llvm dev - Apr 2008 - [LLVMdev] IMPLICIT_DEF

Can someone explain where things like IMPLICIT_DEF_FR64 come from?
I believe something is noticing a use before def and inserting some kind of
bogus code to compensate.  The machine instructions look like this (x86):

	%reg1069<def> = IMPLICIT_DEF_FR64
	FsMOVLPDmr %reg0, 1, %reg0, 0, %reg1069

This is no good -- it stores to zero.

Thanks.

                                           -Dave

IMPLICIT_DEF is a value defined by an 'undef' instruction:

http://llvm.org/docs/LangRef.html#undefvalues

It can come from source program or produced as result of optimization.

Evan

On Apr 1, 2008, at 2:58 PM, David Greene wrote:
> Can someone explain where things like IMPLICIT_DEF_FR64 come from?
> I believe something is noticing a use before def and inserting some  
> kind of
> bogus code to compensate.  The machine instructions look like this  
> (x86):
>
> 	%reg1069<def> = IMPLICIT_DEF_FR64
> 	FsMOVLPDmr %reg0, 1, %reg0, 0, %reg1069
>
> This is no good -- it stores to zero.
>
> Thanks.
>
>                                           -Dave
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