[LLVMdev] Proposed modification of AsmPrinter::EmitConstantValueOnly()

Nick Lewycky nicholas at mxc.ca
Thu Aug 14 23:17:37 PDT 2008

Richard Pennington wrote:
> I have a problem (bug 2672) in which I have a static initializer that 
> contains a GEP. The GEP itself has arguments that do casts and another GEP.
> This fails in EmitConstantValueOnly() because the GEP arguments are not 
> recognized as constant (link time) expressions.

> It looks to me that a proper solution could be to make 
> EmitConstantValueOnly() return its constant value and call it for each 
> argument of the GEP which will then be passed to getIndexedOffset().

I don't think that's necessary. You should be able to emit the '((', 
EmitConstantValueOnly(innerConstant), then emit ') * ' followed by 
TD->getIndexedOffset(innerConstant). Something like that, substitute 
whatever pattern you need.

> Does this sound like a reasonable solution?

I'm concerned about the case where the parameter to the GEP really is 
something that can't be folded down to an integer. A GEP can be written 
out in terms of addition and multiplies, even if you don't know what 
each field is.

So as long as:

   i32 ptrtoint (i32* getelementptr (i32* @global, i32 sub (ptrtoint 
(i32* @symb1 to i32), (ptrtoint (i32* @symb2 to i32)))))

really does become:

  .global + ((.symb1 - .symb2) * 4)

then I'd be happy. Or if I got the example wrong, whatever the right 
answer really is ;-)


> -Rich
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