With f32 denormals flushed, every f32 multiply and fma checks whether the
exact product is below 2^-126 (#1850): two exponent extracts, a 32-bit
significand multiply-high and a chain of compares and selects. When a
factor is a literal or inline constant, its exponent settles the check
during translation:
- a factor that is zero (after the input flush), infinite, NaN or at least
1 in magnitude never gives a tiny product, so the check is not emitted;
- a power-of-two factor never carries out of the significand product, so
the multiply-high is not emitted.
Constant sources are folded with their abs, neg and denormal flush, so the
factor reaches the check as a constant.
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