[GitHub] [tvm] Lunderberg commented on a diff in pull request #12266: [TIR] Add tir::builtin::undef

[GitBox <gi...@apache.org>]

Lunderberg commented on code in PR #12266:
URL: https://github.com/apache/tvm/pull/12266#discussion_r935648157


##########
src/tir/op/builtin.cc:
##########
@@ -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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