On 10.06.2017 01:44, Connor Abbott wrote:
From: Connor Abbott <cwabbo...@gmail.com>
These optimizations happened to work with derivatives, but they won't
with upcoming shader_ballot and group_vote instructions.
Signed-off-by: Connor Abbott <cwabbo...@gmail.com>
---
src/compiler/nir/nir_instr_set.c | 22 ++++++++++++++++++++++
src/compiler/nir/nir_opt_peephole_select.c | 11 +++++++++++
2 files changed, 33 insertions(+)
diff --git a/src/compiler/nir/nir_instr_set.c b/src/compiler/nir/nir_instr_set.c
index 9cb9ed4..4bd0717 100644
--- a/src/compiler/nir/nir_instr_set.c
+++ b/src/compiler/nir/nir_instr_set.c
@@ -178,6 +178,14 @@ hash_instr(const void *data)
const nir_instr *instr = data;
uint32_t hash = _mesa_fnv32_1a_offset_bias;
+ /*
+ * In nir_instrs_equal(), we compare the instruction's basic blocks in this
+ * case. See the comment there for the explanation.
+ */
+ if (nir_instr_is_cross_thread(instr) && !nir_instr_is_convergent(instr)) {
+ HASH(hash, instr->block);
+ }
+
switch (instr->type) {
case nir_instr_type_alu:
hash = hash_alu(hash, nir_instr_as_alu(instr));
@@ -256,6 +264,20 @@ nir_instrs_equal(const nir_instr *instr1, const nir_instr
*instr2)
if (instr1->type != instr2->type)
return false;
+ /*
+ * If the instructions are cross-thread, then they must have the same
+ * execution mask. If they are convergent, then we can always replace one
+ * invocation with another since every invocation is guaranteed convergent.
+ * But not so for non-convergent instructions, since different invocations
+ * may be called with different execution maskes and therefore have
+ * different results. Conservatively enforce that the instructions are in
+ * the same basic block.
+ */
+ if (nir_instr_is_cross_thread(instr1) && !nir_instr_is_convergent(instr1)) {
Hmm... this is another reason not to like the definition of "cross
thread" and "convergent". It seems like crossthread + convergent is a
weaker restriction that only crossthread, which I'd say is inherently
unintuitive. Can we make it so that each attribute only makes things
more restrictive?
Cheers,
Nicolai
+ if (instr1->block != instr2->block)
+ return false;
+ }
+
switch (instr1->type) {
case nir_instr_type_alu: {
nir_alu_instr *alu1 = nir_instr_as_alu(instr1);
diff --git a/src/compiler/nir/nir_opt_peephole_select.c
b/src/compiler/nir/nir_opt_peephole_select.c
index 4ca4f80..ce41781 100644
--- a/src/compiler/nir/nir_opt_peephole_select.c
+++ b/src/compiler/nir/nir_opt_peephole_select.c
@@ -61,6 +61,17 @@ static bool
block_check_for_allowed_instrs(nir_block *block, unsigned *count, bool alu_ok)
{
nir_foreach_instr(instr, block) {
+ if (nir_instr_is_cross_thread(instr) && !nir_instr_is_convergent(instr))
{
+ /* If the instruction is cross-thread, then we can't execute it
+ * conditionally when we would've executed it unconditionally before,
+ * except when the condition is uniform. If the instruction is
+ * convergent, though, we're already guaranteed that the entire
+ * region is convergent (including the condition) so we can go ahead.
+ *
+ * TODO: allow when the if-condition is uniform
+ */
+ return false;
+ }
switch (instr->type) {
case nir_instr_type_intrinsic: {
nir_intrinsic_instr *intrin = nir_instr_as_intrinsic(instr);
--
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.
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