bug#62069: Support xz parallel compression in dist-xz

[Mike Frysinger]

On 09 Mar 2023 08:47, Sam James wrote:
> xz supports parallel compression which obviously speeds up
> the time taken to run the 'dist-xz' rule, but it also
> speeds up *decompression* too, as the parallel compressor
> creates different output which can be decompressed in parallel.
> 
> There's two options, I think, for automake:
> 1. Pass -T+1 to xz if xz is >= 5.4.0 (a runtime check
> would be needed, but it's cheap to do) as this uses
> 1 thread, but uses the parallel compressor to do it,
> so allows decompression in parallel by users;
> 
> 2. Pass -T0 (or -Tn, n >= 1) to allow autodetection
> of the number of available CPU cores (or n if specified)
> which also then naturally uses the threaded compressor.

it should be a configure check when dist-xz is enabled in the call to
AM_INIT_AUTOMAKE, and we should default to `-T0`.  this would be the
default XZ_OPT value if it hasn't been set/overridden (where we use
-e now).
-mike

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