On Fri, Nov 24, 2023 at 5:01 PM Roger Heflin <rogerheflin@xxxxxxxxx> wrote:
I have seen the bug in old code that a new compiler
optimization/library fix exposed.
[...]
I also previously worked for a company that compiled their code with 3
different bought linux compilers, and a SGI and Sun compiler and
examined the warnings from all and resolved them and found that
warnings on one or the other compiler would flag something and upon
examination of the code they often could not understand why the code
"appeared" work well enough even with what was clearly a bug in the
code.
Just because your code "appears" to work in your test case says little
or nothing about the code being "perfect". I have found significant
bugs that made me question how the code was returning correct results
too many times.
Never assume that old code is right. there are many ways for it to be
wrong and still return correct results until someone else fixes their
external code that was allowing your bad code to work.
There are some codes where there are ways to cross-check a computation,
but others where you have to rely on everything being done correctly.
For 25 years my job included maintaining and porting a Fortran code that
started on a CDC Cyber and did not have ways to cross-check the results, so
I relied on test data sets. We were able to get test data to produce bit-for-bit
identical results across linux, Windows, and macOS, but, especially in the early
years, often required some effort to install toolchains that didn't have bugs,
including building GCC with upstream bug fixes that hadn't yet made it into
the versions available on user systems. In recent years most of the work has
been finding replacements for libraries that have been dropped from linux
distros.
George N. White III
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