llvm/llvm-project
Voir sur GitHub[InstCombine] Missed optimization: fail to fold `(A >> C1) Pred C2` if `shr` is used multiple times
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#83 430 ouverte le 29 févr. 2024
good first issuellvm:instcombinemissed-optimization
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Description
Alive2 proof: https://alive2.llvm.org/ce/z/4JKW9d
Motivating example
define i1 @src(i32 %a){
entry:
%div = ashr exact i32 %a, 3
call void @use(i32 %div)
%cmp = icmp slt i32 %div, 15
ret i1 %cmp
}
can be folded to:
define i1 @tgt(i32 %a) {
%div = ashr exact i32 %a, 3
call void @use(i32 %div)
%cmp = icmp slt i32 %a, 120
ret i1 %cmp
}
This enables more optimizations for the other user of shr as I observe in benchmark.
Unsigned case is in alive2 proof.
Real-world motivation
Signed case is derived from z3/src/sat/smt/pb_solver.cpp (after O3 pipeline). Unsigned case is derived from z3/src/tactic/core/collect_occs.cpp (after O3 pipeline). The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR, email me please.
Let me know if you can confirm that it's an optimization opportunity, thanks.