The change implements constant folding of ‘llvm.experimental.constrained.fcmp’
and ‘llvm.experimental.constrained.fcmps’ intrinsics.
This still worries me. Are _analysis_ passes allowed to change the input IR? What if the caller decides to not do a fold after calling this function to see if a fold is possible? And why is the ebIgnore case different from the other two?
Constant folding won't add this ReadNone attribute when the constant folding code doesn't have an Instruction to alter. I don't know what to do about that.
Besides moving this code to a new function, are there any changes to it? It's hard to tell.
There are no "maytrap" tests here. For the sake of future readers it would probably be useful to show that "maytrap" was taken into account and is working correctly.
Constant folding is not an analysis, it changes IR. I don't know why this file is in Analysis directory.
Nothing bad would happen. Only side effect would be removed, but we know for sure that it is absent, we just evaluated the operation. The function call would not be removed if its result is used.
If exception behavior is ebIgnore, such calls will get attribute SDNodeFlags::NoFPExcept in DAG and such instructions do not have side effects. But setting ReadNone for instructions with ebIgnore allows removal of these instructions earlier, at IR level, which could have positive effect.
Comparison of two floating point numbers do not use memory access. But it can change bits in the floating point state register (only Invalid bit can be set). This change is emulated as memory access so that instruction be ordered correctly. This is why constrained intrinsics declared with attribute IntrInaccessibleMemOnly. As no actual memory access occurs, it is harmless to set ReadNone in this case.
No, this is only a code moving.
Added tests with "maytrap".
The constant folding analysis does not change IR. Users of the constant folding analysis change IR based on the analysis result.
My point here was that adding a readnone attribute invalidates MemorySSA, because it means that the instruction should no longer have a MemoryAccess -- it would result in a verification failure. Just calling ConstFold/InstSimplify should never have this kind of effect.