[Bugfix] Add check to avoid calling back() on an empty container

[syang-ng]

This is related to an issue here.

In a word, function PrimExpr VisitExpr_(const LoadNode* op) forgets to check the size of the vector scope_[op->buffer_var.get()], while calling std::vector::back on an empty container will cause undefined behavior.

This PR fixed this problem by adding the vector size check before calling back().

To reproduce the bug:

from tvm import tir
import tvm

zero = tir.const(0)
nop = tir.Evaluate(zero)
v = tir.Var('i1', 'int32')
for_stmt = tir.For(v, zero, zero, tir.ForKind.SERIAL, nop)
load = tir.Evaluate(tir.Load('int32', v, zero))
seq = tir.SeqStmt([for_stmt, for_stmt, load])
func = tir.PrimFunc([], seq)

tvm.build(func)

[tqchen]

Thanks @syang-ng , can you add a regression unit test case to cover the problem(ideally not calling build but directly call the pass that invokes this function)?

[tqchen]

cc @ZihengJiang @vinx13 please help to manage the PR

[syang-ng]

I add a test case to cover the problem in the new commit. Since there is no way to invoke this function directly, I call the pass tir.transform.InjectVirtualThread to trigger this problem. Could you help review this PR :-D @vinx13

[vinx13]

please fix ci error

[syang-ng]

I updated the test case and it has passed all checks. Could you help to review it? Thanks :-P @vinx13