RFR: 8365163: [ubsan] left-shift issue in globalDefinitions.hpp

[Andrew Haley]

On Sun, 17 Aug 2025 06:49:47 GMT, Afshin Zafari <azafari at openjdk.org> wrote:

> There was a left-shift of negative value UB in `set_high` function where the high value sign bit is on and is left-shifted 32 bits to put it in high word of the destination address.
> To address it, first the left 32 bits of the provided `high` arg is cleared and then left-shifted 32 bits. 
> 
> Tests:
> mach5 tiers 1-5 {macosx-aarch64, linux-x64, windows-x64} x {debug, product}

src/hotspot/share/utilities/globalDefinitions.hpp line 1079:

> 1077:   return result;
> 1078: }
> 1079: 

This still has an overflowing left shift, and it's hard for the reader to follow.

I'd do something like this:


inline void set_low (jlong* value, jint low ) {
  union {
    jlong value_s;
    julong value_u;
  };
  value_s = *value;
  value_u = (value_u & ~(julong)0xffffffff) | (julong)low;
  *value = value_s;
}

inline void set_high(jlong* value, jint high) {
  union {
    jlong value_s;
    julong value_u;
  };
  value_s = *value;
  value_u = (value_u & (julong)0xfffffffful) | ((julong)high << 32);
  *value = value_s;
}

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/26809#discussion_r2280787149