RFR: 8341178: TypeRawPtr::add_offset may be "miscompiled" due to UB

[Kim Barrett]

On Thu, 3 Oct 2024 16:38:34 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Please review this change to TypeRawPtr::add_offset to prevent a compiler from
>> inferring things based on prior pointer arithmetic not invoking UB.  As noted in
>> the bug report, clang is actually doing this.
>> 
>> To accomplish this, changed to integral arithmetic.  Also added over/underflow
>> checks.
>> 
>> Also made a couple of minor touchups.  Replaced an implicit conversion to bool
>> with an explicit compare to nullptr (per style guide).  Removed a no longer
>> needed dummy return after a (now) noreturn function.
>> 
>> Testing: mach5 tier1-7
>> That testing was with calls to "fatal" for the over/underflow cases and the
>> sum==0 case.  There were no hits.  I'm not sure how to construct a test that
>> would hit those.
>
> src/hotspot/share/opto/type.cpp line 3136:
> 
>> 3134: 
>> 3135: const TypeRawPtr *TypeRawPtr::make( address bits ) {
>> 3136:   assert( bits != nullptr, "Use TypePtr for null" );
> 
> Please, remove spaces after open and before close `()`.

I'm not fond of those spaces, but they follow the style used throughout this file.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21324#discussion_r1787152954