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… at least with gfortran from gcc version 12.2.1 20221121 (Red Hat 12.2.1-4) (GCC). (Internet research suggests this goes back to GCC 8, probably, so some 5 years back)

I don't know how or where to generally detect that, or set that. Otherwise, I'd make a PR :)

benchees/Makefile.am should make it conditional on ALTIVEC_CFLAGS whether vBigDSP is even included in SUBDIRS

Maybe I'm doing something wrong, but even when I define CFLAGS before invoking configure, the CFLAGS used for compiling the benchees are always those determined by AX_CC_MAXOPT. The only way I manage to "fix" this is by commenting out AX_CC_MAXOPT in configure.ac and re-running autoreconf etc.

The performance penalty caused by the automatically chosen flags is quite dramatic; more than a factor of 2 for some transforms.

Is there some subtlety I'm missing?

it's usual (and more than sensible) to have that bit set for a bootstrap script. Seeing it doesn't use any special shell features, a #!/bin/sh starting line would be good.