Why not use a CMOV in a i686-specific crt0.c?
Then programs compiled for i686 but run on i586 will SIGILL
deterministically at program start-up. It seems to me that
the major problem with SIGILL is that it occurs depending
upon the program execution flow, and thus appears indeterministic
to the user.
This doesn't solve the problem of a i386 executable calling
a i686 library, but solving that problem deterministically
requires a lot of baggage:
- compiler to produce an object file header stating CPU
- run time linker to take union of all CPU features in
object file headers and check against CPU features
returned by CPUID.
Even this isn't perfect, consider multi-processor machines
with differing CPU feature sets or applications which attempt
to implement their own run-time checking:
if (feature.cmov && feature.somethingelse && ...)
This leads inevitably more flags in the object file header
to instruct the run-time linker to skip particular CPU feature
gcc -c -mdisable_cpu_feature_check=cmov -o mytask.o mytask.c
SIGILL starts to look lightweight :-)
-- Glen Turner (08) 8303 3936 or +61 8 8303 3936 Australian Academic and Research Network www.aarnet.edu.au
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