Lunderberg commented on code in PR #12266:
URL: https://github.com/apache/tvm/pull/12266#discussion_r935648157
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src/tir/op/builtin.cc:
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@@ -288,6 +288,10 @@ TIR_DEFINE_BUILTIN_FUNC(texture2d_load)
TIR_DEFINE_BUILTIN_FUNC(mem_copy).set_attr<TCallEffectKind>("TCallEffectKind",
Integer(CallEffectKind::kOpaque));
+TIR_DEFINE_BUILTIN_FUNC(undef)
+ .set_attr<TCallEffectKind>("TCallEffectKind",
Integer(CallEffectKind::kReadState))
Review Comment:
Primarily to avoid simplifications that rely on a pure function returning a
consistent value for consistent inputs. Since `undef` takes no arguments, this
would imply that the return value of `undef` is always identical, which could
result in incorrect simplifications. For example, inlining of a let binding is
valid if the bound value is `kPure`, but requires additional checks if the
bound value is `kReadState`.
I had picked `kReadState` as analogous to a hypothetical
`builtin::system_clock_time`, which takes no arguments, but reads from a hidden
state and may return a different value for each call.
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