RFR: 8240615: is_power_of_2() has Undefined Behaviour and is inconsistent

[Andrew Haley]

On 3/9/20 3:56 PM, Doerr, Martin wrote:
> it doesn't run into the UB case, but log2_jint(x) returns 63 for negative x.
> I'd treat this as bug. A jint has 32 bit and log2 should never return 63.

I guess this bug dates from the days when uintptr_t was 32 bits.

I think you're right, but this has never been a problem because all uses
are either guarded by is_power_of_2() or they occur in a context (such as,
say, os::active_processor_count()) where > 2**31 is extremely unlikely.

I'm happy to fix this, though, if anyone thinks I should. The fix is:

diff -r f9893c227e12 src/hotspot/share/utilities/globalDefinitions.hpp
--- a/src/hotspot/share/utilities/globalDefinitions.hpp	Wed Mar 11 15:02:09 2020 +0000
+++ b/src/hotspot/share/utilities/globalDefinitions.hpp	Wed Mar 11 15:45:26 2020 +0000
@@ -1022,12 +1022,12 @@

 inline int log2_int(int x) {
   STATIC_ASSERT(sizeof(int) <= sizeof(uintptr_t));
-  return log2_intptr((uintptr_t)x);
+  return log2_intptr((uintptr_t)(unsigned int)x);
 }

 inline int log2_jint(jint x) {
   STATIC_ASSERT(sizeof(jint) <= sizeof(uintptr_t));
-  return log2_intptr((uintptr_t)x);
+  return log2_intptr((uintptr_t)(juint)x);
 }

 inline int log2_uint(uint x) {

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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