Alyssa Rosenzweig
ee7aaa27b8
We use essentially the same logic, but we need to treat entire clauses
as large instructions, and spill on clause boundaries instead of
instruction boundaries. So factor out the code a bit, using the new
iterators, removing bi_unwrap_singleton.
A few specific fixes are needed to adapt. One is simple: rewriting
destinations needs to preserve clamps and such. The other is a much more
subtle bug. Consider the clause
{
ADD 0, ...
---unrelated code---
MUL 1, 0, ...
}
Suppose we spill 0. The naive spill code would generate an SSA temporary to
spill to and another SSA temporary to fill from, generating:
{
LOAD.tl 10
}
{
ADD 11, ...
---unrelated code---
MUL 1, 10, ...
}
{
STORE.tl 11
}
But this is wrong; the MUL now reads stale (and for SSA, likely
undefined/uninitialized) data. The simplest fix, employed here, is to
spill/fill within a clause simultaneously, which means the temporary
can't be SSA, generating correct code:
{
LOAD.tl r0
}
{
ADD r0, ...
---unrelated code---
MUL 1, r0, ...
}
{
STORE.tl r0
}
This is suboptimal, since the LOAD is still likely reading garbage that
is unused. But it is still correct, and the next commit will avoid
generating the load in this case.
To make the bug even more subtle, if ADD/MUL are within 2-3 instructions
of each other, the scheduler will optimize the load to a
temporary/passthrough, so the bug isn't noticed. It only happens if they
are 3+ instructions apart yet still in the same clause (<=16
instructions).
Special thanks to macc24 for reporting this bug and to Jason Ekstrand
for allowing me to rubberduck.
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8723>
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