[llvm] [DAGCombiner] Freeze maybe poison operands when folding select to logic (PR #84924)

[Björn Pettersson via llvm-commits]

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@@ -6,21 +6,14 @@
 
 ; (x0 < x1) && (x2 > x3)
 define i32 @cmp_and2(i32 %0, i32 %1, i32 %2, i32 %3) {
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bjope wrote:

I actually started on a patch that simply treated CopyFromReg similar as FREEZE in SelectionDAG::isGuaranteedNotToBeUndefOrPoison. Kind of based on the comments in ISDOpcodes.h about CopyFromReg loading from a register "that is defined outside of the scope of this SelectionDAG". So one could for example assume that we get the same value.

I'm not sure however if there could be other problems. On IR level we only treat arguments as not being poison depending on the noundef attribute. So then I started to think that maybe it was more complicated also for SelectionDAG. But maybe we are saved by only looking at one BB at a time.
I think CopyFromReg can be used on any BB, also dealing with values passed on from one BB to another. So it is at least not only about input arguments to the function (but maybe it can be seen as input arguments to the basic block). Would it for example be a problem if the value tracking would make assumtions based on dominating conditions in predecessor basic blocks? But maybe we avoid such things.

(I know that there were some discussion a couple of years back about poison in SelectionDAG, https://discourse.llvm.org/t/funnel-shift-select-and-poison/51255/25, but that never really landed in any formal decision about how to deal with it.)

https://github.com/llvm/llvm-project/pull/84924