On Tuesday, January 13, 2015 03:35:57 PM Matt Turner wrote:
> This is a fix for a regression introduced in commit a9f8296d ("i965/fs:
> Preserve the CFG in a few more places.").
>
> The errata this code works around is described in a comment before the
> function:
>
> "[DevBW, DevCL] Errata: A destination register from a send can not be
> used as a destination register until after it has been sourced by an
> instruction with a different destination register.
>
> The framebuffer write's sources must be in message registers, which SEND
> instructions cannot have as a destination. There's no way for this
> errata to affect anything at the end of the program. Just remove the
> code.I don't think that's the point. The idea is that code such as SEND g10 ...sources... rlen 4 MUL g10 ... ... needs a workaround - you can't write to the destination of a SEND safely without reading them first. You'd have to do: SEND g10 ...sources... rlen 4 MOV null g10 pointless read of g10, any instruction will do MUL g10 ... Normally, the results of SEND instructions are actually used. However, they aren't always - for example, depth texturing returns 4 values, but we only care about the .X channel. In the past, we attempted to insert DEP_RESOLVE_MOVs to read every register that was written by a SEND. With your patch, we will only insert them for components that were actually used. I imagine this will work fine - if you read the G45 PRM Volume 4 page 516, you'll see all kinds of dependency tracking woes. Switching threads also apparently solves the pre-send problem, so I imagine terminating the thread without ever touching the register again would be safe. I'm kind of tempted to keep the old behavior, out of paranoia, but I can test it, at least. > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84613 > --- > Untested on real hardware. > > src/mesa/drivers/dri/i965/brw_fs.cpp | 10 ---------- > 1 file changed, 10 deletions(-) > > diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp > b/src/mesa/drivers/dri/i965/brw_fs.cpp > index 9dfb7b7..3f9cd68 100644 > --- a/src/mesa/drivers/dri/i965/brw_fs.cpp > +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp > @@ -2925,16 +2925,6 @@ > fs_visitor::insert_gen4_post_send_dependency_workarounds(bblock_t *block, > fs_ins > if (i == write_len) > return; > } > - > - /* If we hit the end of the program, resolve all remaining dependencies > out > - * of paranoia. > - */ > - fs_inst *last_inst = (fs_inst *)this->instructions.get_tail(); > - assert(last_inst->eot); > - for (int i = 0; i < write_len; i++) { > - if (needs_dep[i]) > - last_inst->insert_before(block, DEP_RESOLVE_MOV(first_write_grf + > i)); > - } > } > > void
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