Ignore clobber def when replacing register in WEB pass (was Re: Fix a bug in merging uninitialized refs into a single web)

[Jeff Law]

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On 02/09/11 22:05, Jie Zhang wrote:
> Hi Jeff,
> 
> On 02/05/2011 08:05 PM, Jie Zhang wrote:
>>>> Thanks for review. Yeah. I thought about ignoring the clobber at first.
>>>> But later I found there was a bug in the code which merges
>>>> uninitialized
>>>> refs into a single web and fixing that bug should also fix the issue I
>>>> encountered. So I just try to fix that bug which will be safer and
>>>> easier for me.
>>> So just so I'm certain I understand the problem. In the original
>>> testcase a naked CLOBBER was the "set" that triggered the problem, but
>>> this problem can occur for assignments to any uninitialized pseudo, such
>>> as in examples you provided below.
>>>
>>> When we see a set to an uninitialized pseudo, we're losing the saved
>>> DF_REF_UID which allows us to combine all the uninitialized uses into a
>>> single web. Right?
>>>
>> Yes. My patch just prevents this losing.
>>
> While looking into PR 47622, which was caused by my patch, I found my
> patch was wrong. For example
> 
> reg 134                 <-- is uninitialized
> use (reg 134)           <-- use1
> set (reg 134)           <-- def2
> use (reg 134)           <-- use2
> 
> use1 forms a web, def2 and use2 form another web. In such case, we still
> want (reg 134) in the second web to be renamed. But with my patch, it
> will not. That bug I found in Alex's patch is *not* real and I have
> reverted my patch. Sorry for the huge noise I caused.
> 
> A new patch is attached, which just follows Jeff's suggestion to ignore
> clobber in the web pass. Testing is still going on. Is it OK if the
> testing is good?
Is there some reason you don't just do something like

if (NONDEBUG_INSN_P (insn) && GET_CODE (insn) != CLOBBER)

In the outer conditional?

That's more typical of how I ignore naked clobbers.  Otherwise don't you
run the risk of ignoring a clobber which appears inside a normal insn?

jeff
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