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Re: Language-independent functions-as-trees representation
- From: Jason Merrill <jason at redhat dot com>
- To: Mark Mitchell <mark at codesourcery dot com>
- Cc: Richard Henderson <rth at redhat dot com>,Per Bothner <per at bothner dot com>, Diego Novillo <dnovillo at redhat dot com>,"gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, Jason Merrill <jason at redhat dot com>
- Date: Mon, 22 Jul 2002 00:35:21 +0100
- Subject: Re: Language-independent functions-as-trees representation
- References: <64910000.1027292337@warlock.codesourcery.com>
>>>>> "Mark" == Mark Mitchell <mark@codesourcery.com> writes:
>> Why? We don't currently share expressions in RTL; why would we want to do
>> so in SIMPLE?
> Maybe this will make more sense after my other comment below.
> Besides that, it makes things like rtx_equal_p simple/faster to implement,
> and could save tons of memory.
>> Sorry, I don't follow. Most computations care very much about where they
>> are in the control flow; if I move computations about arbitrarily, their
>> values will tend to change. The value of "x + y" is different before and
>> after I increment x. It's the job of CSE to recognize when we can share
>> values between multiple instances of an expression.
> I was implicitly (sorry!) in SSA form.
I remain unenlightened. Can you give an example of where such sharing
would be useful?
Jason