Cmm optimizer: propagate constants across basic block boundaries

The new constant propagation pass optimizes

fn
{
    bits64 a, b;

    a = 1;
    b = a + a;
    RET_N(b);
}

as

fn()    { []
        }
    cc: R1 = 2;
        jump (I64[Sp + 0]) ();
}

which is good. However, it fails to propagate the constants across a basic block boundary. For example, the following code

fn
{
    bits64 a, b;

    a = 1;
lbl:
    b = a + a;
    RET_N(b);
}

gets optimized as

n()    { []
        }
    cd: _cf::I64 = 1;
        goto ce;
    ce: R1 = _cf::I64 + _cf::I64;
        jump (I64[Sp + 0]) ();
}

To make this work we should ideally work with a better representation of instructions and their use sites than currently used in CmmOpt.hs. For example, see how simple this optimization pass is to implement in LLVM: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/ConstantProp.cpp?view=markup

Trac metadata

Trac field	Value
Version	7.0.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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