[LLVMdev] nested GEP in a static initializer fails

[Joel E. Denny]

Hi Nick,

On Tue, 26 Oct 2010, Nick Lewycky wrote:

> Joel E. Denny wrote:

> > Given this LLVM assembly:
> > 
> >    @a = global i8* getelementptr (i8* null, i64 mul (i64 ptrtoint (i32*
> > getelementptr (i32* null, i32 1) to i64), i64 2))
> > 
> > llc fails an assertion:
> > 
> >    llc: /home/jdenny/llvm-svn/include/llvm/Support/Casting.h:202: typename
> > llvm::cast_retty<To, From>::ret_type llvm::cast(const Y&) [with X =
> > llvm::ConstantInt, Y = llvm::Value*]: Assertion `isa<X>(Val)&&  "cast<Ty>()
> > argument of incompatible type!"' failed.

> The limitations are a product of the ABI and on-disk .o file format. For
> example, given "int X = (int)&G / (int)&H;", llc will happily generate "X:
> .long G/H" which is invalid assembly, but llc doesn't know that.
>
> Maybe with the MC infrastructure we could start to figure out what is and
> isn't representable.

llc is already able to determine that something has gone wrong.  It fails 
an assertion.  Could that be turned into a user-friendly error message?

However, I'm not sure that the target assembly's exact limitations are 
really the issue in the cases I've been encountering.  I think the issue 
is how far LLVM is able to go to fold constant expressions, in general.

For example, for my platform, llc folds the add but not the getelementptr 
in:

  @a = global i8* getelementptr (i8* null, i64 add (i64 3, i64 2))

so we get:

  .quad	0+5

However, it folds neither of those in:

  @b = global i64 add (i64 ptrtoint (i8* getelementptr (i8* null, i64 3) to i64), i64 2)

so we get:

  .quad	((0+1)*3)+2

In other words, it seems llc knows when it must fold an add, but it 
doesn't do it if it doesn't have to.  Why can't it use the same logic for 
getelementptr?  For example, if a getelementptr invocation has the 
following form:

  <result> = getelementptr (<pty>* null{, <ty> <idx>}*)

where every idx is a constant, is there any reason this getelementptr 
cannot be folded?  This would allow more flexibility in building 
expressions that use getelementptr to compute sizeof or offsetof.

In case it isn't clear, my goal is more about understanding constant 
folding in LLVM than in seeing this particular functionality implemented.

Thanks.