B2R : Move Barrier To Register

SPA 5.0:
        {@{!}Pg}   B2R.BAR      Rd, #BarNameU4   {&req_6}   {&rdN}   {&wrN}   {?sched}   ;   
        {@{!}Pg}   B2R.WARP     Rd               {&req_6}   {&rdN}   {&wrN}   {?sched}   ;   
        {@{!}Pg}   B2R.RESULT   Rd{, Pd}         {&req_6}   {&rdN}   {&wrN}   {?sched}   ;   

   mode: { .BAR*, .WARP, .RESULT }

B2R.BAR and B2R.WARP can be used to read out in-progress barrier state for context save. Use R2B to restore barrier state. B2R.BAR reads the state associated with Barrier specified by #BarNameU4 while B2R.WARP reads per-warp barrier state.

B2R.RESULT is used to read the result of BAR.RED, BAR.ARV, or BAR.SCAN.

B2R.BAR returns the barrier state BarrierState_t that contains counts associated with the given barrier. These counts will be for those threads that contributed to the barrier but did not stop (BAR.ARV/BAR.SCAN as opposed to BAR.SYNC/BAR.RED).

If the BarInUse field is 0, the rest of the values are 0.

Note: Behavior when mixed with execution of target barriers in the same region is UNPREDICTABLE.

B2R.WARP returns per warp barrier state which includes:

• a 1-bit value indicating whether the warp is waiting on a barrier completion,
• a 4-bit value indicating the logical barrier the warp is waiting upon (if any),
• a 11-bit barrier result (the same result returned by B2R.RESULT).

The per-warp WaitingOnSyncAllBarrier state is not exposed. The state written back to Rd is formatted as data structure WarpBarrierState_t.

B2R.RESULT writes the previous barrier reduction result for that warp into a register and/or predicate. BAR.SYNC and BAR.RED generate barrier results upon barrier release. For boolean .RED operations, RedCntResult is 0xffff_ffff if .AND/.OR are true, and 0x0000_0000 otherwise. BAR.SCAN generates a barrier result upon barrier execution. BAR.ARV contributes to the reduction but does not return a result. The barrier result format is defined in the BAR instruction. B2R.RESULT returns the barrier result in the following format:

    31:0   RedCntResult

B2R     R5, 0;                            // R5 = Barrier[0];
B2R     R0, 3;                            // R0 = Barrier[3];