On 30 January 2016 at 07:00, Ilia Mirkin <imir...@alum.mit.edu> wrote: > We use this logic to detect live ranges and then do plain renaming > across the whole codebase. As such, to prevent WaW hazards, we have to > treat a write as if it were also a read. > > For example, the following sequence was observed before this patch: > > 13: UIF TEMP[6].xxxx :0 > 14: ADD TEMP[6].x, CONST[6].xxxx, -IN[3].yyyy > 15: RCP TEMP[7].x, TEMP[3].xxxx > 16: MUL TEMP[3].x, TEMP[6].xxxx, TEMP[7].xxxx > 17: ADD TEMP[6].x, CONST[7].xxxx, -IN[3].yyyy > 18: RCP TEMP[7].x, TEMP[3].xxxx > 19: MUL TEMP[4].x, TEMP[6].xxxx, TEMP[7].xxxx > > While after this patch it becomes: > > 13: UIF TEMP[7].xxxx :0 > 14: ADD TEMP[7].x, CONST[6].xxxx, -IN[3].yyyy > 15: RCP TEMP[8].x, TEMP[3].xxxx > 16: MUL TEMP[4].x, TEMP[7].xxxx, TEMP[8].xxxx > 17: ADD TEMP[7].x, CONST[7].xxxx, -IN[3].yyyy > 18: RCP TEMP[8].x, TEMP[3].xxxx > 19: MUL TEMP[5].x, TEMP[7].xxxx, TEMP[8].xxxx > > Most importantly note that in the first example, the second RCP is done > on the result of the MUL while in the second, the second RCP should have > the same value as the first. Looking at the GLSL source, it is apparent > that both of the RCP's should have had the same source. > > Looking at what's going on, the GLSL looks something like > > float tmin_8; > float tmin_10; > tmin_10 = tmin_8; > ... lots of code ... > tmin_8 = tmpvar_17; > ... more code that never looks at tmin_8 ... > > And so we end up with a last_read somewhere at the beginning, and a > first_write somewhere at the bottom. For some reason DCE doesn't remove > it, but even if that were fixed, DCE doesn't handle 100% of cases, esp > including loops. > > With the last_read somewhere high up, we overwrite the previously > correct (and large) last_read with a low one, and then proceed to decide > to merge all kinds of junk onto this temp. > > As a result, we should treat a write as a last_read for the purpose of > determining the live range. > > Signed-off-by: Ilia Mirkin <imir...@alum.mit.edu> > Cc: mesa-sta...@lists.freedesktop.org
I think you've convinced me this is right, I'd like to know why DCE doesn't trash that shader better, but I'm happy for this to go in. Signed-off-by: Dave Airlie <airl...@redhat.com> _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
