nak: Add latency_upper_bound to ShaderModel

Signed-off-by: Lorenzo Rossi <git@rossilorenzo.dev>
Reviewed-by: Mel Henning <mhenning@darkrefraction.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37108>

src/nouveau/compiler/nak/ir.rs

@@ -9215,6 +9215,14 @@ pub trait ShaderModel {
     /// Worst-case access-after-write latency
     fn worst_latency(&self, write: &Op, dst_idx: usize) -> u32;
 
+    /// Upper bound on latency
+    ///
+    /// Every '*_latency' function must return latencies that are
+    /// bounded.  Ex: self.war_latency() <= self.latency_upper_bound().
+    /// This is only used for compile-time optimization.  If unsure, be
+    /// conservative.
+    fn latency_upper_bound(&self) -> u32;
+
     /// Maximum encodable instruction delay
     fn max_instr_delay(&self) -> u8;
 

src/nouveau/compiler/nak/sm20.rs

@@ -7,7 +7,7 @@ use crate::legalize::{
 };
 use crate::sm30_instr_latencies::{
     encode_kepler_shader, instr_exec_latency, instr_latency,
-    KeplerInstructionEncoder,
+    latency_upper_bound, KeplerInstructionEncoder,
 };
 use bitview::*;
 
@@ -104,6 +104,10 @@ impl ShaderModel for ShaderModel20 {
         13
     }
 
+    fn latency_upper_bound(&self) -> u32 {
+        latency_upper_bound()
+    }
+
     fn worst_latency(&self, write: &Op, dst_idx: usize) -> u32 {
         instr_latency(self.sm, write, dst_idx)
     }

src/nouveau/compiler/nak/sm30_instr_latencies.rs

@@ -26,6 +26,10 @@ pub fn instr_latency(_sm: u8, op: &Op, _dst_idx: usize) -> u32 {
     }
 }
 
+pub fn latency_upper_bound() -> u32 {
+    24
+}
+
 pub fn instr_exec_latency(sm: u8, op: &Op) -> u32 {
     let is_kepler_a = sm == 30;
     match op {

src/nouveau/compiler/nak/sm32.rs

@@ -8,7 +8,7 @@ use crate::legalize::{
 };
 use crate::sm30_instr_latencies::{
     encode_kepler_shader, instr_exec_latency, instr_latency,
-    KeplerInstructionEncoder,
+    latency_upper_bound, KeplerInstructionEncoder,
 };
 use bitview::{
     BitMutView, BitMutViewable, BitView, BitViewable, SetBit, SetField,
@@ -110,6 +110,10 @@ impl ShaderModel for ShaderModel32 {
         instr_latency(self.sm, write, dst_idx)
     }
 
+    fn latency_upper_bound(&self) -> u32 {
+        latency_upper_bound()
+    }
+
     fn max_instr_delay(&self) -> u8 {
         32
     }

src/nouveau/compiler/nak/sm50.rs

@@ -151,6 +151,10 @@ impl ShaderModel for ShaderModel50 {
         13
     }
 
+    fn latency_upper_bound(&self) -> u32 {
+        14
+    }
+
     fn worst_latency(&self, write: &Op, dst_idx: usize) -> u32 {
         instr_latency(self.sm, write, dst_idx)
     }

src/nouveau/compiler/nak/sm70.rs

@@ -260,6 +260,18 @@ impl ShaderModel for ShaderModel70 {
         }
     }
 
+    fn latency_upper_bound(&self) -> u32 {
+        if self.is_blackwell() {
+            30
+        } else if self.is_ampere() || self.is_ada() {
+            30
+        } else if self.is_turing() {
+            25
+        } else {
+            15
+        }
+    }
+
     fn worst_latency(&self, write: &Op, dst_idx: usize) -> u32 {
         if self.is_blackwell() {
             SM120Latency::raw(write, dst_idx, None, 0)