[PATCH] Optimize redundant insertvalue instructions

[Michael Zolotukhin]

The first one formally isn’t dead, because its result is used in the second (the first argument). The patch actually allows considering it dead.

I’m trying to reproduce this in a small C/C++ test, but that’s not that easy.

Thanks,
Michael

On May 6, 2014, at 9:24 PM, Eric Christopher <echristo at gmail.com> wrote:

> This is weird, how'd we get here? How aren't we deleting the first one
> as dead code? Etc.
> 
> -eric
> 
> On Tue, May 6, 2014 at 7:42 AM, Michael Zolotukhin
> <mzolotukhin at apple.com> wrote:
>> Hi,
>> 
>> This patch implements a simple optimization of redundant insertvalue
>> instructions.
>> 
>> Here is a small example where it will help:
>>  %0 = insertvalue { i8, i32 } undef, i8 %x, 0
>>  %1 = insertvalue { i8, i32 } %0,    i8 %y, 0
>> 
>> In this example, the second instruction inserts values at the same indices,
>> as the first one, and consequently, the first instruction is redundant.
>> This code will be transformed to:
>>  %0 = insertvalue { i8, i32 } undef, i8 %y, 0
>> 
>> This optimization catches nothing on specs and test-suite, but the example
>> was inspired by a real world application.
>> 
>> As an experiment, I tried a more sophisticated implementation, which was
>> capable of dealing with several users and with partially overlapping
>> indices, but that also caught nothing on specs (and for my application the
>> simple version was enough). If there is an interest, I could share a patch
>> with this more complicated implementation as well.
>> 
>> Is it ok to commit this patch?
>> 
>> 
>> 
>> Thanks,
>> Michael
>> 
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