https://bugs.kde.org/show_bug.cgi?id=436398
Bug ID: 436398
Summary: R NAs and NaN payload when converting 80-bit and
64-bit numbers on x87
Product: valgrind
Version: 3.15 SVN
Platform: Compiled Sources
OS: Linux
Status: REPORTED
Severity: wishlist
Priority: NOR
Component: vex
Assignee: jsew...@acm.org
Reporter: tomas.kalib...@gmail.com
Target Milestone: ---
Created attachment 138026
--> https://bugs.kde.org/attachment.cgi?id=138026&action=edit
Quick hack to preserve NaN payload which fixes observed issue.
VEX does not preserve NaN payload bits when converting between 80-bit and
64-bit floating point numbers in convert_f80le_to_f64le and
convert_f64le_to_f80le.
This causes trouble when using valgrind to run R. R uses a special NaN payload
to distinguish R NA ("missing") values from usual NaNs that result from
floating point computations.
One way to reproduce is to run
R -d valgrind --quiet -e "sum(c(1, NA))"
with R compiled with gcc -O0 (with -O2, different code is generated and the
problem does not appear). I tried on Ubuntu 20.04 (gcc 9.3, valgrind 3.15.0)
When R is run this way, the result is "NaN". When it is run without valgrind,
it is "NA" (which it should be). It would be nice, if possible, if valgrind
could match the underlying platform in this case.
The attached patch is an experiment/hack which confirms the cause is
convert_f80le_to_f64le/convert_f64le_to_f80le: with the patch applied, R
returns "NA", matching the platform behavior, when running the example.
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