[PATCH PR62151]Fix uninitialized register issue caused by distribute_notes in combine pass

Jeff Law law@redhat.com
Mon Sep 1 16:52:00 GMT 2014


On 08/31/14 06:18, Segher Boessenkool wrote:
> On Fri, Aug 29, 2014 at 11:58:37PM -0600, Jeff Law wrote:
>> One could argue that this mess is a result of trying to optimize a reg
>> that is set more than once.    Though I guess that might be a bit of a
>> big hammer.
>
> It works fine in other cases, and is quite beneficial for e.g. optimising
> instruction sequences that set a fixed carry register twice.
How common is that?

While we don't have any formal SSA-like properties in RTL, we're 
certainly better off if we can avoid unnecessary cases where a single 
pseudo is set more than once and these days I wouldn't expect too many 
cases where have multiple sets appearing in a dep chain that can be 
processed by combine (and if we do one could easily argue those dep 
chains should be simplified).

Jeff



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