On December 23, 2019 6:30:31 PM GMT+01:00, Erick Ochoa
<erick.oc...@theobroma-systems.com> wrote:
>Hi,
>
>I am working on an LTO pass which drops unused fields on structs. On my
>tests, I found that the gimple generated for `sizeof` is a constant.
>For
>example, for the following C code:
>
>```
>struct astruct_s { _Bool c; _Bool a; _Bool b; };
>struct astruct_s astruct;
>
>int
>main()
>{
> int size = sizeof(astruct);
> return 0;
>}
>```
>
>Produces the following gimple code:
>```
>size_1 = 3;
>_2 = 0;
><L0>:
>return _2;
>```
>
>Is there a way to determine where the value assigned to size_1 is
>obtained from? I would like to
>1. inspect sizeof statements
>2. determine whether the argument of sizeof is struct astruct_s
>3. substitute struct astruct_s with a modified version of struct
>astruct_s which has field `a` removed.
>
>Therefore, at the end of this transformation we would have size_1 = 2;
>Similary, I would like pointer arithmetic to be affected. For example:
>
>```
>struct astruct_s { _Bool c; _Bool a; _Bool b; };
>struct astruct_s astruct;
>
>int
>main()
>{
> _Bool *c = &astruct.c;
> _Bool *b = &astruct.b;
> ptrdiff_t d = b - c;
> printf("%d\n", d);
>}
>```
>
>Produces the following gimple code:
>```
>c_3 = &astruct.c;
>b_4 = &astruct.b;
>d_5 = b_4 - c_3;
>```
>
>Running the code results in the value 2 being printed . After running
>the transformation, the result should be 1.
>
>Can anyone point me in the right direction?
I believe there is no convenient point in the compilation to do all of this in
GCC.
Note that users can very well write a literal 8 (substitute correct value) for a
Sizeof expression. Likewise writing pointer subtraction in terms of integers is
possible.
>So far I have tried changing the TYPE_FIELDS to drop field a, but that
>doesn't work and it is not general. I also have some code which
>creates a copy of RECORD_TYPE tree and modifies the tree and creates a
>new identifier for this modified RECORD_TYPE tree. However, I believe
>this may still not produce the intended behaviour.
>
>Any help is appreciated. Thanks!
You need to analyze and change all actual accesses since not all of them will
necessarily use component refs refering to the FIELD_DECLs you change.
Richard.
>-Erick
