[InstCombine] Missed optimization: fold other_cond && (a s> -1) && a s< b to other_cond && a u< b when b is a specific constant

[XChy]

Alive2 proof: https://alive2.llvm.org/ce/z/DT9eDZ

Motivating example:

define i1 @src(i32 %a, i1 %other_cond) {
entry:                                      
  %cmp1 = icmp sgt i32 %a, -1
  %or.cond = select i1 %other_cond, i1 %cmp1, i1 false
  %cmp2 = icmp slt i32 %a, 10086
  %ret = select i1 %or.cond, i1 %cmp2, i1 false
  ret i1 %ret
}

can be folded to:

define i1 @tgt(i32 %a, i1 %other_cond) {
entry:                                      
  %or.cond = icmp ult i32 %a, 10086
  %ret = select i1 %other_cond, i1 %or.cond, i1 false
  ret i1 %ret
}

However, it folds if we replace the logical and with a bitwise and. I thought that Reassociate should be to blame, but godbolt told me that similar fold for bitwise and is handled by InstCombine. Therefore, I tag InstCombine here.

Real-world motivation

This snippet of IR is derived from qemu/net/util.c@net_parse_macaddr (after O3 pipeline).
The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR, see also:https://godbolt.org/z/Ea8GGxKba

Let me know if you can confirm that it's an optimization opportunity, thanks.

[dtcxzyw]

It would be fixed by #76621 if we know that select (icmp sgt i32 %a, -1), (icmp slt i32 %a, 10086), false can be simplified into icmp ult i32 %a, 10086.

[nikic]

For cases with a bitwise outer operation, this is handled by

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

Line 2565 in 2c2de4b

// TODO: Make this recursive; it's a little tricky because an arbitrary

. We're missing handling for the case of a logical outer operation, I believe.

[XChy]

I think this issue can be handled easier. Note that:
(A && B) && C --> A && (B & C) if "C is poison" implies "B is poison": https://alive2.llvm.org/ce/z/nb2W5i
I realized this after I handicrafted a bunch of proofs on the general reassociation...

[XChy]

I will try to file a simple solution for it. Though I also have one much complex patch.