media-libs/openjpeg generates slightly different images depending on whether it was compiled with "normal" instructions or with some "advanced" ones, e.g. using fused-multiply-add (FMA), FP387.
This lets the test suite of openjpeg fail.
For more details see bug #628430 and the references therein.
The FMA instructions are implied by -march=<ABC> in combination with -O2 (or higher) in the CFLAGS, which are very common if not recommended for users.
GCC allows to control the usage of FMA with the -ffp-contract= option 
In https://bugs.gentoo.org/628430#c4 there is also the -mfpmath= setting mentioned which might or might not disable FMA or similar instructions if set to -mfpmath=sse.
A possible fix/workaround would be to filter the offending flags from CFLAGS and/or add their disabled counterparts (e.g. -ffp-contract=off, -mfpmath=sse, ...).
The bug has been closed via the following commit(s):
Author: Sam James <firstname.lastname@example.org>
AuthorDate: 2020-12-29 02:56:24 +0000
Commit: Sam James <email@example.com>
CommitDate: 2020-12-29 02:56:24 +0000
media-libs/openjpeg: security bump to 2.4.0
Package-Manager: Portage-3.0.9, Repoman-3.0.2
Signed-off-by: Sam James <firstname.lastname@example.org>
media-libs/openjpeg/Manifest | 2 +
.../files/openjpeg-2.4.0-gnuinstalldirs.patch | 420 +++++++++++++++++++++
media-libs/openjpeg/openjpeg-2.4.0.ebuild | 139 +++++++
3 files changed, 561 insertions(+)