RFR: 8145096: Undefined behaviour in HotSpot

[Andrew Haley]

On 12/17/2015 05:13 PM, Kim Barrett wrote:
> Nice.
> 
> There's another that looks like that here:
> src/share/vm/opto/mulnode.cpp
>  577       jlong t1_support = java_subtract(((jlong)1 << (1 + log2_long(t1->_hi))), (jlong)1);
> 
> Everything else looks good.

Ah, I didn't want to touch that one.  :-)

Here's the original:

      jlong t1_support = ((jlong)1 << (1 + log2_long(t1->_hi))) - 1;

I think that the equivalent is

      int type_bit_count = log2_long(t1->_hi) + 1;
      jlong t1_support = jlong((~ UCONST64(0)) >> (BitsPerJavaLong - type_bit_count));

I've broken it apart for clarity.

However, I'm having trouble analysing this to prove that all the
corner cases are correct.

Andrew.