[llvm-commits] PATCH: remove VICmp and VFCmp.

[Duncan Sands]

Hi Eli,

>> I think this is a misunderstanding of the roles of type and
>> operation legalization.  The getTypeToTransformTo method exists
>> entirely for the benefit of type legalization.  Since <4 x i1> is
>> an illegal type, it needs to be transformed into the legal type
>> given by getTypeToTransformTo, for example <4 x i8>.
> 
> We don't support that kind of promotion at the moment, although I
> suppose that's an implementation detail rather than a fundamental
> issue.

exactly.  The fact that this might not actually work right now is
an implementation problem, not a conceptual problem.

>> After type
>> legalization has finished you will have vector comparison operations
>> returning <4 x i8>.  Now operation legalization kicks in.  Since the
>> target machine cannot handle vector comparisons returning <4 x i8>,
>> operation legalization needs to turn that into a comparison returning
>> <4 x i32> or <4 x i64>.  It of course needs some way to know which
>> type to use, but this has nothing to do with getTypeToTransformTo.
> 
> Well, suppose a comparison of two <4 x i32> vectors.  Your natural
> promotion for the result on x86 would be to a <4 x i16>.  But yes, you
> could put off the subsequent adjustments to operation legalization,
> although if it weren't custom-lowered, it would likely end up
> producing something really ugly.

Operation legalization = turning illegal operations into legal
operations.  This may be via custom lowering or via some kind of
generic expansion, it doesn't matter.  Since a comparison of two
<4 x i32> vectors that returns a <4 x i16> is an illegal operation
for x86, but with legal types, it is by definition the job of the
operation legalization code to take care of this.

Ciao,

Duncan.