diff options
| -rw-r--r-- | src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp b/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp index 16480ad8a90..392691dd47e 100644 --- a/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp +++ b/src/mesa/drivers/dri/i965/brw_fs_live_variables.cpp @@ -221,6 +221,30 @@ fs_visitor::calculate_live_intervals() } this->live_intervals_valid = true; + + /* Note in the non-control-flow code above, that we only take def[] as the + * first store, and use[] as the last use. We use this in dead code + * elimination, to determine when a store never gets used. However, we + * also use these arrays to answer the virtual_grf_interferes() question + * (live interval analysis), which is used for register coalescing and + * register allocation. + * + * So, there's a conflict over what the array should mean: if use[] + * considers a def after the last use, then the dead code elimination pass + * never does anything (and it's an important pass!). But if we don't + * include dead code, then virtual_grf_interferes() lies and we'll do + * horrible things like coalesce the register that is dead-code-written + * into another register that was live across the dead write (causing the + * use of the second register to take the dead write's source value instead + * of the coalesced MOV's source value). + * + * To resolve the conflict, immediately after calculating live intervals, + * detect dead code, nuke it, and if we changed anything, calculate again + * before returning to the caller. Now we happen to produce def[] and + * use[] arrays that will work for virtual_grf_interferes(). + */ + if (dead_code_eliminate()) + calculate_live_intervals(); } bool |