[Mesa-dev] [PATCH 08/10] i965/fs: Calculate the (un)spill block size correctly.

[Francisco Jerez]

Currently the spilling code attempts to guess the scratch message
block size from the dispatch width of the shader, which is plain wrong
for SIMD-lowered instructions (frequently but not exclusively
encountered in SIMD32 shaders) or for instructions with register
region data types of size other than 32 bit.

Instead try to use the SIMD component size of the instruction which in
some cases will allow the dataport to apply the correct channel mask
to the scratch data read or written.  In the spill case the block size
needs to be clamped to the number of MRF registers reserved for
spilling.  In the unspill case I didn't even bother because we
currently have no 100% accurate way to determine whether a source
region is per-channel or whether it contains things like headers that
don't respect channel boundaries -- That's fine, because the unspill
is marked force_writemask_all we can just use the largest allowable
scratch message size.
---
 src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp b/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp
index 5cb2013..ff40c42 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp
@@ -942,8 +942,14 @@ fs_visitor::spill_reg(int spill_reg)
             inst->src[i].nr = unspill_dst.nr;
             inst->src[i].reg_offset = 0;
 
+            /* We read the largest power-of-two divisor of the register count
+             * (because only POT scratch read blocks are allowed by the
+             * hardware) up to the maximum supported block size.
+             * XXX - Bump the limit when the generator code is ready for
+             *       32-wide spills.
+             */
             const unsigned width =
-               dispatch_width == 16 && regs_read % 2 == 0 ? 16 : 8;
+               MIN2(16, 1u << (ffs(MAX2(1, regs_read) * 8) - 1));
 
             /* Set exec_all() on unspill messages under the (rather
              * pessimistic) assumption that there is no one-to-one
@@ -974,8 +980,16 @@ fs_visitor::spill_reg(int spill_reg)
          inst->no_dd_clear = false;
          inst->no_dd_check = false;
 
-         const unsigned width =
-            dispatch_width == 16 && inst->regs_written % 2 == 0 ? 16 : 8;
+         /* Calculate the execution width of the scratch messages (which work
+          * in terms of 32 bit components so we have a fixed number of eight
+          * channels per spilled register).  We attempt to write one
+          * exec_size-wide component of the variable at a time without
+          * exceeding the maximum number of (fake) MRF registers reserved for
+          * spills.
+          */
+         const unsigned width = 8 * MIN2(
+            DIV_ROUND_UP(inst->dst.component_size(inst->exec_size), REG_SIZE),
+            spill_max_size(this));
 
          /* Spills should only write data initialized by the instruction for
           * whichever channels are enabled in the excution mask.  If that's
-- 
2.7.3