llvm dev - Nov 2017 - Expanding SDNodes

Hello all,

I’ve been working on writing a backend, I was testing zext, and during the first
run of combining llc does the following thing:
> === zext
> Creating new node: t3: i16 = truncate t2
> Creating new node: t5: ch = CopyToReg t0, Register:i32 %vreg1, t2
> Creating new node: t6: i32 = zero_extend t3
> Creating new node: t9: ch = MyArchISD::RET_FLAG t8, Register:i32 %R0, t8:1
> Initial selection DAG: BB#0 'zext:entry'
> SelectionDAG has 10 nodes:
>   t0: ch = EntryToken
>   t2: i32,ch = CopyFromReg t0, Register:i32 %vreg0
>     t5: ch = CopyToReg t0, Register:i32 %vreg1, t2
>       t3: i16 = truncate t2
>     t6: i32 = zero_extend t3
>   t8: ch,glue = CopyToReg t5, Register:i32 %R0, t6
>   t9: ch =MyArchISD::RET_FLAG t8, Register:i32 %R0, t8:1
> 
> 
> 
> Combining: t9: ch =MyArchISD::RET_FLAG t8, Register:i32 %R0, t8:1
> 
> Combining: t8: ch,glue = CopyToReg t5, Register:i32 %R0, t6
> 
> Combining: t7: i32 = Register %R0
> 
> Combining: t6: i32 = zero_extend t3
> Creating constant: t10: i32 = Constant<65535>
> Creating new node: t11: i32 = and t2, Constant:i32<65535>
>  ... into: t11: i32 = and t2, Constant:i32<65535>
I was wondering if there is a way to expand the t6 node to “zext t3” so it won’t
combine?
(Maybe defining some pattern on my backend’s tablegen or something like that)
Thanks,
Liad.
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On 11/20/2017 5:31 AM, Liad Mordekoviz via llvm-dev
wrote:
> 
>> Combining: t6: i32 = zero_extend t3
>> Creating constant: t10: i32 = Constant<65535>
>> Creating new node: t11: i32 = and t2, Constant:i32<65535>
>>  ... into: t11: i32 = and t2, Constant:i32<65535>
> 
> I was wondering if there is a way to expand the t6 node to “zext t3” so 
> it won’t combine?
> (Maybe defining some pattern on my backend’s tablegen or something like 
> that)
The DAG combiner will expand zero-extends into ands, and in your 
selection patterns you can simply match a corresponding and with the 
bitmask instead of a zext. You can even write a PatFrag to serve as a 
macro for that.
Another option is that you can lower zero-extend to a custom operation 
for your target. DAG combiner won't do anything with it, and then you 
can match it directly in the patterns.

-Krzysztof

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