Hi Kenneth,
El 2014-07-09 17:19, Kenneth Graunke escribió:
On Tuesday, July 08, 2014 11:19:38 AM Iago Toral wrote:
Hi,
I have some code that first initializes a register and then
overwrites a
specific subregister. However, after the optimization passes in
brw_vec4.cpp the initialization instruction goes away. I see that
live
intervals are computed for the destination register without
considering
if we are writing only to a specific subregister, so I guess that is
the
reason for this behaviour, but this strikes me as odd, as it defeats
the
purpose of overwriting only specific subregs, so I guess there is
something else that I am missing.
This an example of what is happening:
mov(8) g6<1>.xUD g0<4,4,1>UD {
align16 WE_all 1Q };
mov(1) g6.2<1>UD 0x00000007UD { align1
WE_all compacted };
The first MOV is removed from the instruction set when I put the
second
MOV, however the second MOV should only be writing to subreg 6.2 and
keep the rest of register 6 intact, or at least that is what I am
trying
to do...
Can someone explain what is wrong with those two instructions? how
should I overwrite only a specific subregister without causing this
behavior?
Thanks,
Iago
This sounds a lot like the bug I fixed in:
commit d0575d98fc595dcc17706dc73d1eb461027ca17a
Author: Kenneth Graunke <kenn...@whitecape.org>
Date: Sat Jun 14 03:53:07 2014 -0700
i965/vec4: Fix dead code elimination for VGRFs of size > 1.
where it would see those two as the same register, and delete the
first MOV.
You probably already have that in your tree though, right? If not,
update :)
Yeah, I have it already, so I guess my problem was different.
If so, you might try turning off other optimization passes in
brw_vec4.cpp and see if it helps - sounds like a bug in one of them.
The one causing that behaviour was the dead code elimination pass in
brw_vec4.cpp. Anyway, this is not happening consistently, I have done
some changes to my code that are unrelated to these two instructions and
now this is not happening any more...
Iago
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