[DA] Does not account for wrapping accesses #149501
Description
Input:
define void @f(ptr %a, i64 %n, i64 %m) { entry: %bound = sub i64 %m, %n br label %loop loop: %i = phi i64 [ 0, %entry ], [ %i.next, %loop ] %subscript.0 = add i64 %i, %n %subscript.1 = add i64 %i, %m %idx.0 = getelementptr i8, ptr %a, i64 %subscript.0 %idx.1 = getelementptr i8, ptr %a, i64 %subscript.1 store i8 42, ptr %idx.0 store i8 42, ptr %idx.1 %i.next = add i64 %i, 1 %cond.exit = icmp eq i64 %i.next, %bound br i1 %cond.exit, label %exit, label %loop exit: ret void }Result:
$ opt -passes='print<da>' -disable-output test.ll Printing analysis 'Dependence Analysis' for function 'f': Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.0, align 1 da analyze - none! Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.1, align 1 da analyze - none! Src: store i8 42, ptr %idx.1, align 1 --> Dst: store i8 42, ptr %idx.1, align 1 da analyze - none! godbolt: https://godbolt.org/z/jvascaoo7
At the very least, when both %n and %m are zero, there is a dependency between the two stores within a single iteration.
Here is a portion of the debug output related to the analysis between them:
testing subscript 0, SIV src = {(%n + %a),+,1}<%loop> dst = {(%m + %a),+,1}<%loop> Strong SIV test Coeff = 1, i64 SrcConst = (%n + %a), ptr DstConst = (%m + %a), ptr Delta = ((-1 * %m) + %n), i64 UpperBound = (-1 + (-1 * %n) + %m), i64 none! The root cause seems to be that DA uses the backedge-taken count as the iteration count without considering any wrapping accesses (ref: strongSIVtest). In this case, the issue is caused by strongSIVtest, but I suspect similar issues may exist elsewhere in DA.