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Re: [tree-ssa] reaching def. question
- From: law at redhat dot com
- To: Devang Patel <dpatel at apple dot com>
- Cc: "gcc at gcc dot gnu dot org list" <gcc at gcc dot gnu dot org>, Diego Novillo <dnovillo at redhat dot com>
- Date: Tue, 16 Mar 2004 20:32:24 -0700
- Subject: Re: [tree-ssa] reaching def. question
- Reply-to: law at redhat dot com
In message <11A7C99E-77AD-11D8-9053-000393A91CAA@apple.com>, Devang Patel write
s:
>
>On Mar 16, 2004, at 4:27 PM, law@redhat.com wrote:
>
>> You also have to be damn careful about marking something to be
>> rewritten
>> which is used in a mixed PHI node. ie
>>
>> a_3 = PHI (a_2, b_1)
>
>This is new to me. When do I expect to see mixed PHI node? An example
>would help me to understand it.
It happens on a regular basis due to copy propagation.
For example if you had
if (cond)
a_4 = b_1;
else
a_2 = p + q;
a_3 = PHI (a_2, a_4);
Copy propagation would turn the PHI into:
a_3 = PHI (a_2, b_1);
jeff