[tree-ssa] reaching def. question

[Devang Patel]

On Mar 16, 2004, at 1:47 PM, Diego Novillo wrote:

> On Tue, 2004-03-16 at 16:39, Devang Patel wrote:
>> How do I find out reaching definitions of 'a' at statement S1?
>>
>> S1:      a = b;
>>
>> What I want to know is whether S1 is defining 'a' first time
>> or killing earlier def of 'a'. compute_reaching_defs () is not
>> yet implemented. Is there any other alternative API?
>>
> compute_reaching_defs() wouldn't help you.  There are no uses of 'a' in
> S1.
>
> You need to do a dominator walk and stop when you find the first
> definition.  But, there can be more than one:
>
> 	if (...)
> 	   a = b;
> 	else
> 	   a = c;
>
> Both defs to 'a' are defininig 'a' for the "first time".  Besides, in
> our current framework.  Every definition is a different variable.
>
> What are you trying to solve?

during if-conversion

S1: a_1 = x;
S2: if (...)
S3:   a_2 = b;

Now, to if-convert S3, I need to know if a_2 is first def. of variable 
'a'
or not. Based on that info I can replace S3 with

S3: MODIFY_EXPR <a_2, COND_EXPR < c, b, a_1>>

OR

S3: MODIFY_EXPR <a_2, COND_EXPR < c, b, NULL>>

--
Devang