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

[Julian Brown <julian@codesourcery.com>]

Ian Lance Taylor wrote:
> Julian Brown <julian@codesourcery.com> 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
>>immediates).
> 
> 
> 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.

Thanks,

Julian