Unprofitable ctpop vectorization #57476
Description
Example from rust-lang/rust#101060:
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128" target triple = "x86_64-unknown-linux-gnu" define i64 @test(ptr %arr) { entry: br label %loop loop: %accum = phi i64 [ %accum.next, %loop ], [ 0, %entry ] %iv = phi i64 [ %iv.next, %loop ], [ 0, %entry ] %iv.next = add nuw i64 %iv, 1 %gep = getelementptr inbounds i64, ptr %arr, i64 %iv %value = load i64, ptr %gep, align 8 %ctpop = tail call i64 @llvm.ctpop.i64(i64 %value) %accum.next = add i64 %accum, %ctpop %exitcond = icmp eq i64 %iv.next, 2 br i1 %exitcond, label %exit, label %loop exit: %lcssa = phi i64 [ %accum.next, %loop ] ret i64 %lcssa } declare i64 @llvm.ctpop.i64(i64)This two-iteration loop gets vectorized by opt -loop-vectorize -mcpu=znver2 (https://llvm.godbolt.org/z/M8qTdTbfE), because we assign cost 1 to scalar ctpop and cost 3 to the vector ctpop, so it's nominally "profitable". At least for low iteration count, this is not actually the case.