x563baf0afb18 <line:10:3, col:13> 'void'
        `-ImplicitCastExpr 0x563baf0afb00 <col:3> 'void (*)()' <FunctionToPointerDecay>
          `-DeclRefExpr 0x563baf0afae0 <col:3> 'void ()' lvalue Function 0x563baf0af668 'wrong_asm' 'void ()'

As you might see, you don't see any hint of the declare variant stuff that will eventually transform the `wrong_asm` call into a `cpu` call.

In the proposed scheme, the AST looks like this:

  `-FunctionDecl 0x1e53398 <line:8:1, line:11:1> line:8:6 foo 'void ()'
    `-CompoundStmt 0x1e53580 <col:12, line:11:1>
      |-CallExpr 0x1e534b8 <line:9:3, col:7> 'void'
      | `-ImplicitCastExpr 0x1e534a0 <col:3> 'void (*)()' <FunctionToPointerDecay>
      |   `-DeclRefExpr 0x1e53480 <col:3> 'void ()' lvalue Function 0x1e52e98 'cpu' 'void ()'
      `-CallExpr 0x1e53560 <line:10:3, col:13> 'void'
        `-ImplicitCastExpr 0x1e53548 <col:3> 'void (*)()' <FunctionToPointerDecay>
          `-DeclRefExpr 0x1e53520 <col:3> 'void ()' lvalue Function 0x1e52e98 'cpu' 'void ()' (Function 0x1e530a8 'wrong_asm' 'void ()')

Here, both the original callee (`wrong_ast`) and the actual callee `cpu` are shown at the call site.

Why would we not want that?


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