[llvm-commits] [PATCH] Don't PRE compares

[Chris Lattner]

On Mar 8, 2012, at 10:53 AM, Jakob Stoklund Olesen wrote:

> The attached patch stops GVN from eliminating partially redundant compare instructions.
> 
> CodeGenPrepare sinks compares to their uses so that they can be fused with branches in isel. The phi node inserted by PRE prevents the sinking, and increases register pressure by forcing the i1 value into a general purpose register.
> 
> There is no way for CodeGenPrepare to figure this out on its own. Run the included test case through opt -O2 to see how value propagation messes with the i1 phi:
> 
>  %cmp3.pre-phi = phi i1 [ %cmp1, %if.then ], [ false, %entry ]
> 
> Owen suggested a more radical approach: Don't PRE instructions that have a single use in the same block. It solves a more general version of the same problem: PRE separates instructions that the code generator might have been able to fuse.

Makes sense to me.  Are there any cases where PRE'ing the compare could cause it to constant fold to true or false in all predecessors?  If so, then we'd like to keep PRE'ing in that case to enable jump threading.

-Chris