What's going on with exp() and --math-mode=fast?

General Usage
4

The beginning of the code for exp looks something like:

const a = 369.3299304675746
const b = 6.755399441055744e15

function f(x::Float64)
    z = muladd(x, a, b)
    z -= b
end

Without --math-mode=fast this gives the result 369.0, with it it gives 369.3299304675746. Looking at the generated code, in fast mode, it has simplified this code to x*a:

julia> @code_llvm f(1.0)
...
  %1 = fmul fast double %0, 0x40771547652B82FE
  ret double %1
}

while it otherwise computes things as given:

julia> @code_llvm f(1.0)
...
   %1 = fmul contract double %0, 0x40771547652B82FE
   %2 = fadd contract double %1, 0x4338000000000000
   %3 = fadd double %2, 0xC338000000000000
  ret double %3
}

There can be an arbitrary error in the final result from such a change.

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