[Bug] [阻塞] PTODSL: variable reassignment inside pto.if_ is not carried out correctly and may cause dominance errors

[W4termy]

Component

Python bindings (python/)

Description

PTODSL 中，如果一个变量在 pto.if_ 的 then/else region 内被重新赋值，并且这个变量在 pto.if_ 之后继续使用，目前看起来没有被正确 lower 成 scf.if 的 result/yield。

这会导致两类问题：

1. 简单赋值可能被错误地当成无条件赋值，scf.if 变成空 region，语义错误。
2. 如果赋值 RHS 是在 pto.if_ region 内新产生的 SSA value，则会在后续使用时报 MLIR dominance error。

这和 #973 类似，都是 PTODSL AST rewrite / SSA carry 对 Python rebinding 的处理问题；但这里触发点是 pto.if_，不是 for loop 里的 list subscript self assignment。

Reproduction (minimal)

Case 1: assignment inside `pto.if_` is silently treated as if it were unconditional


from ptodsl import pto

@pto.jit(name="repro_if_reassign_wrong_semantics", target="a5")
def kernel(n: pto.i32) -> None:
    mask = pto.pset_b32("PAT_ALL")

    x = pto.const(0, dtype=pto.i32)
    with pto.if_((n > 0)) as br, br.then_:
        x = n

    # Expected: vdup uses (n if n > 0 else 0)
    # Actual: vdup uses n unconditionally
    v = pto.vdup(x, mask)
    return

print(kernel.compile().mlir_text())

Actual generated MLIR excerpt:
%1 = arith.cmpi sgt, %arg0, %c0_i32_0 : i32
scf.if %1 {
}
%2 = pto.vdup %arg0, %0 : i32, !pto.mask<b32> -> !pto.vreg<64xi32>

The scf.if is empty, and %arg0 is used unconditionally after the if.

Case 2: assignment to a value created inside pto.if_ causes MLIR dominance failure

from ptodsl import pto

@pto.jit(name="repro_if_reassign_dominance", target="a5")
def kernel(n: pto.i32) -> None:
    mask = pto.pset_b32("PAT_ALL")

    x = pto.const(0, dtype=pto.i32)
    with pto.if_((n > 0)) as br, br.then_:
        x = n + 1

    # Expected: vdup uses if-result (n + 1 if n > 0 else 0)
    # Actual: value from child region is used outside, causing dominance error
    v = pto.vdup(x, mask)
    return

print(kernel.compile().mlir_text())

Expected behavior

Variables reassigned inside pto.if_ should be modeled as values yielded from the scf.if region, similar to:
%new_x = scf.if %cond -> (i32) {
%then_x = arith.addi %arg0, %c1_i32 : i32
scf.yield %then_x : i32
} else {
scf.yield %old_x : i32
}

%v = pto.vdup %new_x, %mask : i32, !pto.mask -> !pto.vreg<64xi32>

For pto.if_ without an explicit else branch, the else value should preserve the previous value of the variable.

Actual behavior / error logs

ptoas.mlir._mlir_libs._site_initialize.<locals>.MLIRError: Verification failed:
error: unknown: operand #0 does not dominate this use
 note: unknown: see current operation: %4 = "pto.vdup"(%6, %0) : (i32, !pto.mask<b32>) -> !pto.vreg<64xi32>
 note: unknown: operand defined here (op in a child region)

Git commit

8cd7219

Host platform

None

Target Ascend arch (if relevant)

a5

PTOAS build level (if relevant)

None