Compile-time check for zero alpha in matmul

[jishnub]

Currently, we check for iszero(_add.alpha) in various matrix multiplication functions, where _add::MulAddMul. However, a MulAddMul(alpha, beta) stores isone(alpha) as a type parameter, and if alpha is one, we know at compile-time that iszero(_add.alpha) is false. Using the type parameter in dispatch would allow us to eliminate the iszero(alpha) branches.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.03%. Comparing base (61e444d) to head (6fc77c1).

Files with missing lines	Patch %	Lines
src/generic.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1293      +/-   ##
==========================================
- Coverage   92.03%   92.03%   -0.01%     
==========================================
  Files          34       34              
  Lines       15501    15503       +2     
==========================================
+ Hits        14267    14268       +1     
- Misses       1234     1235       +1     

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[dkarrasch]

Does this improve TTFX visibly?

[jishnub]

It helps a bit when the branch calls _rmul_or_fill!, which may be skipped.

julia> n = 4; T = t = Bidiagonal(ones(n), ones(n-1), :U); C = similar(T); D = Diagonal(T);

julia> @time mul!(C, T, D);
  0.124931 seconds (398.97 k allocations: 20.078 MiB, 99.98% compilation time) # master
  0.110213 seconds (387.34 k allocations: 19.559 MiB, 99.98% compilation time) # this PR

Often, the different would be small, as the branch is trivial.