llvm dev - Nov 2008 - [LLVMdev] computeMaskedBitsforTargetNode

Can someone explain what this function is supposed to do? I've looked at
PowerPC, Sparc, etc.. and can't really figure out what I should be doing
here.

 

Thanks,

 

Micah Villmow

Systems Engineer

Advanced Technology & Performance

Advanced Micro Devices Inc.

4555 Great America Pkwy,

Santa Clara, CA. 95054

P: 408-572-6219

F: 408-572-6596

 

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On Thu, Nov 20, 2008 at 6:19 PM, Villmow, Micah <Micah.Villmow at amd.com>
wrote:
> Can someone explain what this function is supposed to do? I've looked
at
> PowerPC, Sparc, etc.. and can't really figure out what I should be
doing
> here.
>
Here's what the comment in X86/X86ISelLowering.h has to say:

    /// computeMaskedBitsForTargetNode - Determine which of the bits
specified
    /// in Mask are known to be either zero or one and return them in
the
    /// KnownZero/KnownOne bitsets.


-bw

On Thu, Nov 20, 2008 at 6:19 PM, Villmow, Micah <Micah.Villmow at amd.com>
wrote:
> Can someone explain what this function is supposed to do? I've looked
at
> PowerPC, Sparc, etc.. and can't really figure out what I should be
doing
> here.
It's an optimization: it allows the target-independent passes like
DAGCombine to get information about which bits of a value are known at
compile-time to be set or known to be unset.  The conservative
assumption is that nothing is known, so the default implementation is
always correct.

-Eli