[PATCH] D29014: [SelDag][MIR] Add FREEZE

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune marked an inline comment as done.
aqjune added inline comments.


================
Comment at: llvm/include/llvm/Target/Target.td:1067
+  let hasNoSchedulingInfo = 1;
+  let isNotDuplicable = 1;
+}
----------------
arsenm wrote:
> arsenm wrote:
> > aqjune wrote:
> > > arsenm wrote:
> > > > Why isNotDuplicable? I don't think this is the best maintained instruction property
> > > It is because different freeze defs can yield different values. For example, consider following transformation:
> > > ```
> > > %undef = IMPLICIT_DEF
> > > %x = FREEZE %undef
> > > use(%x)
> > > use(%x) // these two uses should see the same freezed value
> > > ->
> > > %undef = IMPLICIT_DEF
> > > %x = FREEZE %undef
> > > use(%x)
> > > %x' = FREEZE %undef
> > > use(%x') // It is possible that %x and %x' are assigned differently freezed values.
> > > ```
> > > This transformation is incorrect, because use()s after the transformation can see different values.
> > > To prevent this class of optimizations, isNotDuplicable is set to 1.
> > But these both are using the same undef vreg? The second freeze is still using the original undef, so this should be fine?
> To clarify, I think the property you are really looking for is not the property given to you by isNotDuplicable. What you really care about is the input operand not using a new instance of undef. The "nonduplicability" is a function of the input value and not the instruction itself
The second use of the undef register can be assigned a different physical register at (perhaps) register allocation, after seeing that its definition is IMPLICIT_DEF.
```
%undef = IMPLICIT_DEF
%x = FREEZE %undef
use(%x)
%x' = FREEZE %undef
use(%x')
=>
%x = FREEZE $eax
use(%x)
%x' = FREEZE $ebx
use(%x')
```
This transformation can happen if there is a function call between them. `test/CodeGen/X86/freeze-call.ll` checks that it never happens.



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  https://reviews.llvm.org/D29014/new/

https://reviews.llvm.org/D29014