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From GitBox <...@apache.org>
Subject [GitHub] [incubator-tvm] dpankratz opened a new pull request #5156: [TIR] Add bound check for make_const
Date Sun, 26 Apr 2020 14:30:10 GMT

dpankratz opened a new pull request #5156:
URL: https://github.com/apache/incubator-tvm/pull/5156


   ## Bug 
   
   Casting values which are known at compile time results in strange behaviour. For example
`1 * tir.Cast("bool", 77)` becomes `77` after the rewrite_simplify pass instead of `1`. 
   
   However if that value was passed in at runtime then the expect result of `1` would be printed.
   
   ## Example
   ```
   print(tvm.arith.Analyzer().rewrite_simplify(1 * tir.Cast('bool', 77))) #prints 77
   
   a =te.var('a')
   shape = (1,)
   c = te.compute(shape,lambda i: 1 * tir.Cast('bool',a))
   s = te.create_schedule([c.op])
   f = tvm.build(s,[a,c])
   c_tvm = tvm.nd.array(np.zeros(shape,dtype='int32'))
   f(77,c_tvm) 
   
   assert c_tvm.asnumpy()[0] == 1
   ```
   
   ## Explanation
   
   When a `Cast(dtype, value)` call is used on a compile time value then it is replaced with
an `IntImm` with the `dtype` and `value` from the call. However, the `value` is stored as
a C++ `int64_t` and thus does not limit the `value` to a sensible range . If the resulting
`IntImm` is later used in an expression that results in a type promotion unexpected behaviour
such as the example above occurs where the `Cast` effectively does nothing. 
   
   ## Fix 
   
   My fix is to ensure that as the `IntImm` is created it takes the type bounds into account.
After the fix the above example would behave as expected.
   ```
   print(tvm.arith.Analyzer().rewrite_simplify(1 * tir.Cast('bool', 77))) #prints 1
   ```
   
   A review would be much appreciated!
   
   @tqchen @vinx13  @ZihengJiang 
   


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