[LLVMdev] Question regarding ReplaceValueWith and ReplaceNodeResults

Pranav Bhandarkar pranavb at codeaurora.org
Sun Sep 2 16:51:33 PDT 2012

Hi Duncan,

Thanks for the information. Another question inline below.
>    /// ReplaceNodeResults - This callback is invoked when a node result
type is
>    /// illegal for the target, and the operation was registered to use
>    /// lowering for that result type.  The target places new result values
>    /// the node in Results (their number and types must exactly match
> of
>    /// the original return values of the node), or leaves Results empty,
>    /// indicates that the node is not to be custom lowered after all.
>    ///
>    /// If the target has no operations that require custom lowering, it
need not
>    /// implement this.  The default implementation aborts.

Right.  Now, we have a unique case in Hexagon, wherein we have some
intrinsics that return i8.
"i8" itself is not a supported type in general on hexagon, except for
instructions that return predicates. These 8bit results from intrinsics are
stored in predicate registers.
Typically, such intrinsics are for comparisons and bit-field testing /
setting /clearing. There are no 8 bit registers for arithmetic though.

The problem is that if we add "i8" to the PredRegsClass then Isel lowering
thinks "i8" is a legal type on Hexagon, which is not necessarily the case.
We decided to fake the PredRegsClass as taking "i32" too and then use
instruction patterns (PredRegs:$src1 vs IntRegs:$src1) to disambiguate
between the use of PredRegs and IntRegs (32 bit GPR).

So, the intrinsic in question is defined to return "i8" (llvm) or 'c' (in
clang) and we hope to use the type legalizer to promote the result to 'i32'.
This way we can then write a pattern for i32 that uses PredRegs.
But, the problem is I cannot setOperationAction for INTRINSIC_WO_CHAIN to
promote because I do not want to do that for all intrinsics. So, I set it to
"custom" and tried to, as it turns out now, quite wrongly, use
ReplaceNodeResults.  How do I get the type of the result to be promoted to
i32 then ? Any suggestions would be much appreciated.


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