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Re: [PATCH, libiberty] Fix segfault in floatformat.c:get_field on 64-bit hosts

Ian Lance Taylor wrote:
Julian Brown <> writes:

This patch fixes a problem with floatformat.c:get_field on 64-bit (on
at least x86_64), when cross-assembling to arm-none-eabi. The line
which reads:

result = *(data + cur_byte) >> (-cur_bitshift);

was executed with cur_byte = -1 (start + len == 0 and order ==
floatformat_little), which happily segfaulted (during printing of FP

I don't understand how start + len == 0 could ever be true.  What was
calling the function?  I note that put_field has the exact same
problem if start + len == 0.

Sorry, I botched my explanation a bit. I should have said:

(start + len) / FLOATFORMAT_CHAR_BIT == 0

which is true when e.g. extracting the sign bit of a single-precision IEEE float -- in that case, start will be 0 and len will be 1 (with big-endian bit numbering used elsewhere in floatformat.c). The function is called by opcodes/arm-dis.c to print out the ARM Neon "quarter-precision" floating-point immediates.

! return result & ((2 << (total_len - 1)) - 1);

Why do you need to do this?  And if you do need to do it, why use 2?
Why not ((1 << total_len) - 1)?

It was to attempt to maintain the original semantics of the function, as I understood them: the result is truncated to total_len after being built up <=8 bits at a time (though looking again, that might not have been the original intention at all, or at least not at that level of granularity... I suspect that bit should be removed).

The reason for using 2 << ... rather than 1 << .. was so that, e.g., the total_len == 32 case works properly. AIUI, shift amounts must be strictly less than the width of the operand, and the total_len==0 case isn't interesting. But that's irrelevant if I'm getting rid of that bit anyway :-)

So, I'll remove that bit and do the following...

Please compile the file as a standalone program with -DIEEE_DEBUG to
make sure those tests still work.  Ideally on both a big- and
little-endian system, if possible.



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