Fix return types of SInt::Unsigned and SInt::*Divide - #156
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This utility function is useful for things like applying constant folding to types like 'Bits(4+1)'
Apply constant folding to types like 'Bits(4+1)'. Apart from being simpler, this is essential when using the -O0 compilation flag because it requires that bitwidths are literal constants. In particular, this is needed in tests of functions whose return type is of the form 'Bits(x * y)', 'Bits(x + 1)', etc.
This was incorrectly calculating bounds such as {3..3}
when calculating the bounds of an expression like 'Log2(x) + 3'
This was wrong because the result of Log2 has type {0..}
so the most accurate bounds we could calculate should be {3..} (with no upper bound)
This was incorrectly saying that Bits(N) can be stored in an N-bit signed value. Obviously, we need an (N+1)-bit signed value to represent an unsigned N-bit number.
This had been added to the runtime but, for some reason, it had not been added to the standard library.
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The return types of Std::SInt::Unsigned and Std::SInt::*Divide need one more bit to represent the result than the size of the input.
More specifically:
Also adds support for compiling Std::SInt::Ceiling_Divide.
This has been supported in the runtime_*.ml files for a while but we had not added support in the C backend.
To be able to write tests that use -O0 for these operations, I had to tweak the typechecker so that it does not infer types like "__sint(8+1)" (which the backend will not accept) and, instead infers the type "__sint(9)".