[LLVMdev] type legalizer promoting BUILD_VECTORs

[Duncan Sands]

Hi Bob,

> LLVM's type legalizer is changing the types of BUILD_VECTORs in a way  
> that seems wrong to me, but I'm not sure if this is a bug or if some  
> targets may be relying on it.
> 
> On a 32-bit target, the default action for legalizing i8 and i16 types  
> is to promote them.  If you then have a BUILD_VECTOR to construct a  
> legal vector type composed of i8 or i16 values, the type legalizer  
> will look at the BUILD_VECTOR operands and decide that it needs to  
> promote them to i32 types.  You end up with a BUILD_VECTOR that  
> constructs a vector of i32 values that are then bitcast to the  
> original vector type.
> 
> This works fine for SSE, where it appears that BUILD_VECTORs are  
> intentionally canonicalized to use i32 elements for the benefit of  
> CSE.  I'm looking at implementing something where I think I'd like to  
> keep the original vector types.  Is this behavior in the type  
> legalizer something that should be changed?

another way this could be done is to say that the operands of a
BUILD_VECTOR don't have to have the same type as the element type
of the built vector.  Then when the type legalizer sees a
v4i16 = BUILD_VECTOR(i16, i16, i16, i16) it can turn this into a
v4i16 = BUILD_VECTOR(i32, i32, i32, i32) and it will be happy
(all result and operand types are legal).  This requires changing
the definition of BUILD_VECTOR slightly.  Targets will need to
understand that only the bottom 16 bits of the operands are to
be used, but I doubt that's a problem.  Would this solve your
problem?

Ciao,

Duncan.

PS: Can you please give a concrete example where the current
behavior causes trouble for you?