llvm/llvm-project
GitHub で見るMissed Optimization: max(max(x, c1) << c2, c3) —> max(x << c2, c3) when c3 >= c1 * 2 ^ c2
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#139,786 opened on 2025年5月13日
good first issuellvm:instcombinemissed-optimization
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説明
define i8 @src(i8 %arg0) {
%1 = call i8 @llvm.umax.i8(i8 %arg0, i8 1)
%2 = shl nuw i8 %1, 1
%3 = call i8 @llvm.umax.i8(i8 %2, i8 16)
%4 = icmp sgt i8 %1, -1
%5 = select i1 %4, i8 %3, i8 -1
ret i8 %3
}
define i8 @tgt(i8 %arg0) {
%1 = shl nuw i8 %arg0, 1
%2 = call i8 @llvm.umax.i8(i8 %1, i8 16)
%3 = icmp sgt i8 %2, -1
%4 = select i1 %3, i8 %2, i8 -1
ret i8 %2
}
Alive2: https://alive2.llvm.org/ce/z/on2tJE
Found this pattern at: https://github.com/dtcxzyw/llvm-opt-benchmark/blob/94dab89f901fdfae3e47dc6696c06d591fdd7993/bench/openssl/optimized/packet.ll#L1086-L1094
I think this transformation can be generalized to operations other than shift and also to min().