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Re: Typecasting information in MEM[...] GIMPLE

On Mon, Jul 27, 2015 at 12:20 PM, Jakub Jelinek <> wrote:
> On Mon, Jul 27, 2015 at 03:35:45PM +0530, Uday Khedker wrote:
>> We are interested in extracting the type of a tree node that appears within
>> Given a C program:
>>     struct node * * pvar;
>>     struct node qvar;
>>     pvar = (struct node * *) malloc (sizeof (struct node *));
>>     *pvar = &qvar;
>> It is transformed into the following GIMPLE code:
>>     void * * pvar.1;
>>     void * pvar.0;
>>     pvar.0_1 = malloc (4);
>>     pvar = pvar.0_1;
>>     MEM[(struct node * *)pvar.0_1] = &qvar;
>> We wish to discover the type of the argument of MEM[...] in the last
>> GIMPLE statement. We can see from the GIMPLE dump that the argument's
>> type is "struct node * *". How do we extract this from the tree
>> definition of MEM[...]?
>> We speculate the following solution: Given a variable var (whose tree is
>> tree_of_var) and a tree, say t,
>>     if (TREE_CODE(t) is MEM_REF) and (TREE_OPERAND(t, 0) is tree_of_var)
>>     then
>>         the type of the expression inside MEM[...] of tree t is
>>         POINTER_TYPE to TREE_TYPE(t).
>> Is is correct? It is general enough?
> A MEM_REF has 3 possibly distinct types.  If TREE_CODE (t) == MEM_REF,
> one type, TREE_TYPE (t), is the type of the access, struct node *
> in the above case.  Another type is one for alias analysis purposes,
> stored in TREE_TYPE (TREE_OPERAND (t, 1)), this one will be
> struct node ** in your case.  And yet another type is the type of the
> pointer, TREE_TYPE (TREE_OPERAND (t, 0)), which usually is the same
> as pointer to TREE_TYPE (t) initially, but as most of pointer conversions
> are regarded as useless, after optimization passes you often can end up
> there with very different type.

Note that TREE_TYPE (t) is the type of the access while the other two
types are pointer types.  Thus TREE_TYPE (t) might be 'int' for example
while the other two are always pointer types.

Also TREE_TYPE (t) is used to constrain the minimum alignment of
the access - TYPE_ALIGN (t) is supposed to hold at runtime.


> The type for alias analysis purposes can also differ from pointer to
> TREE_TYPE (t), consider e.g.
>   short *p = ...;
>   int i = 26;
>   memcpy (p, &i, sizeof (int));
> which is folded (depending on alignment behavior) as MEM_REF[(int *)p] = 26;
> and here TREE_TYPE (t) will be int, TREE_TYPE (TREE_OPERAND (t, 0)) likely
> short * and TREE_TYPE (TREE_OPERAND (t, 1)) likely char * (ref_all).
>         Jakub

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