Accessing const parameter of GIMPLE_CALL

David Malcolm dmalcolm@redhat.com
Sun Jan 16 23:53:13 GMT 2022


On Sun, 2022-01-16 at 18:52 +0530, Shubham Narlawar via Gcc wrote:
> Hello,

Hi; various notes inline below...

> 
> My aim is to iterate over gimple call stmt parameters and check
> whether it is constant or constant expression and mark/store them for
> some gimple transformation.
> 
> I have an intrinsic function call of the following -
> 
> __builtin_xyz(void*, 7, addr + 10);
> 
> I want to find its parameters which are either constant or constant
> expression i.e. 7 and addr + 10 from above case.

Gimple "flattens" all tree-like operations into a sequence of simple
operations, so I would expect the gimple for this to look something
like this:

   _tmp = addr + 10;
   __builtin_xyx (7, _tmp);

Your email doesn't specify *when* your code runs.

The IR for a function goes through several stages:

- an initial gimple IR without a CFG
- gimple with a CFG, but not in SSA
- gimple-SSA with a CFG
  (most of the gimple optimization passes operate in this form of the
IR)
- gimple with a CFG, but no longer in CFG form, immediately before
conversion to RTL-with-CFG form
- RTL-with-CFG
- RTL-without a CFG
- assembler

Are you doing it as part of a plugin, or modifying an existing pass? 
In either case, it's a good idea to dump the gimple and see what the
code has been turned into.  You'll probably find the following options
useful:
  -fdump-tree-all -fdump-gimple-all

or alternatively just turn it on for the pass that you're working on.

> 
> [1] I tried below macro but there is very less usage in the entire
> source code -
> 
> tree fn_ptr = gimple_call_fn (dyn_cast<gcall *> (stmt));        //stmt

gimple_call_fn returns the function that will be called, a pointer. 
This is very general, for handling things like jumps through function
pointers, but here you have the common case of a callsite that calls a
specific function, so "fn_ptr" here is:
   &__builtin_xyx
i.e. an ADDR_EXPR where operand 0 is the FUNCTION_DECL for the builtin.

> = gimple_call
> function_args_iterator iter;
> tree argtype;
> 
> if (TREE_CODE (fn_ptr) == ADDR_EXPR)
> {
>   FOREACH_FUNCTION_ARGS (fn_ptr, argtype, iter)

Looking in tree.h, FOREACH_FUNCTION_ARGS takes a FUNCTION_TYPE as its
first argument, but the code above is passing it the ADDR_EXPR wrapping
the FUNCTION_DECL.

Unfortunately, because these things are all of type "tree", this kind
of type mismatch doesn't get caught - unless you build gcc from source
(with --enable-checking=debug) in which case all these accesses are
checked at the compiler's run time (which is probably a good thing to
do if you're hoping to work on gcc for GSoC).

You can get the FUNCTION_TYPE of a FUNCTION_DECL via TREE_TYPE
(fndecl), or alternatively, gimple_call_fntype (call) will get the type
of the function expected at the call stmt (useful if there was a type
mismatch).

That said, FOREACH_FUNCTION_ARGS iterates through the types of the
params of the FUNCTION_TYPE, but it sounds like you want to be
iterating through the arguments at this particular *callsite*.

For that you can use
  gimple_call_num_args (call);
and
  gimple_call_arg (call, idx);

>     {
>         if (TREE_CONSTANT (argtype))
>            // Found a constant expression parameter
>     }
> }
> 
> The problem is I am getting only one parameter tree but there are 2
> constants in the above function call. Even if "addr + 10" is treated
> differently, I want to mark it for the transformation.

I think you're seeing the function pointer being called, ather than the
params.

> 
> a. Is the above correct method to iterate over function call
> parameters?

As noted above, it depends on whether you want to iterate over the
types of the parameters in the function's decl, or over the expressions
of the arguments at the callsite.  I believe the above explains how to
do each of these.

> b. Is there a different way to achieve the above goal?

If you're looking to get familiar with GCC's insides, I recommend
stepping through it in the debugger, rather than relying on injecting
print statements and recompiling, since the former makes it much easier
to spot mistakes like the one above (which we all make).

I've written a guide to debugging GCC here:

https://dmalcolm.fedorapeople.org/gcc/newbies-guide/debugging.html


Hope this is helpful
Dave



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