int(long(-sys.maxint-1)) fails on Linux

[Christian Tismer]

Chad Netzer wrote:
> 
> Here I go, replying to my own posts again...
...
> Here is some assembly of the same region, compiled with the -O1 flag, which DOES
> exhibit the incorrect behavior.  Notice that it is quite different from the previous code,
> and I think the bug is that it uses the "LEA" instruction improperly:
> 
>  <PyLong_AsLong+89>: test   %edx,%edx                            ; x < 0
>  <PyLong_AsLong+91>: jge    0x806c22a <PyLong_AsLong+106>
>  <PyLong_AsLong+93>: test   %ebx,%ebx                            ; sign > 0
>  <PyLong_AsLong+95>: jg     0x806c231 <PyLong_AsLong+113> ; goto overflow
>  <PyLong_AsLong+97>: lea    0x0(,%edx,2),%eax             ; ?? Where is (x << 1)?

lea doesn't change any flags. It optimizes the shift to an add
which is fine. The implied test of the result is just not done
and this is a false optimization.

Somehing similar happened to me when I hand-optimized assembly
code. It is tempting to replace
   add ax, 1
by
   inc ax

but beware, that doesn't set flags, and it's likely that you forget
about it when changing tests.

>  <PyLong_AsLong+104>: jne    0x806c231 <PyLong_AsLong+113> ; goto overflow
>  <PyLong_AsLong+106>: mov    %edx,%eax
>  <PyLong_AsLong+108>: imul   %ebx,%eax
>  <PyLong_AsLong+111>: jmp    0x806c246 <PyLong_AsLong+134>
> 
> As you can see, the original code had add %eax, %eax instruction to shift
> left by one, the optimized one doesn't.  I don't know what the heck it
> was trying to do, and I don't know what condition codes "lea" sets, so
> I can't know what it intended.  In any case, I speculate that line +104
> was causing a jump to the overflow handler when it should not.
> 
> Anyway, I apologize for cluttering the newsgroup with assembly;  clearly the
> problem is with pgcc w/ optimization turned on, which is a Mandrake 6.0 issue.

I can't see any bad about assembly. This is just another data type,
and Python can be a fine tool as a generic, 
table driven disassembler ;-)

but that's another project which I will continue next year - chris

-- 
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