[LLVMdev] Stop MachineCSE on certain instructions

Jim Grosbach grosbach at apple.com
Mon Dec 19 09:03:30 PST 2011

Hi Johannes,

You may be interested in the (very) recently added explicit instruction bundle support. For an example of their usage, have a look at the ARM backend's IT-block (Thumb2 predication support) pass, which uses them to tie instructions together.


On Dec 17, 2011, at 12:24 PM, Johannes Birgmeier wrote:

> Hello,
> I'm writing for a backend and have a complicated instruction bundle (3 
> instructions) that has to be executed like a single block (meaning: if 
> the first instruction is executed, all three have to be executed to 
> obtain the result, though not necessarily without other instructions in 
> between). Unfortunately, MachineCSE gets in the way sometimes and rips 
> it apart.
> Is there a way to stop CSE from doing its thing (common subexpression 
> elimination) for certain instructions?
> I've already tried glueing (gluing?) them together, but that doesn't 
> seem to make a difference.
> Regards,
> Johannes Birgmeier
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